This is tar.info, produced by makeinfo version 4.8 from tar.texi. This manual is for GNU `tar' (version 1.16, 16 October 2006), which creates and extracts files from archives. Copyright (C) 1992, 1994, 1995, 1996, 1997, 1999, 2000, 2001, 2003, 2004, 2005, 2006 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with the Front-Cover Texts being "A GNU Manual," and with the Back-Cover Texts as in (a) below. A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Back-Cover Text is: "You are free to copy and modify this GNU Manual. Buying copies from GNU Press supports the FSF in developing GNU and promoting software freedom." INFO-DIR-SECTION Archiving START-INFO-DIR-ENTRY * Tar: (tar). Making tape (or disk) archives. END-INFO-DIR-ENTRY INFO-DIR-SECTION Individual utilities START-INFO-DIR-ENTRY * tar: (tar)tar invocation. Invoking GNU `tar'. END-INFO-DIR-ENTRY  File: tar.info, Node: dereference, Next: old, Prev: Portable Names, Up: Portability 8.3.2 Symbolic Links -------------------- Normally, when `tar' archives a symbolic link, it writes a block to the archive naming the target of the link. In that way, the `tar' archive is a faithful record of the file system contents. `--dereference' (`-h') is used with `--create' (`-c'), and causes `tar' to archive the files symbolic links point to, instead of the links themselves. When this option is used, when `tar' encounters a symbolic link, it will archive the linked-to file, instead of simply recording the presence of a symbolic link. The name under which the file is stored in the file system is not recorded in the archive. To record both the symbolic link name and the file name in the system, archive the file under both names. If all links were recorded automatically by `tar', an extracted file might be linked to a file name that no longer exists in the file system. If a linked-to file is encountered again by `tar' while creating the same archive, an entire second copy of it will be stored. (This _might_ be considered a bug.) So, for portable archives, do not archive symbolic links as such, and use `--dereference' (`-h'): many systems do not support symbolic links, and moreover, your distribution might be unusable if it contains unresolved symbolic links.  File: tar.info, Node: old, Next: ustar, Prev: dereference, Up: Portability 8.3.3 Old V7 Archives --------------------- Certain old versions of `tar' cannot handle additional information recorded by newer `tar' programs. To create an archive in V7 format (not ANSI), which can be read by these old versions, specify the `--format=v7' option in conjunction with the `--create' (`-c') (`tar' also accepts `--portability' or `--old-archive' for this option). When you specify it, `tar' leaves out information about directories, pipes, fifos, contiguous files, and device files, and specifies file ownership by group and user IDs instead of group and user names. When updating an archive, do not use `--format=v7' unless the archive was created using this option. In most cases, a _new_ format archive can be read by an _old_ `tar' program without serious trouble, so this option should seldom be needed. On the other hand, most modern `tar's are able to read old format archives, so it might be safer for you to always use `--format=v7' for your distributions. Notice, however, that `ustar' format is a better alternative, as it is free from many of `v7''s drawbacks.  File: tar.info, Node: ustar, Next: gnu, Prev: old, Up: Portability 8.3.4 Ustar Archive Format -------------------------- Archive format defined by POSIX.1-1988 specification is called `ustar'. Although it is more flexible than the V7 format, it still has many restrictions (*Note ustar: Formats, for the detailed description of `ustar' format). Along with V7 format, `ustar' format is a good choice for archives intended to be read with other implementations of `tar'. To create archive in `ustar' format, use `--format=ustar' option in conjunction with the `--create' (`-c').  File: tar.info, Node: gnu, Next: posix, Prev: ustar, Up: Portability 8.3.5 GNU and old GNU `tar' format ---------------------------------- GNU `tar' was based on an early draft of the POSIX 1003.1 `ustar' standard. GNU extensions to `tar', such as the support for file names longer than 100 characters, use portions of the `tar' header record which were specified in that POSIX draft as unused. Subsequent changes in POSIX have allocated the same parts of the header record for other purposes. As a result, GNU `tar' format is incompatible with the current POSIX specification, and with `tar' programs that follow it. In the majority of cases, `tar' will be configured to create this format by default. This will change in the future releases, since we plan to make `POSIX' format the default. To force creation a GNU `tar' archive, use option `--format=gnu'.  File: tar.info, Node: posix, Next: Checksumming, Prev: gnu, Up: Portability 8.3.6 GNU `tar' and POSIX `tar' ------------------------------- Starting from version 1.14 GNU `tar' features full support for POSIX.1-2001 archives. A POSIX conformant archive will be created if `tar' was given `--format=posix' (`--format=pax') option. No special option is required to read and extract from a POSIX archive. * Menu: * PAX keywords:: Controlling Extended Header Keywords.  File: tar.info, Node: PAX keywords, Up: posix 8.3.6.1 Controlling Extended Header Keywords ............................................ `--pax-option=KEYWORD-LIST' Handle keywords in PAX extended headers. This option is equivalent to `-o' option of the `pax' utility. KEYWORD-LIST is a comma-separated list of keyword options, each keyword option taking one of the following forms: `delete=PATTERN' When used with one of archive-creation commands, this option instructs `tar' to omit from extended header records that it produces any keywords matching the string PATTERN. When used in extract or list mode, this option instructs tar to ignore any keywords matching the given PATTERN in the extended header records. In both cases, matching is performed using the pattern matching notation described in POSIX 1003.2, 3.13 (*note wildcards::). For example: --pax-option delete=security.* would suppress security-related information. `exthdr.name=STRING' This keyword allows user control over the name that is written into the ustar header blocks for the extended headers. The name is obtained from STRING after making the following substitutions: Meta-character Replaced By -------------------------------------------------------- %d The directory name of the file, equivalent to the result of the `dirname' utility on the translated pathname. %f The filename of the file, equivalent to the result of the `basename' utility on the translated pathname. %p The process ID of the `tar' process. %% A `%' character. Any other `%' characters in STRING produce undefined results. If no option `exthdr.name=string' is specified, `tar' will use the following default value: %d/PaxHeaders.%p/%f `globexthdr.name=STRING' This keyword allows user control over the name that is written into the ustar header blocks for global extended header records. The name is obtained from the contents of STRING, after making the following substitutions: Meta-character Replaced By -------------------------------------------------------- %n An integer that represents the sequence number of the global extended header record in the archive, starting at 1. %p The process ID of the `tar' process. %% A `%' character. Any other `%' characters in STRING produce undefined results. If no option `globexthdr.name=string' is specified, `tar' will use the following default value: $TMPDIR/GlobalHead.%p.%n where `$TMPDIR' represents the value of the TMPDIR environment variable. If TMPDIR is not set, `tar' uses `/tmp'. `KEYWORD=VALUE' When used with one of archive-creation commands, these keyword/value pairs will be included at the beginning of the archive in a global extended header record. When used with one of archive-reading commands, `tar' will behave as if it has encountered these keyword/value pairs at the beginning of the archive in a global extended header record. `KEYWORD:=VALUE' When used with one of archive-creation commands, these keyword/value pairs will be included as records at the beginning of an extended header for each file. This is effectively equivalent to KEYWORD=VALUE form except that it creates no global extended header records. When used with one of archive-reading commands, `tar' will behave as if these keyword/value pairs were included as records at the end of each extended header; thus, they will override any global or file-specific extended header record keywords of the same names. For example, in the command: tar --format=posix --create \ --file archive --pax-option gname:=user . the group name will be forced to a new value for all files stored in the archive.  File: tar.info, Node: Checksumming, Next: Large or Negative Values, Prev: posix, Up: Portability 8.3.7 Checksumming Problems --------------------------- SunOS and HP-UX `tar' fail to accept archives created using GNU `tar' and containing non-ASCII file names, that is, file names having characters with the eight bit set, because they use signed checksums, while GNU `tar' uses unsigned checksums while creating archives, as per POSIX standards. On reading, GNU `tar' computes both checksums and accept any. It is somewhat worrying that a lot of people may go around doing backup of their files using faulty (or at least non-standard) software, not learning about it until it's time to restore their missing files with an incompatible file extractor, or vice versa. GNU `tar' compute checksums both ways, and accept any on read, so GNU tar can read Sun tapes even with their wrong checksums. GNU `tar' produces the standard checksum, however, raising incompatibilities with Sun. That is to say, GNU `tar' has not been modified to _produce_ incorrect archives to be read by buggy `tar''s. I've been told that more recent Sun `tar' now read standard archives, so maybe Sun did a similar patch, after all? The story seems to be that when Sun first imported `tar' sources on their system, they recompiled it without realizing that the checksums were computed differently, because of a change in the default signing of `char''s in their compiler. So they started computing checksums wrongly. When they later realized their mistake, they merely decided to stay compatible with it, and with themselves afterwards. Presumably, but I do not really know, HP-UX has chosen that their `tar' archives to be compatible with Sun's. The current standards do not favor Sun `tar' format. In any case, it now falls on the shoulders of SunOS and HP-UX users to get a `tar' able to read the good archives they receive.  File: tar.info, Node: Large or Negative Values, Next: Other Tars, Prev: Checksumming, Up: Portability 8.3.8 Large or Negative Values ------------------------------ _(This message will disappear, once this node revised.)_ The above sections suggest to use `oldest possible' archive format if in doubt. However, sometimes it is not possible. If you attempt to archive a file whose metadata cannot be represented using required format, GNU `tar' will print error message and ignore such a file. You will than have to switch to a format that is able to handle such values. The format summary table (*note Formats::) will help you to do so. In particular, when trying to archive files larger than 8GB or with timestamps not in the range 1970-01-01 00:00:00 through 2242-03-16 12:56:31 UTC, you will have to chose between GNU and POSIX archive formats. When considering which format to choose, bear in mind that the GNU format uses two's-complement base-256 notation to store values that do not fit into standard ustar range. Such archives can generally be read only by a GNU `tar' implementation. Moreover, they sometimes cannot be correctly restored on another hosts even by GNU `tar'. For example, using two's complement representation for negative time stamps that assumes a signed 32-bit `time_t' generates archives that are not portable to hosts with differing `time_t' representations. On the other hand, POSIX archives, generally speaking, can be extracted by any tar implementation that understands older ustar format. The only exception are files larger than 8GB.  File: tar.info, Node: Other Tars, Prev: Large or Negative Values, Up: Portability 8.3.9 How to Extract GNU-Specific Data Using Other `tar' Implementations ------------------------------------------------------------------------ In previous sections you became acquainted with various quirks necessary to make your archives portable. Sometimes you may need to extract archives containing GNU-specific members using some third-party `tar' implementation or an older version of GNU `tar'. Of course your best bet is to have GNU `tar' installed, but if it is for some reason impossible, this section will explain how to cope without it. When we speak about "GNU-specific" members we mean two classes of them: members split between the volumes of a multi-volume archive and sparse members. You will be able to always recover such members if the archive is in PAX format. In addition split members can be recovered from archives in old GNU format. The following subsections describe the required procedures in detail. * Menu: * Split Recovery:: Members Split Between Volumes * Sparse Recovery:: Sparse Members  File: tar.info, Node: Split Recovery, Next: Sparse Recovery, Up: Other Tars 8.3.9.1 Extracting Members Split Between Volumes ................................................ If a member is split between several volumes of an old GNU format archive most third party `tar' implementation will fail to extract it. To extract it, use `tarcat' program (*note Tarcat::). This program is available from GNU `tar' home page (http://www.gnu.org/software/tar/utils/tarcat.html). It concatenates several archive volumes into a single valid archive. For example, if you have three volumes named from `vol-1.tar' to `vol-3.tar', you can do the following to extract them using a third-party `tar': $ tarcat vol-1.tar vol-2.tar vol-3.tar | tar xf - You could use this approach for most (although not all) PAX format archives as well. However, extracting split members from a PAX archive is a much easier task, because PAX volumes are constructed in such a way that each part of a split member is extracted to a different file by `tar' implementations that are not aware of GNU extensions. More specifically, the very first part retains its original name, and all subsequent parts are named using the pattern: %d/GNUFileParts.%p/%f.%n where symbols preceeded by `%' are "macro characters" that have the following meaning: Meta-character Replaced By ------------------------------------------------------------ %d The directory name of the file, equivalent to the result of the `dirname' utility on its full name. %f The file name of the file, equivalent to the result of the `basename' utility on its full name. %p The process ID of the `tar' process that created the archive. %n Ordinal number of this particular part. For example, if the file `var/longfile' was split during archive creation between three volumes, and the creator `tar' process had process ID `27962', then the member names will be: var/longfile var/GNUFileParts.27962/longfile.1 var/GNUFileParts.27962/longfile.2 When you extract your archive using a third-party `tar', these files will be created on your disk, and the only thing you will need to do to restore your file in its original form is concatenate them in the proper order, for example: $ cd var $ cat GNUFileParts.27962/longfile.1 \ GNUFileParts.27962/longfile.2 >> longfile $ rm -f GNUFileParts.27962 Notice, that if the `tar' implementation you use supports PAX format archives, it will probably emit warnings about unknown keywords during extraction. They will look like this: Tar file too small Unknown extended header keyword 'GNU.volume.filename' ignored. Unknown extended header keyword 'GNU.volume.size' ignored. Unknown extended header keyword 'GNU.volume.offset' ignored. You can safely ignore these warnings. If your `tar' implementation is not PAX-aware, you will get more warnings and more files generated on your disk, e.g.: $ tar xf vol-1.tar var/PaxHeaders.27962/longfile: Unknown file type 'x', extracted as normal file Unexpected EOF in archive $ tar xf vol-2.tar tmp/GlobalHead.27962.1: Unknown file type 'g', extracted as normal file GNUFileParts.27962/PaxHeaders.27962/sparsefile.1: Unknown file type 'x', extracted as normal file Ignore these warnings. The `PaxHeaders.*' directories created will contain files with "extended header keywords" describing the extracted files. You can delete them, unless they describe sparse members. Read further to learn more about them.  File: tar.info, Node: Sparse Recovery, Prev: Split Recovery, Up: Other Tars 8.3.9.2 Extracting Sparse Members ................................. Any `tar' implementation will be able to extract sparse members from a PAX archive. However, the extracted files will be "condensed", i.e., any zero blocks will be removed from them. When we restore such a condensed file to its original form, by adding zero bloks (or "holes") back to their original locations, we call this process "expanding" a compressed sparse file. To expand a file, you will need a simple auxiliary program called `xsparse'. It is available in source form from GNU `tar' home page (http://www.gnu.org/software/tar/utils/xsparse.html). Let's begin with archive members in "sparse format version 1.0"(1), which are the easiest to expand. The condensed file will contain both file map and file data, so no additional data will be needed to restore it. If the original file name was `DIR/NAME', then the condensed file will be named `DIR/GNUSparseFile.N/NAME', where N is a decimal number(2). To expand a version 1.0 file, run `xsparse' as follows: $ xsparse `cond-file' where `cond-file' is the name of the condensed file. The utility will deduce the name for the resulting expanded file using the following algorithm: 1. If `cond-file' does not contain any directories, `../cond-file' will be used; 2. If `cond-file' has the form `DIR/T/NAME', where both T and NAME are simple names, with no `/' characters in them, the output file name will be `DIR/NAME'. 3. Otherwise, if `cond-file' has the form `DIR/NAME', the output file name will be `NAME'. In the unlikely case when this algorithm does not suite your needs, you can explicitly specify output file name as a second argument to the command: $ xsparse `cond-file' `out-file' It is often a good idea to run `xsparse' in "dry run" mode first. In this mode, the command does not actually expand the file, but verbosely lists all actions it would be taking to do so. The dry run mode is enabled by `-n' command line argument: $ xsparse -n /home/gray/GNUSparseFile.6058/sparsefile Reading v.1.0 sparse map Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to `/home/gray/sparsefile' Finished dry run To actually expand the file, you would run: $ xsparse /home/gray/GNUSparseFile.6058/sparsefile The program behaves the same way all UNIX utilities do: it will keep quiet unless it has simething important to tell you (e.g. an error condition or something). If you wish it to produce verbose output, similar to that from the dry run mode, use `-v' option: $ xsparse -v /home/gray/GNUSparseFile.6058/sparsefile Reading v.1.0 sparse map Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to `/home/gray/sparsefile' Done Additionally, if your `tar' implementation has extracted the "extended headers" for this file, you can instruct `xstar' to use them in order to verify the integrity of the expanded file. The option `-x' sets the name of the extended header file to use. Continuing our example: $ xsparse -v -x /home/gray/PaxHeaders.6058/sparsefile \ /home/gray/GNUSparseFile.6058/sparsefile Reading extended header file Found variable GNU.sparse.major = 1 Found variable GNU.sparse.minor = 0 Found variable GNU.sparse.name = sparsefile Found variable GNU.sparse.realsize = 217481216 Reading v.1.0 sparse map Expanding file `/home/gray/GNUSparseFile.6058/sparsefile' to `/home/gray/sparsefile' Done An "extended header" is a special `tar' archive header that precedes an archive member and contains a set of "variables", describing the member properties that cannot be stored in the standard `ustar' header. While optional for expanding sparse version 1.0 members, use of extended headers is mandatory when expanding sparse members in older sparse formats: v.0.0 and v.0.1 (The sparse formats are described in detail in *note Sparse Formats::.) So, for this format, the question is: how to obtain extended headers from the archive? If you use a `tar' implementation that does not support PAX format, extended headers for each member will be extracted as a separate file. If we represent the member name as `DIR/NAME', then the extended header file will be named `DIR/PaxHeaders.N/NAME', where N is an integer number. Things become more difficult if your `tar' implementation does support PAX headers, because in this case you will have to manually extract the headers. We recommend the following algorithm: 1. Consult the documentation of your `tar' implementation for an option that prints "block numbers" along with the archive listing (analogous to GNU `tar''s `-R' option). For example, `star' has `-block-number'. 2. Obtain verbose listing using the `block number' option, and find block numbers of the sparse member in question and the member immediately following it. For example, running `star' on our archive we obtain: $ star -t -v -block-number -f arc.tar ... star: Unknown extended header keyword 'GNU.sparse.size' ignored. star: Unknown extended header keyword 'GNU.sparse.numblocks' ignored. star: Unknown extended header keyword 'GNU.sparse.name' ignored. star: Unknown extended header keyword 'GNU.sparse.map' ignored. block 56: 425984 -rw-r--r-- gray/users Jun 25 14:46 2006 GNUSparseFile.28124/sparsefile block 897: 65391 -rw-r--r-- gray/users Jun 24 20:06 2006 README ... (as usual, ignore the warnings about unknown keywords.) 3. Let SIZE be the size of the sparse member, BS be its block number and BN be the block number of the next member. Compute: N = BS - BN - SIZE/512 - 2 This number gives the size of the extended header part in tar "blocks". In our example, this formula gives: `897 - 56 - 425984 / 512 - 2 = 7'. 4. Use `dd' to extract the headers: dd if=ARCHIVE of=HNAME bs=512 skip=BS count=N where ARCHIVE is the archive name, HNAME is a name of the file to store the extended header in, BS and N are computed in previous steps. In our example, this command will be $ dd if=arc.tar of=xhdr bs=512 skip=56 count=7 Finally, you can expand the condensed file, using the obtained header: $ xsparse -v -x xhdr GNUSparseFile.6058/sparsefile Reading extended header file Found variable GNU.sparse.size = 217481216 Found variable GNU.sparse.numblocks = 208 Found variable GNU.sparse.name = sparsefile Found variable GNU.sparse.map = 0,2048,1050624,2048,... Expanding file `GNUSparseFile.28124/sparsefile' to `sparsefile' Done ---------- Footnotes ---------- (1) *Note PAX 1::. (2) technically speaking, N is a "process ID" of the `tar' process which created the archive (*note PAX keywords::).  File: tar.info, Node: cpio, Prev: Portability, Up: Formats 8.4 Comparison of `tar' and `cpio' ================================== _(This message will disappear, once this node revised.)_ The `cpio' archive formats, like `tar', do have maximum pathname lengths. The binary and old ASCII formats have a max path length of 256, and the new ASCII and CRC ASCII formats have a max path length of 1024. GNU `cpio' can read and write archives with arbitrary pathname lengths, but other `cpio' implementations may crash unexplainedly trying to read them. `tar' handles symbolic links in the form in which it comes in BSD; `cpio' doesn't handle symbolic links in the form in which it comes in System V prior to SVR4, and some vendors may have added symlinks to their system without enhancing `cpio' to know about them. Others may have enhanced it in a way other than the way I did it at Sun, and which was adopted by AT&T (and which is, I think, also present in the `cpio' that Berkeley picked up from AT&T and put into a later BSD release--I think I gave them my changes). (SVR4 does some funny stuff with `tar'; basically, its `cpio' can handle `tar' format input, and write it on output, and it probably handles symbolic links. They may not have bothered doing anything to enhance `tar' as a result.) `cpio' handles special files; traditional `tar' doesn't. `tar' comes with V7, System III, System V, and BSD source; `cpio' comes only with System III, System V, and later BSD (4.3-tahoe and later). `tar''s way of handling multiple hard links to a file can handle file systems that support 32-bit inumbers (e.g., the BSD file system); `cpio's way requires you to play some games (in its "binary" format, i-numbers are only 16 bits, and in its "portable ASCII" format, they're 18 bits--it would have to play games with the "file system ID" field of the header to make sure that the file system ID/i-number pairs of different files were always different), and I don't know which `cpio's, if any, play those games. Those that don't might get confused and think two files are the same file when they're not, and make hard links between them. `tar's way of handling multiple hard links to a file places only one copy of the link on the tape, but the name attached to that copy is the _only_ one you can use to retrieve the file; `cpio's way puts one copy for every link, but you can retrieve it using any of the names. What type of check sum (if any) is used, and how is this calculated. See the attached manual pages for `tar' and `cpio' format. `tar' uses a checksum which is the sum of all the bytes in the `tar' header for a file; `cpio' uses no checksum. If anyone knows why `cpio' was made when `tar' was present at the unix scene, It wasn't. `cpio' first showed up in PWB/UNIX 1.0; no generally-available version of UNIX had `tar' at the time. I don't know whether any version that was generally available _within AT&T_ had `tar', or, if so, whether the people within AT&T who did `cpio' knew about it. On restore, if there is a corruption on a tape `tar' will stop at that point, while `cpio' will skip over it and try to restore the rest of the files. The main difference is just in the command syntax and header format. `tar' is a little more tape-oriented in that everything is blocked to start on a record boundary. Is there any differences between the ability to recover crashed archives between the two of them. (Is there any chance of recovering crashed archives at all.) Theoretically it should be easier under `tar' since the blocking lets you find a header with some variation of `dd skip=NN'. However, modern `cpio''s and variations have an option to just search for the next file header after an error with a reasonable chance of resyncing. However, lots of tape driver software won't allow you to continue past a media error which should be the only reason for getting out of sync unless a file changed sizes while you were writing the archive. If anyone knows why `cpio' was made when `tar' was present at the unix scene, please tell me about this too. Probably because it is more media efficient (by not blocking everything and using only the space needed for the headers where `tar' always uses 512 bytes per file header) and it knows how to archive special files. You might want to look at the freely available alternatives. The major ones are `afio', GNU `tar', and `pax', each of which have their own extensions with some backwards compatibility. Sparse files were `tar'red as sparse files (which you can easily test, because the resulting archive gets smaller, and GNU `cpio' can no longer read it).  File: tar.info, Node: Media, Next: Changes, Prev: Formats, Up: Top 9 Tapes and Other Archive Media ******************************* _(This message will disappear, once this node revised.)_ A few special cases about tape handling warrant more detailed description. These special cases are discussed below. Many complexities surround the use of `tar' on tape drives. Since the creation and manipulation of archives located on magnetic tape was the original purpose of `tar', it contains many features making such manipulation easier. Archives are usually written on dismountable media--tape cartridges, mag tapes, or floppy disks. The amount of data a tape or disk holds depends not only on its size, but also on how it is formatted. A 2400 foot long reel of mag tape holds 40 megabytes of data when formatted at 1600 bits per inch. The physically smaller EXABYTE tape cartridge holds 2.3 gigabytes. Magnetic media are re-usable--once the archive on a tape is no longer needed, the archive can be erased and the tape or disk used over. Media quality does deteriorate with use, however. Most tapes or disks should be discarded when they begin to produce data errors. EXABYTE tape cartridges should be discarded when they generate an "error count" (number of non-usable bits) of more than 10k. Magnetic media are written and erased using magnetic fields, and should be protected from such fields to avoid damage to stored data. Sticking a floppy disk to a filing cabinet using a magnet is probably not a good idea. * Menu: * Device:: Device selection and switching * Remote Tape Server:: * Common Problems and Solutions:: * Blocking:: Blocking * Many:: Many archives on one tape * Using Multiple Tapes:: Using Multiple Tapes * label:: Including a Label in the Archive * verify:: * Write Protection::  File: tar.info, Node: Device, Next: Remote Tape Server, Up: Media 9.1 Device Selection and Switching ================================== _(This message will disappear, once this node revised.)_ `-f [HOSTNAME:]FILE' `--file=[HOSTNAME:]FILE' Use archive file or device FILE on HOSTNAME. This option is used to specify the file name of the archive `tar' works on. If the file name is `-', `tar' reads the archive from standard input (when listing or extracting), or writes it to standard output (when creating). If the `-' file name is given when updating an archive, `tar' will read the original archive from its standard input, and will write the entire new archive to its standard output. If the file name contains a `:', it is interpreted as `hostname:file name'. If the HOSTNAME contains an "at" sign (`@'), it is treated as `user@hostname:file name'. In either case, `tar' will invoke the command `rsh' (or `remsh') to start up an `/usr/libexec/rmt' on the remote machine. If you give an alternate login name, it will be given to the `rsh'. Naturally, the remote machine must have an executable `/usr/libexec/rmt'. This program is free software from the University of California, and a copy of the source code can be found with the sources for `tar'; it's compiled and installed by default. The exact path to this utility is determined when configuring the package. It is `PREFIX/libexec/rmt', where PREFIX stands for your installation prefix. This location may also be overridden at runtime by using `rmt-command=COMMAND' option (*Note --rmt-command: Option Summary, for detailed description of this option. *Note Remote Tape Server::, for the description of `rmt' command). If this option is not given, but the environment variable `TAPE' is set, its value is used; otherwise, old versions of `tar' used a default archive name (which was picked when `tar' was compiled). The default is normally set up to be the "first" tape drive or other transportable I/O medium on the system. Starting with version 1.11.5, GNU `tar' uses standard input and standard output as the default device, and I will not try anymore supporting automatic device detection at installation time. This was failing really in too many cases, it was hopeless. This is now completely left to the installer to override standard input and standard output for default device, if this seems preferable. Further, I think _most_ actual usages of `tar' are done with pipes or disks, not really tapes, cartridges or diskettes. Some users think that using standard input and output is running after trouble. This could lead to a nasty surprise on your screen if you forget to specify an output file name--especially if you are going through a network or terminal server capable of buffering large amounts of output. We had so many bug reports in that area of configuring default tapes automatically, and so many contradicting requests, that we finally consider the problem to be portably intractable. We could of course use something like `/dev/tape' as a default, but this is _also_ running after various kind of trouble, going from hung processes to accidental destruction of real tapes. After having seen all this mess, using standard input and output as a default really sounds like the only clean choice left, and a very useful one too. GNU `tar' reads and writes archive in records, I suspect this is the main reason why block devices are preferred over character devices. Most probably, block devices are more efficient too. The installer could also check for `DEFTAPE' in `'. `--force-local' Archive file is local even if it contains a colon. `--rsh-command=COMMAND' Use remote COMMAND instead of `rsh'. This option exists so that people who use something other than the standard `rsh' (e.g., a Kerberized `rsh') can access a remote device. When this command is not used, the shell command found when the `tar' program was installed is used instead. This is the first found of `/usr/ucb/rsh', `/usr/bin/remsh', `/usr/bin/rsh', `/usr/bsd/rsh' or `/usr/bin/nsh'. The installer may have overridden this by defining the environment variable `RSH' _at installation time_. `-[0-7][lmh]' Specify drive and density. `-M' `--multi-volume' Create/list/extract multi-volume archive. This option causes `tar' to write a "multi-volume" archive--one that may be larger than will fit on the medium used to hold it. *Note Multi-Volume Archives::. `-L NUM' `--tape-length=NUM' Change tape after writing NUM x 1024 bytes. This option might be useful when your tape drivers do not properly detect end of physical tapes. By being slightly conservative on the maximum tape length, you might avoid the problem entirely. `-F FILE' `--info-script=FILE' `--new-volume-script=FILE' Execute `file' at end of each tape. This implies `--multi-volume' (`-M'). *Note info-script::, for a detailed description of this option.  File: tar.info, Node: Remote Tape Server, Next: Common Problems and Solutions, Prev: Device, Up: Media 9.2 The Remote Tape Server ========================== In order to access the tape drive on a remote machine, `tar' uses the remote tape server written at the University of California at Berkeley. The remote tape server must be installed as `PREFIX/libexec/rmt' on any machine whose tape drive you want to use. `tar' calls `rmt' by running an `rsh' or `remsh' to the remote machine, optionally using a different login name if one is supplied. A copy of the source for the remote tape server is provided. It is Copyright (C) 1983 by the Regents of the University of California, but can be freely distributed. It is compiled and installed by default. Unless you use the `--absolute-names' (`-P') option, GNU `tar' will not allow you to create an archive that contains absolute file names (a file name beginning with `/'.) If you try, `tar' will automatically remove the leading `/' from the file names it stores in the archive. It will also type a warning message telling you what it is doing. When reading an archive that was created with a different `tar' program, GNU `tar' automatically extracts entries in the archive which have absolute file names as if the file names were not absolute. This is an important feature. A visitor here once gave a `tar' tape to an operator to restore; the operator used Sun `tar' instead of GNU `tar', and the result was that it replaced large portions of our `/bin' and friends with versions from the tape; needless to say, we were unhappy about having to recover the file system from backup tapes. For example, if the archive contained a file `/usr/bin/computoy', GNU `tar' would extract the file to `usr/bin/computoy', relative to the current directory. If you want to extract the files in an archive to the same absolute names that they had when the archive was created, you should do a `cd /' before extracting the files from the archive, or you should either use the `--absolute-names' option, or use the command `tar -C / ...'. Some versions of Unix (Ultrix 3.1 is known to have this problem), can claim that a short write near the end of a tape succeeded, when it actually failed. This will result in the -M option not working correctly. The best workaround at the moment is to use a significantly larger blocking factor than the default 20. In order to update an archive, `tar' must be able to backspace the archive in order to reread or rewrite a record that was just read (or written). This is currently possible only on two kinds of files: normal disk files (or any other file that can be backspaced with `lseek'), and industry-standard 9-track magnetic tape (or any other kind of tape that can be backspaced with the `MTIOCTOP' `ioctl'. This means that the `--append', `--concatenate', and `--delete' commands will not work on any other kind of file. Some media simply cannot be backspaced, which means these commands and options will never be able to work on them. These non-backspacing media include pipes and cartridge tape drives. Some other media can be backspaced, and `tar' will work on them once `tar' is modified to do so. Archives created with the `--multi-volume', `--label', and `--incremental' (`-G') options may not be readable by other version of `tar'. In particular, restoring a file that was split over a volume boundary will require some careful work with `dd', if it can be done at all. Other versions of `tar' may also create an empty file whose name is that of the volume header. Some versions of `tar' may create normal files instead of directories archived with the `--incremental' (`-G') option.  File: tar.info, Node: Common Problems and Solutions, Next: Blocking, Prev: Remote Tape Server, Up: Media 9.3 Some Common Problems and their Solutions ============================================ errors from system: permission denied no such file or directory not owner errors from `tar': directory checksum error header format error errors from media/system: i/o error device busy  File: tar.info, Node: Blocking, Next: Many, Prev: Common Problems and Solutions, Up: Media 9.4 Blocking ============ _(This message will disappear, once this node revised.)_ "Block" and "record" terminology is rather confused, and it is also confusing to the expert reader. On the other hand, readers who are new to the field have a fresh mind, and they may safely skip the next two paragraphs, as the remainder of this manual uses those two terms in a quite consistent way. John Gilmore, the writer of the public domain `tar' from which GNU `tar' was originally derived, wrote (June 1995): The nomenclature of tape drives comes from IBM, where I believe they were invented for the IBM 650 or so. On IBM mainframes, what is recorded on tape are tape blocks. The logical organization of data is into records. There are various ways of putting records into blocks, including `F' (fixed sized records), `V' (variable sized records), `FB' (fixed blocked: fixed size records, N to a block), `VB' (variable size records, N to a block), `VSB' (variable spanned blocked: variable sized records that can occupy more than one block), etc. The `JCL' `DD RECFORM=' parameter specified this to the operating system. The Unix man page on `tar' was totally confused about this. When I wrote `PD TAR', I used the historically correct terminology (`tar' writes data records, which are grouped into blocks). It appears that the bogus terminology made it into POSIX (no surprise here), and now Franc,ois has migrated that terminology back into the source code too. The term "physical block" means the basic transfer chunk from or to a device, after which reading or writing may stop without anything being lost. In this manual, the term "block" usually refers to a disk physical block, _assuming_ that each disk block is 512 bytes in length. It is true that some disk devices have different physical blocks, but `tar' ignore these differences in its own format, which is meant to be portable, so a `tar' block is always 512 bytes in length, and "block" always mean a `tar' block. The term "logical block" often represents the basic chunk of allocation of many disk blocks as a single entity, which the operating system treats somewhat atomically; this concept is only barely used in GNU `tar'. The term "physical record" is another way to speak of a physical block, those two terms are somewhat interchangeable. In this manual, the term "record" usually refers to a tape physical block, _assuming_ that the `tar' archive is kept on magnetic tape. It is true that archives may be put on disk or used with pipes, but nevertheless, `tar' tries to read and write the archive one "record" at a time, whatever the medium in use. One record is made up of an integral number of blocks, and this operation of putting many disk blocks into a single tape block is called "reblocking", or more simply, "blocking". The term "logical record" refers to the logical organization of many characters into something meaningful to the application. The term "unit record" describes a small set of characters which are transmitted whole to or by the application, and often refers to a line of text. Those two last terms are unrelated to what we call a "record" in GNU `tar'. When writing to tapes, `tar' writes the contents of the archive in chunks known as "records". To change the default blocking factor, use the `--blocking-factor=512-SIZE' (`-b 512-SIZE') option. Each record will then be composed of 512-SIZE blocks. (Each `tar' block is 512 bytes. *Note Standard::.) Each file written to the archive uses at least one full record. As a result, using a larger record size can result in more wasted space for small files. On the other hand, a larger record size can often be read and written much more efficiently. Further complicating the problem is that some tape drives ignore the blocking entirely. For these, a larger record size can still improve performance (because the software layers above the tape drive still honor the blocking), but not as dramatically as on tape drives that honor blocking. When reading an archive, `tar' can usually figure out the record size on itself. When this is the case, and a non-standard record size was used when the archive was created, `tar' will print a message about a non-standard blocking factor, and then operate normally. On some tape devices, however, `tar' cannot figure out the record size itself. On most of those, you can specify a blocking factor (with `--blocking-factor') larger than the actual blocking factor, and then use the `--read-full-records' (`-B') option. (If you specify a blocking factor with `--blocking-factor' and don't use the `--read-full-records' option, then `tar' will not attempt to figure out the recording size itself.) On some devices, you must always specify the record size exactly with `--blocking-factor' when reading, because `tar' cannot figure it out. In any case, use `--list' (`-t') before doing any extractions to see whether `tar' is reading the archive correctly. `tar' blocks are all fixed size (512 bytes), and its scheme for putting them into records is to put a whole number of them (one or more) into each record. `tar' records are all the same size; at the end of the file there's a block containing all zeros, which is how you tell that the remainder of the last record(s) are garbage. In a standard `tar' file (no options), the block size is 512 and the record size is 10240, for a blocking factor of 20. What the `--blocking-factor' option does is sets the blocking factor, changing the record size while leaving the block size at 512 bytes. 20 was fine for ancient 800 or 1600 bpi reel-to-reel tape drives; most tape drives these days prefer much bigger records in order to stream and not waste tape. When writing tapes for myself, some tend to use a factor of the order of 2048, say, giving a record size of around one megabyte. If you use a blocking factor larger than 20, older `tar' programs might not be able to read the archive, so we recommend this as a limit to use in practice. GNU `tar', however, will support arbitrarily large record sizes, limited only by the amount of virtual memory or the physical characteristics of the tape device. * Menu: * Format Variations:: Format Variations * Blocking Factor:: The Blocking Factor of an Archive  File: tar.info, Node: Format Variations, Next: Blocking Factor, Up: Blocking 9.4.1 Format Variations ----------------------- _(This message will disappear, once this node revised.)_ Format parameters specify how an archive is written on the archive media. The best choice of format parameters will vary depending on the type and number of files being archived, and on the media used to store the archive. To specify format parameters when accessing or creating an archive, you can use the options described in the following sections. If you do not specify any format parameters, `tar' uses default parameters. You cannot modify a compressed archive. If you create an archive with the `--blocking-factor' option specified (*note Blocking Factor::), you must specify that blocking-factor when operating on the archive. *Note Formats::, for other examples of format parameter considerations.  File: tar.info, Node: Blocking Factor, Prev: Format Variations, Up: Blocking 9.4.2 The Blocking Factor of an Archive --------------------------------------- _(This message will disappear, once this node revised.)_ The data in an archive is grouped into blocks, which are 512 bytes. Blocks are read and written in whole number multiples called "records". The number of blocks in a record (i.e., the size of a record in units of 512 bytes) is called the "blocking factor". The `--blocking-factor=512-SIZE' (`-b 512-SIZE') option specifies the blocking factor of an archive. The default blocking factor is typically 20 (i.e., 10240 bytes), but can be specified at installation. To find out the blocking factor of an existing archive, use `tar --list --file=ARCHIVE-NAME'. This may not work on some devices. Records are separated by gaps, which waste space on the archive media. If you are archiving on magnetic tape, using a larger blocking factor (and therefore larger records) provides faster throughput and allows you to fit more data on a tape (because there are fewer gaps). If you are archiving on cartridge, a very large blocking factor (say 126 or more) greatly increases performance. A smaller blocking factor, on the other hand, may be useful when archiving small files, to avoid archiving lots of nulls as `tar' fills out the archive to the end of the record. In general, the ideal record size depends on the size of the inter-record gaps on the tape you are using, and the average size of the files you are archiving. *Note create::, for information on writing archives. Archives with blocking factors larger than 20 cannot be read by very old versions of `tar', or by some newer versions of `tar' running on old machines with small address spaces. With GNU `tar', the blocking factor of an archive is limited only by the maximum record size of the device containing the archive, or by the amount of available virtual memory. Also, on some systems, not using adequate blocking factors, as sometimes imposed by the device drivers, may yield unexpected diagnostics. For example, this has been reported: Cannot write to /dev/dlt: Invalid argument In such cases, it sometimes happen that the `tar' bundled by the system is aware of block size idiosyncrasies, while GNU `tar' requires an explicit specification for the block size, which it cannot guess. This yields some people to consider GNU `tar' is misbehaving, because by comparison, `the bundle `tar' works OK'. Adding `-b 256', for example, might resolve the problem. If you use a non-default blocking factor when you create an archive, you must specify the same blocking factor when you modify that archive. Some archive devices will also require you to specify the blocking factor when reading that archive, however this is not typically the case. Usually, you can use `--list' (`-t') without specifying a blocking factor--`tar' reports a non-default record size and then lists the archive members as it would normally. To extract files from an archive with a non-standard blocking factor (particularly if you're not sure what the blocking factor is), you can usually use the `--read-full-records' (`-B') option while specifying a blocking factor larger then the blocking factor of the archive (i.e., `tar --extract --read-full-records --blocking-factor=300'. *Note list::, for more information on the `--list' (`-t') operation. *Note Reading::, for a more detailed explanation of that option. `--blocking-factor=NUMBER' `-b NUMBER' Specifies the blocking factor of an archive. Can be used with any operation, but is usually not necessary with `--list' (`-t'). Device blocking `-b BLOCKS' `--blocking-factor=BLOCKS' Set record size to BLOCKS * 512 bytes. This option is used to specify a "blocking factor" for the archive. When reading or writing the archive, `tar', will do reads and writes of the archive in records of BLOCK*512 bytes. This is true even when the archive is compressed. Some devices requires that all write operations be a multiple of a certain size, and so, `tar' pads the archive out to the next record boundary. The default blocking factor is set when `tar' is compiled, and is typically 20. Blocking factors larger than 20 cannot be read by very old versions of `tar', or by some newer versions of `tar' running on old machines with small address spaces. With a magnetic tape, larger records give faster throughput and fit more data on a tape (because there are fewer inter-record gaps). If the archive is in a disk file or a pipe, you may want to specify a smaller blocking factor, since a large one will result in a large number of null bytes at the end of the archive. When writing cartridge or other streaming tapes, a much larger blocking factor (say 126 or more) will greatly increase performance. However, you must specify the same blocking factor when reading or updating the archive. Apparently, Exabyte drives have a physical block size of 8K bytes. If we choose our blocksize as a multiple of 8k bytes, then the problem seems to disappear. Id est, we are using block size of 112 right now, and we haven't had the problem since we switched... With GNU `tar' the blocking factor is limited only by the maximum record size of the device containing the archive, or by the amount of available virtual memory. However, deblocking or reblocking is virtually avoided in a special case which often occurs in practice, but which requires all the following conditions to be simultaneously true: * the archive is subject to a compression option, * the archive is not handled through standard input or output, nor redirected nor piped, * the archive is directly handled to a local disk, instead of any special device, * `--blocking-factor' is not explicitly specified on the `tar' invocation. If the output goes directly to a local disk, and not through stdout, then the last write is not extended to a full record size. Otherwise, reblocking occurs. Here are a few other remarks on this topic: * `gzip' will complain about trailing garbage if asked to uncompress a compressed archive on tape, there is an option to turn the message off, but it breaks the regularity of simply having to use `PROG -d' for decompression. It would be nice if gzip was silently ignoring any number of trailing zeros. I'll ask Jean-loup Gailly, by sending a copy of this message to him. * `compress' does not show this problem, but as Jean-loup pointed out to Michael, `compress -d' silently adds garbage after the result of decompression, which tar ignores because it already recognized its end-of-file indicator. So this bug may be safely ignored. * `gzip -d -q' will be silent about the trailing zeros indeed, but will still return an exit status of 2 which tar reports in turn. `tar' might ignore the exit status returned, but I hate doing that, as it weakens the protection `tar' offers users against other possible problems at decompression time. If `gzip' was silently skipping trailing zeros _and_ also avoiding setting the exit status in this innocuous case, that would solve this situation. * `tar' should become more solid at not stopping to read a pipe at the first null block encountered. This inelegantly breaks the pipe. `tar' should rather drain the pipe out before exiting itself. `-i' `--ignore-zeros' Ignore blocks of zeros in archive (means EOF). The `--ignore-zeros' (`-i') option causes `tar' to ignore blocks of zeros in the archive. Normally a block of zeros indicates the end of the archive, but when reading a damaged archive, or one which was created by concatenating several archives together, this option allows `tar' to read the entire archive. This option is not on by default because many versions of `tar' write garbage after the zeroed blocks. Note that this option causes `tar' to read to the end of the archive file, which may sometimes avoid problems when multiple files are stored on a single physical tape. `-B' `--read-full-records' Reblock as we read (for reading 4.2BSD pipes). If `--read-full-records' is used, `tar' will not panic if an attempt to read a record from the archive does not return a full record. Instead, `tar' will keep reading until it has obtained a full record. This option is turned on by default when `tar' is reading an archive from standard input, or from a remote machine. This is because on BSD Unix systems, a read of a pipe will return however much happens to be in the pipe, even if it is less than `tar' requested. If this option was not used, `tar' would fail as soon as it read an incomplete record from the pipe. This option is also useful with the commands for updating an archive. Tape blocking When handling various tapes or cartridges, you have to take care of selecting a proper blocking, that is, the number of disk blocks you put together as a single tape block on the tape, without intervening tape gaps. A "tape gap" is a small landing area on the tape with no information on it, used for decelerating the tape to a full stop, and for later regaining the reading or writing speed. When the tape driver starts reading a record, the record has to be read whole without stopping, as a tape gap is needed to stop the tape motion without loosing information. Using higher blocking (putting more disk blocks per tape block) will use the tape more efficiently as there will be less tape gaps. But reading such tapes may be more difficult for the system, as more memory will be required to receive at once the whole record. Further, if there is a reading error on a huge record, this is less likely that the system will succeed in recovering the information. So, blocking should not be too low, nor it should be too high. `tar' uses by default a blocking of 20 for historical reasons, and it does not really matter when reading or writing to disk. Current tape technology would easily accommodate higher blockings. Sun recommends a blocking of 126 for Exabytes and 96 for DATs. We were told that for some DLT drives, the blocking should be a multiple of 4Kb, preferably 64Kb (`-b 128') or 256 for decent performance. Other manufacturers may use different recommendations for the same tapes. This might also depends of the buffering techniques used inside modern tape controllers. Some imposes a minimum blocking, or a maximum blocking. Others request blocking to be some exponent of two. So, there is no fixed rule for blocking. But blocking at read time should ideally be the same as blocking used at write time. At one place I know, with a wide variety of equipment, they found it best to use a blocking of 32 to guarantee that their tapes are fully interchangeable. I was also told that, for recycled tapes, prior erasure (by the same drive unit that will be used to create the archives) sometimes lowers the error rates observed at rewriting time. I might also use `--number-blocks' instead of `--block-number', so `--block' will then expand to `--blocking-factor' unambiguously.  File: tar.info, Node: Many, Next: Using Multiple Tapes, Prev: Blocking, Up: Media 9.5 Many Archives on One Tape ============================= Most tape devices have two entries in the `/dev' directory, or entries that come in pairs, which differ only in the minor number for this device. Let's take for example `/dev/tape', which often points to the only or usual tape device of a given system. There might be a corresponding `/dev/nrtape' or `/dev/ntape'. The simpler name is the _rewinding_ version of the device, while the name having `nr' in it is the _no rewinding_ version of the same device. A rewinding tape device will bring back the tape to its beginning point automatically when this device is opened or closed. Since `tar' opens the archive file before using it and closes it afterwards, this means that a simple: $ tar cf /dev/tape DIRECTORY will reposition the tape to its beginning both prior and after saving DIRECTORY contents to it, thus erasing prior tape contents and making it so that any subsequent write operation will destroy what has just been saved. So, a rewinding device is normally meant to hold one and only one file. If you want to put more than one `tar' archive on a given tape, you will need to avoid using the rewinding version of the tape device. You will also have to pay special attention to tape positioning. Errors in positioning may overwrite the valuable data already on your tape. Many people, burnt by past experiences, will only use rewinding devices and limit themselves to one file per tape, precisely to avoid the risk of such errors. Be fully aware that writing at the wrong position on a tape loses all information past this point and most probably until the end of the tape, and this destroyed information _cannot_ be recovered. To save DIRECTORY-1 as a first archive at the beginning of a tape, and leave that tape ready for a second archive, you should use: $ mt -f /dev/nrtape rewind $ tar cf /dev/nrtape DIRECTORY-1 "Tape marks" are special magnetic patterns written on the tape media, which are later recognizable by the reading hardware. These marks are used after each file, when there are many on a single tape. An empty file (that is to say, two tape marks in a row) signal the logical end of the tape, after which no file exist. Usually, non-rewinding tape device drivers will react to the close request issued by `tar' by first writing two tape marks after your archive, and by backspacing over one of these. So, if you remove the tape at that time from the tape drive, it is properly terminated. But if you write another file at the current position, the second tape mark will be erased by the new information, leaving only one tape mark between files. So, you may now save DIRECTORY-2 as a second archive after the first on the same tape by issuing the command: $ tar cf /dev/nrtape DIRECTORY-2 and so on for all the archives you want to put on the same tape. Another usual case is that you do not write all the archives the same day, and you need to remove and store the tape between two archive sessions. In general, you must remember how many files are already saved on your tape. Suppose your tape already has 16 files on it, and that you are ready to write the 17th. You have to take care of skipping the first 16 tape marks before saving DIRECTORY-17, say, by using these commands: $ mt -f /dev/nrtape rewind $ mt -f /dev/nrtape fsf 16 $ tar cf /dev/nrtape DIRECTORY-17 In all the previous examples, we put aside blocking considerations, but you should do the proper things for that as well. *Note Blocking::. * Menu: * Tape Positioning:: Tape Positions and Tape Marks * mt:: The `mt' Utility  File: tar.info, Node: Tape Positioning, Next: mt, Up: Many 9.5.1 Tape Positions and Tape Marks ----------------------------------- _(This message will disappear, once this node revised.)_ Just as archives can store more than one file from the file system, tapes can store more than one archive file. To keep track of where archive files (or any other type of file stored on tape) begin and end, tape archive devices write magnetic "tape marks" on the archive media. Tape drives write one tape mark between files, two at the end of all the file entries. If you think of data as a series of records "rrrr"'s, and tape marks as "*"'s, a tape might look like the following: rrrr*rrrrrr*rrrrr*rr*rrrrr**------------------------- Tape devices read and write tapes using a read/write "tape head"--a physical part of the device which can only access one point on the tape at a time. When you use `tar' to read or write archive data from a tape device, the device will begin reading or writing from wherever on the tape the tape head happens to be, regardless of which archive or what part of the archive the tape head is on. Before writing an archive, you should make sure that no data on the tape will be overwritten (unless it is no longer needed). Before reading an archive, you should make sure the tape head is at the beginning of the archive you want to read. You can do it manually via `mt' utility (*note mt::). The `restore' script does that automatically (*note Scripted Restoration::). If you want to add new archive file entries to a tape, you should advance the tape to the end of the existing file entries, backspace over the last tape mark, and write the new archive file. If you were to add two archives to the example above, the tape might look like the following: rrrr*rrrrrr*rrrrr*rr*rrrrr*rrr*rrrr**----------------  File: tar.info, Node: mt, Prev: Tape Positioning, Up: Many 9.5.2 The `mt' Utility ---------------------- _(This message will disappear, once this node revised.)_ *Note Blocking Factor::. You can use the `mt' utility to advance or rewind a tape past a specified number of archive files on the tape. This will allow you to move to the beginning of an archive before extracting or reading it, or to the end of all the archives before writing a new one. The syntax of the `mt' command is: mt [-f TAPENAME] OPERATION [NUMBER] where TAPENAME is the name of the tape device, NUMBER is the number of times an operation is performed (with a default of one), and OPERATION is one of the following: `eof' `weof' Writes NUMBER tape marks at the current position on the tape. `fsf' Moves tape position forward NUMBER files. `bsf' Moves tape position back NUMBER files. `rewind' Rewinds the tape. (Ignores NUMBER). `offline' `rewoff1' Rewinds the tape and takes the tape device off-line. (Ignores NUMBER). `status' Prints status information about the tape unit. If you don't specify a TAPENAME, `mt' uses the environment variable `TAPE'; if `TAPE' is not set, `mt' will use the default device specified in your `sys/mtio.h' file (`DEFTAPE' variable). If this is not defined, the program will display a descriptive error message and exit with code 1. `mt' returns a 0 exit status when the operation(s) were successful, 1 if the command was unrecognized, and 2 if an operation failed.  File: tar.info, Node: Using Multiple Tapes, Next: label, Prev: Many, Up: Media 9.6 Using Multiple Tapes ======================== Often you might want to write a large archive, one larger than will fit on the actual tape you are using. In such a case, you can run multiple `tar' commands, but this can be inconvenient, particularly if you are using options like `--exclude=PATTERN' or dumping entire file systems. Therefore, `tar' provides a special mode for creating multi-volume archives. "Multi-volume" archive is a single `tar' archive, stored on several media volumes of fixed size. Although in this section we will often call `volume' a "tape", there is absolutely no requirement for multi-volume archives to be stored on tapes. Instead, they can use whatever media type the user finds convenient, they can even be located on files. When creating a multi-volume archive, GNU `tar' continues to fill current volume until it runs out of space, then it switches to next volume (usually the operator is queried to replace the tape on this point), and continues working on the new volume. This operation continues until all requested files are dumped. If GNU `tar' detects end of media while dumping a file, such a file is archived in split form. Some very big files can even be split across several volumes. Each volume is itself a valid GNU `tar' archive, so it can be read without any special options. Consequently any file member residing entirely on one volume can be extracted or otherwise operated upon without needing the other volume. Sure enough, to extract a split member you would need all volumes its parts reside on. Multi-volume archives suffer from several limitations. In particular, they cannot be compressed. GNU `tar' is able to create multi-volume archives of two formats (*note Formats::): `GNU' and `POSIX'. * Menu: * Multi-Volume Archives:: Archives Longer than One Tape or Disk * Tape Files:: Tape Files * Tarcat:: Concatenate Volumes into a Single Archive  File: tar.info, Node: Multi-Volume Archives, Next: Tape Files, Up: Using Multiple Tapes 9.6.1 Archives Longer than One Tape or Disk ------------------------------------------- To create an archive that is larger than will fit on a single unit of the media, use the `--multi-volume' (`-M') option in conjunction with the `--create' option (*note create::). A "multi-volume" archive can be manipulated like any other archive (provided the `--multi-volume' option is specified), but is stored on more than one tape or disk. When you specify `--multi-volume', `tar' does not report an error when it comes to the end of an archive volume (when reading), or the end of the media (when writing). Instead, it prompts you to load a new storage volume. If the archive is on a magnetic tape, you should change tapes when you see the prompt; if the archive is on a floppy disk, you should change disks; etc. `--multi-volume' `-M' Creates a multi-volume archive, when used in conjunction with `--create' (`-c'). To perform any other operation on a multi-volume archive, specify `--multi-volume' in conjunction with that operation. For example: $ tar --create --multi-volume --file=/dev/tape FILES The method `tar' uses to detect end of tape is not perfect, and fails on some operating systems or on some devices. If `tar' cannot detect the end of the tape itself, you can use `--tape-length' option to inform it about the capacity of the tape: `--tape-length=SIZE' `-L SIZE' Set maximum length of a volume. The SIZE argument should then be the usable size of the tape in units of 1024 bytes. This option selects `--multi-volume' automatically. For example: $ tar --create --tape-length=41943040 --file=/dev/tape FILES When GNU `tar' comes to the end of a storage media, it asks you to change the volume. The built-in prompt for POSIX locale is(1): Prepare volume #N for `ARCHIVE' and hit return: where N is the ordinal number of the volume to be created and ARCHIVE is archive file or device name. When prompting for a new tape, `tar' accepts any of the following responses: `?' Request `tar' to explain possible responses `q' Request `tar' to exit immediately. `n FILE-NAME' Request `tar' to write the next volume on the file FILE-NAME. `!' Request `tar' to run a subshell. This option can be disabled by giving `--restrict' command line option to `tar'(2). `y' Request `tar' to begin writing the next volume. (You should only type `y' after you have changed the tape; otherwise `tar' will write over the volume it just finished.) The volume number used by `tar' in its tape-changing prompt can be changed; if you give the `--volno-file=FILE-OF-NUMBER' option, then FILE-OF-NUMBER should be an non-existing file to be created, or else, a file already containing a decimal number. That number will be used as the volume number of the first volume written. When `tar' is finished, it will rewrite the file with the now-current volume number. (This does not change the volume number written on a tape label, as per *note label::, it _only_ affects the number used in the prompt.) If you want more elaborate behavior than this, you can write a special "new volume script", that will be responsible for changing the volume, and instruct `tar' to use it instead of its normal prompting procedure: `--info-script=SCRIPT-NAME' `--new-volume-script=SCRIPT-NAME' `-F SCRIPT-NAME' Specify the full name of the volume script to use. The script can be used to eject cassettes, or to broadcast messages such as `Someone please come change my tape' when performing unattended backups. The SCRIPT-NAME is executed without any command line arguments. It inherits `tar''s shell environment. Additional data is passed to it via the following environment variables: `TAR_VERSION' GNU `tar' version number. `TAR_ARCHIVE' The name of the archive `tar' is processing. `TAR_VOLUME' Ordinal number of the volume `tar' is about to start. `TAR_SUBCOMMAND' Short option describing the operation `tar' is executing *Note Operations::, for a complete list of subcommand options. `TAR_FORMAT' Format of the archive being processed. *Note Formats::, for a complete list of archive format names. The volume script can instruct `tar' to use new archive name, by writing in to file descriptor 3 (see below for an example). If the info script fails, `tar' exits; otherwise, it begins writing the next volume. If you want `tar' to cycle through a series of files or tape drives, there are three approaches to choose from. First of all, you can give `tar' multiple `--file' options. In this case the specified files will be used, in sequence, as the successive volumes of the archive. Only when the first one in the sequence needs to be used again will `tar' prompt for a tape change (or run the info script). For example, suppose someone has two tape drives on a system named `/dev/tape0' and `/dev/tape1'. For having GNU `tar' to switch to the second drive when it needs to write the second tape, and then back to the first tape, etc., just do either of: $ tar --create --multi-volume --file=/dev/tape0 --file=/dev/tape1 FILES $ tar cMff /dev/tape0 /dev/tape1 FILES The second method is to use the `n' response to the tape-change prompt. Finally, the most flexible approach is to use a volume script, that writes new archive name to the file descriptor #3. For example, the following volume script will create a series of archive files, named `ARCHIVE-VOL', where ARCHIVE is the name of the archive being created (as given by `--file' option) and VOL is the ordinal number of the archive being created: #! /bin/sh echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE. name=`expr $TAR_ARCHIVE : '\(.*\)-.*'` case $TAR_SUBCOMMAND in -c) ;; -d|-x|-t) test -r ${name:-$TAR_ARCHIVE}-$TAR_VOLUME || exit 1 ;; *) exit 1 esac echo ${name:-$TAR_ARCHIVE}-$TAR_VOLUME >&3 The same script cant be used while listing, comparing or extracting from the created archive. For example: # Create a multi-volume archive: $ tar -c -L1024 -f archive.tar -F new-volume . # Extract from the created archive: $ tar -x -f archive.tar -F new-volume . Notice, that the first command had to use `-L' option, since otherwise GNU `tar' will end up writing everything to file `archive.tar'. You can read each individual volume of a multi-volume archive as if it were an archive by itself. For example, to list the contents of one volume, use `--list', without `--multi-volume' specified. To extract an archive member from one volume (assuming it is described that volume), use `--extract', again without `--multi-volume'. If an archive member is split across volumes (i.e., its entry begins on one volume of the media and ends on another), you need to specify `--multi-volume' to extract it successfully. In this case, you should load the volume where the archive member starts, and use `tar --extract --multi-volume'--`tar' will prompt for later volumes as it needs them. *Note extracting archives::, for more information about extracting archives. Multi-volume archives can be modified like any other archive. To add files to a multi-volume archive, you need to only mount the last volume of the archive media (and new volumes, if needed). For all other operations, you need to use the entire archive. If a multi-volume archive was labeled using `--label=ARCHIVE-LABEL' (*note label::) when it was created, `tar' will not automatically label volumes which are added later. To label subsequent volumes, specify `--label=ARCHIVE-LABEL' again in conjunction with the `--append', `--update' or `--concatenate' operation. Notice that multi-volume support is a GNU extension and the archives created in this mode should be read only using GNU `tar'. If you absolutely have to process such archives using a third-party `tar' implementation, read *note Split Recovery::. ---------- Footnotes ---------- (1) If you run GNU `tar' under a different locale, the translation to the locale's language will be used. (2) *Note --restrict::, for more information about this option  File: tar.info, Node: Tape Files, Next: Tarcat, Prev: Multi-Volume Archives, Up: Using Multiple Tapes 9.6.2 Tape Files ---------------- _(This message will disappear, once this node revised.)_ To give the archive a name which will be recorded in it, use the `--label=VOLUME-LABEL' (`-V VOLUME-LABEL') option. This will write a special block identifying VOLUME-LABEL as the name of the archive to the front of the archive which will be displayed when the archive is listed with `--list'. If you are creating a multi-volume archive with `--multi-volume' (*note Using Multiple Tapes::), then the volume label will have `Volume NNN' appended to the name you give, where NNN is the number of the volume of the archive. (If you use the `--label=VOLUME-LABEL') option when reading an archive, it checks to make sure the label on the tape matches the one you give. *Note label::. When `tar' writes an archive to tape, it creates a single tape file. If multiple archives are written to the same tape, one after the other, they each get written as separate tape files. When extracting, it is necessary to position the tape at the right place before running `tar'. To do this, use the `mt' command. For more information on the `mt' command and on the organization of tapes into a sequence of tape files, see *note mt::. People seem to often do: --label="SOME-PREFIX `date +SOME-FORMAT`" or such, for pushing a common date in all volumes or an archive set.  File: tar.info, Node: Tarcat, Prev: Tape Files, Up: Using Multiple Tapes 9.6.3 Concatenate Volumes into a Single Archive ----------------------------------------------- Sometimes it is necessary to convert existing GNU `tar' multi-volume archive to a single `tar' archive. Simply concatenating all volumes into one will not work, since each volume carries an additional information at the beginning. GNU `tar' is shipped with the shell script `tarcat' designed for this purpose. The script takes a list of files comprising a multi-volume archive and creates the resulting archive at the standard output. For example: tarcat vol.1 vol.2 vol.3 | tar tf - The script implements a simple heuristics to determine the format of the first volume file and to decide how to process the rest of the files. However, it makes no attempt to verify whether the files are given in order or even if they are valid `tar' archives. It uses `dd' and does not filter its standard error, so you will usually see lots of spurious messages.  File: tar.info, Node: label, Next: verify, Prev: Using Multiple Tapes, Up: Media 9.7 Including a Label in the Archive ==================================== _(This message will disappear, once this node revised.)_ To avoid problems caused by misplaced paper labels on the archive media, you can include a "label" entry--an archive member which contains the name of the archive--in the archive itself. Use the `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') option in conjunction with the `--create' operation to include a label entry in the archive as it is being created. `--label=ARCHIVE-LABEL' `-V ARCHIVE-LABEL' Includes an "archive-label" at the beginning of the archive when the archive is being created, when used in conjunction with the `--create' operation. Checks to make sure the archive label matches the one specified (when used in conjunction with any other operation. If you create an archive using both `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') and `--multi-volume' (`-M'), each volume of the archive will have an archive label of the form `ARCHIVE-LABEL Volume N', where N is 1 for the first volume, 2 for the next, and so on. *Note Using Multiple Tapes::, for information on creating multiple volume archives. The volume label will be displayed by `--list' along with the file contents. If verbose display is requested, it will also be explicitly marked as in the example below: $ tar --verbose --list --file=iamanarchive V--------- 0 0 0 1992-03-07 12:01 iamalabel--Volume Header-- -rw-r--r-- ringo user 40 1990-05-21 13:30 iamafilename However, `--list' option will cause listing entire contents of the archive, which may be undesirable (for example, if the archive is stored on a tape). You can request checking only the volume by specifying `--test-label' option. This option reads only the first block of an archive, so it can be used with slow storage devices. For example: $ tar --test-label --file=iamanarchive iamalabel If `--test-label' is used with a single command line argument, `tar' compares the volume label with the argument. It exits with code 0 if the two strings match, and with code 2 otherwise. In this case no output is displayed. For example: $ tar --test-label --file=iamanarchive 'iamalable' => 0 $ tar --test-label --file=iamanarchive 'iamalable' alabel => 1 If you request any operation, other than `--create', along with using `--label' option, `tar' will first check if the archive label matches the one specified and will refuse to proceed if it does not. Use this as a safety precaution to avoid accidentally overwriting existing archives. For example, if you wish to add files to `archive', presumably labeled with string `My volume', you will get: $ tar -rf archive --label 'My volume' . tar: Archive not labeled to match `My volume' in case its label does not match. This will work even if `archive' is not labeled at all. Similarly, `tar' will refuse to list or extract the archive if its label doesn't match the ARCHIVE-LABEL specified. In those cases, ARCHIVE-LABEL argument is interpreted as a globbing-style pattern which must match the actual magnetic volume label. *Note exclude::, for a precise description of how match is attempted(1). If the switch `--multi-volume' (`-M') is being used, the volume label matcher will also suffix ARCHIVE-LABEL by ` Volume [1-9]*' if the initial match fails, before giving up. Since the volume numbering is automatically added in labels at creation time, it sounded logical to equally help the user taking care of it when the archive is being read. The `--label' was once called `--volume', but is not available under that name anymore. You can also use `--label' to get a common information on all tapes of a series. For having this information different in each series created through a single script used on a regular basis, just manage to get some date string as part of the label. For example: $ tar cfMV /dev/tape "Daily backup for `date +%Y-%m-%d`" $ tar --create --file=/dev/tape --multi-volume \ --volume="Daily backup for `date +%Y-%m-%d`" Also note that each label has its own date and time, which corresponds to when GNU `tar' initially attempted to write it, often soon after the operator launches `tar' or types the carriage return telling that the next tape is ready. Comparing date labels does give an idea of tape throughput only if the delays for rewinding tapes and the operator switching them were negligible, which is usually not the case. ---------- Footnotes ---------- (1) Previous versions of `tar' used full regular expression matching, or before that, only exact string matching, instead of wildcard matchers. We decided for the sake of simplicity to use a uniform matching device through `tar'.  File: tar.info, Node: verify, Next: Write Protection, Prev: label, Up: Media 9.8 Verifying Data as It is Stored ================================== `-W' `--verify' Attempt to verify the archive after writing. This option causes `tar' to verify the archive after writing it. Each volume is checked after it is written, and any discrepancies are recorded on the standard error output. Verification requires that the archive be on a back-space-able medium. This means pipes, some cartridge tape drives, and some other devices cannot be verified. You can insure the accuracy of an archive by comparing files in the system with archive members. `tar' can compare an archive to the file system as the archive is being written, to verify a write operation, or can compare a previously written archive, to insure that it is up to date. To check for discrepancies in an archive immediately after it is written, use the `--verify' (`-W') option in conjunction with the `--create' operation. When this option is specified, `tar' checks archive members against their counterparts in the file system, and reports discrepancies on the standard error. To verify an archive, you must be able to read it from before the end of the last written entry. This option is useful for detecting data errors on some tapes. Archives written to pipes, some cartridge tape drives, and some other devices cannot be verified. One can explicitly compare an already made archive with the file system by using the `--compare' (`--diff', `-d') option, instead of using the more automatic `--verify' option. *Note compare::. Note that these two options have a slightly different intent. The `--compare' option checks how identical are the logical contents of some archive with what is on your disks, while the `--verify' option is really for checking if the physical contents agree and if the recording media itself is of dependable quality. So, for the `--verify' operation, `tar' tries to defeat all in-memory cache pertaining to the archive, while it lets the speed optimization undisturbed for the `--compare' option. If you nevertheless use `--compare' for media verification, you may have to defeat the in-memory cache yourself, maybe by opening and reclosing the door latch of your recording unit, forcing some doubt in your operating system about the fact this is really the same volume as the one just written or read. The `--verify' option would not be necessary if drivers were indeed able to detect dependably all write failures. This sometimes require many magnetic heads, some able to read after the writes occurred. One would not say that drivers unable to detect all cases are necessarily flawed, as long as programming is concerned. The `--verify' (`-W') option will not work in conjunction with the `--multi-volume' (`-M') option or the `--append' (`-r'), `--update' (`-u') and `--delete' operations. *Note Operations::, for more information on these operations. Also, since `tar' normally strips leading `/' from file names (*note absolute::), a command like `tar --verify -cf /tmp/foo.tar /etc' will work as desired only if the working directory is `/', as `tar' uses the archive's relative member names (e.g., `etc/motd') when verifying the archive.  File: tar.info, Node: Write Protection, Prev: verify, Up: Media 9.9 Write Protection ==================== Almost all tapes and diskettes, and in a few rare cases, even disks can be "write protected", to protect data on them from being changed. Once an archive is written, you should write protect the media to prevent the archive from being accidentally overwritten or deleted. (This will protect the archive from being changed with a tape or floppy drive--it will not protect it from magnet fields or other physical hazards). The write protection device itself is usually an integral part of the physical media, and can be a two position (write enabled/write disabled) switch, a notch which can be popped out or covered, a ring which can be removed from the center of a tape reel, or some other changeable feature.  File: tar.info, Node: Changes, Next: Configuring Help Summary, Prev: Media, Up: Top Appendix A Changes ****************** This appendix lists some important user-visible changes between version GNU `tar' 1.16 and previous versions. An up-to-date version of this document is available at the GNU `tar' documentation page (http://www.gnu.org/software/tar/manual/changes.html). Use of globbing patterns when listing and extracting. Previous versions of GNU tar assumed shell-style globbing when extracting from or listing an archive. For example: $ tar xf foo.tar '*.c' would extract all files whose names end in `.c'. This behavior was not documented and was incompatible with traditional tar implementations. Therefore, starting from version 1.15.91, GNU tar no longer uses globbing by default. For example, the above invocation is now interpreted as a request to extract from the archive the file named `*.c'. To facilitate transition to the new behavior for those users who got used to the previous incorrect one, `tar' will print a warning if it finds out that a requested member was not found in the archive and its name looks like a globbing pattern. For example: $ tar xf foo.tar '*.c' tar: Pattern matching characters used in file names. Please, tar: use --wildcards to enable pattern matching, or --no-wildcards to tar: suppress this warning. tar: *.c: Not found in archive tar: Error exit delayed from previous errors To treat member names as globbing patterns, use -wildcards option. If you want to tar to mimic the behavior of versions prior to 1.15.91, add this option to your `TAR_OPTIONS' variable. *Note wildcards::, for the detailed discussion of the use of globbing patterns by GNU `tar'. Use of short option `-o'. Earlier versions of GNU `tar' understood `-o' command line option as a synonym for `--old-archive'. GNU `tar' starting from version 1.13.90 understands this option as a synonym for `--no-same-owner'. This is compatible with UNIX98 `tar' implementations. However, to facilitate transition, `-o' option retains its old semantics when it is used with one of archive-creation commands. Users are encouraged to use `--format=oldgnu' instead. It is especially important, since versions of GNU Automake up to and including 1.8.4 invoke tar with this option to produce distribution tarballs. *Note v7: Formats, for the detailed discussion of this issue and its implications. . *Note tar-v7: (automake)Options, for a description on how to use various archive formats with `automake'. Future versions of GNU `tar' will understand `-o' only as a synonym for `--no-same-owner'. Use of short option `-l' Earlier versions of GNU `tar' understood `-l' option as a synonym for `--one-file-system'. Since such usage contradicted to UNIX98 specification and harmed compatibility with other implementation, it was declared deprecated in version 1.14. However, to facilitate transition to its new semantics, it was supported by versions 1.15 and 1.15.90. The present use of `-l' as a short variant of `--check-links' was introduced in version 1.15.91. Use of options `--portability' and `--old-archive' These options are deprecated. Please use `--format=v7' instead. Use of option `--posix' This option is deprecated. Please use `--format=posix' instead.  File: tar.info, Node: Configuring Help Summary, Next: Tar Internals, Prev: Changes, Up: Top Appendix B Configuring Help Summary *********************************** Running `tar --help' displays the short `tar' option summary (*note help::). This summary is organized by "groups" of semantically close options. The options within each group are printed in the following order: a short option, eventually followed by a list of corresponding long option names, followed by a short description of the option. For example, here is an excerpt from the actual `tar --help' output: Main operation mode: -A, --catenate, --concatenate append tar files to an archive -c, --create create a new archive -d, --diff, --compare find differences between archive and file system --delete delete from the archive The exact visual representation of the help output is configurable via `ARGP_HELP_FMT' environment variable. The value of this variable is a comma-separated list of "format variable" assignments. There are two kinds of format variables. An "offset variable" keeps the offset of some part of help output text from the leftmost column on the screen. A "boolean" variable is a flag that toggles some output feature on or off. Depending on the type of the corresponding variable, there are two kinds of assignments: Offset assignment The assignment to an offset variable has the following syntax: VARIABLE=VALUE where VARIABLE is the variable name, and VALUE is a numeric value to be assigned to the variable. Boolean assignment To assign `true' value to a variable, simply put this variable name. To assign `false' value, prefix the variable name with `no-'. For example: # Assign `true' value: dup-args # Assign `false' value: no-dup-args Following variables are declared: -- Help Output: boolean dup-args If true, arguments for an option are shown with both short and long options, even when a given option has both forms, for example: -f ARCHIVE, --file=ARCHIVE use archive file or device ARCHIVE If false, then if an option has both short and long forms, the argument is only shown with the long one, for example: -f, --file=ARCHIVE use archive file or device ARCHIVE and a message indicating that the argument is applicable to both forms is printed below the options. This message can be disabled using `dup-args-note' (see below). The default is false. -- Help Output: boolean dup-args-note If this variable is true, which is the default, the following notice is displayed at the end of the help output: Mandatory or optional arguments to long options are also mandatory or optional for any corresponding short options. Setting `no-dup-args-note' inhibits this message. Normally, only one of variables `dup-args' or `dup-args-note' should be set. -- Help Output: offset short-opt-col Column in which short options start. Default is 2. $ tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE $ ARGP_HELP_FMT=short-opt-col=6 tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE -- Help Output: offset long-opt-col Column in which long options start. Default is 6. For example: $ tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE $ ARGP_HELP_FMT=long-opt-col=16 tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE -- Help Output: offset doc-opt-col Column in which "doc options" start. A doc option isn't actually an option, but rather an arbitrary piece of documentation that is displayed in much the same manner as the options. For example, in the description of `--format' option: -H, --format=FORMAT create archive of the given format. FORMAT is one of the following: gnu GNU tar 1.13.x format oldgnu GNU format as per tar <= 1.12 pax POSIX 1003.1-2001 (pax) format posix same as pax ustar POSIX 1003.1-1988 (ustar) format v7 old V7 tar format the format names are doc options. Thus, if you set `ARGP_HELP_FMT=doc-opt-col=6' the above part of the help output will look as follows: -H, --format=FORMAT create archive of the given format. FORMAT is one of the following: gnu GNU tar 1.13.x format oldgnu GNU format as per tar <= 1.12 pax POSIX 1003.1-2001 (pax) format posix same as pax ustar POSIX 1003.1-1988 (ustar) format v7 old V7 tar format -- Help Output: offset opt-doc-col Column in which option description starts. Default is 29. $ tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE $ ARGP_HELP_FMT=opt-doc-col=19 tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE $ ARGP_HELP_FMT=opt-doc-col=9 tar --help|grep ARCHIVE -f, --file=ARCHIVE use archive file or device ARCHIVE Notice, that the description starts on a separate line if `opt-doc-col' value is too small. -- Help Output: offset header-col Column in which "group headers" are printed. A group header is a descriptive text preceding an option group. For example, in the following text: Main operation mode: -A, --catenate, --concatenate append tar files to an archive -c, --create create a new archive `Main operation mode:' is the group header. The default value is 1. -- Help Output: offset usage-indent Indentation of wrapped usage lines. Affects `--usage' output. Default is 12. -- Help Output: offset rmargin Right margin of the text output. Used for wrapping.  File: tar.info, Node: Tar Internals, Next: Genfile, Prev: Configuring Help Summary, Up: Top Appendix C Tar Internals ************************ * Menu: * Standard:: Basic Tar Format * Extensions:: GNU Extensions to the Archive Format * Sparse Formats:: Storing Sparse Files * Snapshot Files:: * Dumpdir::  File: tar.info, Node: Standard, Next: Extensions, Up: Tar Internals Basic Tar Format ================ _(This message will disappear, once this node revised.)_ While an archive may contain many files, the archive itself is a single ordinary file. Like any other file, an archive file can be written to a storage device such as a tape or disk, sent through a pipe or over a network, saved on the active file system, or even stored in another archive. An archive file is not easy to read or manipulate without using the `tar' utility or Tar mode in GNU Emacs. Physically, an archive consists of a series of file entries terminated by an end-of-archive entry, which consists of two 512 blocks of zero bytes. A file entry usually describes one of the files in the archive (an "archive member"), and consists of a file header and the contents of the file. File headers contain file names and statistics, checksum information which `tar' uses to detect file corruption, and information about file types. Archives are permitted to have more than one member with the same member name. One way this situation can occur is if more than one version of a file has been stored in the archive. For information about adding new versions of a file to an archive, see *note update::. In addition to entries describing archive members, an archive may contain entries which `tar' itself uses to store information. *Note label::, for an example of such an archive entry. A `tar' archive file contains a series of blocks. Each block contains `BLOCKSIZE' bytes. Although this format may be thought of as being on magnetic tape, other media are often used. Each file archived is represented by a header block which describes the file, followed by zero or more blocks which give the contents of the file. At the end of the archive file there are two 512-byte blocks filled with binary zeros as an end-of-file marker. A reasonable system should write such end-of-file marker at the end of an archive, but must not assume that such a block exists when reading an archive. In particular GNU `tar' always issues a warning if it does not encounter it. The blocks may be "blocked" for physical I/O operations. Each record of N blocks (where N is set by the `--blocking-factor=512-SIZE' (`-b 512-SIZE') option to `tar') is written with a single `write ()' operation. On magnetic tapes, the result of such a write is a single record. When writing an archive, the last record of blocks should be written at the full size, with blocks after the zero block containing all zeros. When reading an archive, a reasonable system should properly handle an archive whose last record is shorter than the rest, or which contains garbage records after a zero block. The header block is defined in C as follows. In the GNU `tar' distribution, this is part of file `src/tar.h': /* tar Header Block, from POSIX 1003.1-1990. */ /* POSIX header. */ struct posix_header { /* byte offset */ char name[100]; /* 0 */ char mode[8]; /* 100 */ char uid[8]; /* 108 */ char gid[8]; /* 116 */ char size[12]; /* 124 */ char mtime[12]; /* 136 */ char chksum[8]; /* 148 */ char typeflag; /* 156 */ char linkname[100]; /* 157 */ char magic[6]; /* 257 */ char version[2]; /* 263 */ char uname[32]; /* 265 */ char gname[32]; /* 297 */ char devmajor[8]; /* 329 */ char devminor[8]; /* 337 */ char prefix[155]; /* 345 */ /* 500 */ }; #define TMAGIC "ustar" /* ustar and a null */ #define TMAGLEN 6 #define TVERSION "00" /* 00 and no null */ #define TVERSLEN 2 /* Values used in typeflag field. */ #define REGTYPE '0' /* regular file */ #define AREGTYPE '\0' /* regular file */ #define LNKTYPE '1' /* link */ #define SYMTYPE '2' /* reserved */ #define CHRTYPE '3' /* character special */ #define BLKTYPE '4' /* block special */ #define DIRTYPE '5' /* directory */ #define FIFOTYPE '6' /* FIFO special */ #define CONTTYPE '7' /* reserved */ #define XHDTYPE 'x' /* Extended header referring to the next file in the archive */ #define XGLTYPE 'g' /* Global extended header */ /* Bits used in the mode field, values in octal. */ #define TSUID 04000 /* set UID on execution */ #define TSGID 02000 /* set GID on execution */ #define TSVTX 01000 /* reserved */ /* file permissions */ #define TUREAD 00400 /* read by owner */ #define TUWRITE 00200 /* write by owner */ #define TUEXEC 00100 /* execute/search by owner */ #define TGREAD 00040 /* read by group */ #define TGWRITE 00020 /* write by group */ #define TGEXEC 00010 /* execute/search by group */ #define TOREAD 00004 /* read by other */ #define TOWRITE 00002 /* write by other */ #define TOEXEC 00001 /* execute/search by other */ /* tar Header Block, GNU extensions. */ /* In GNU tar, SYMTYPE is for to symbolic links, and CONTTYPE is for contiguous files, so maybe disobeying the `reserved' comment in POSIX header description. I suspect these were meant to be used this way, and should not have really been `reserved' in the published standards. */ /* *BEWARE* *BEWARE* *BEWARE* that the following information is still boiling, and may change. Even if the OLDGNU format description should be accurate, the so-called GNU format is not yet fully decided. It is surely meant to use only extensions allowed by POSIX, but the sketch below repeats some ugliness from the OLDGNU format, which should rather go away. Sparse files should be saved in such a way that they do *not* require two passes at archive creation time. Huge files get some POSIX fields to overflow, alternate solutions have to be sought for this. */ /* Descriptor for a single file hole. */ struct sparse { /* byte offset */ char offset[12]; /* 0 */ char numbytes[12]; /* 12 */ /* 24 */ }; /* Sparse files are not supported in POSIX ustar format. For sparse files with a POSIX header, a GNU extra header is provided which holds overall sparse information and a few sparse descriptors. When an old GNU header replaces both the POSIX header and the GNU extra header, it holds some sparse descriptors too. Whether POSIX or not, if more sparse descriptors are still needed, they are put into as many successive sparse headers as necessary. The following constants tell how many sparse descriptors fit in each kind of header able to hold them. */ #define SPARSES_IN_EXTRA_HEADER 16 #define SPARSES_IN_OLDGNU_HEADER 4 #define SPARSES_IN_SPARSE_HEADER 21 /* Extension header for sparse files, used immediately after the GNU extra header, and used only if all sparse information cannot fit into that extra header. There might even be many such extension headers, one after the other, until all sparse information has been recorded. */ struct sparse_header { /* byte offset */ struct sparse sp[SPARSES_IN_SPARSE_HEADER]; /* 0 */ char isextended; /* 504 */ /* 505 */ }; /* The old GNU format header conflicts with POSIX format in such a way that POSIX archives may fool old GNU tar's, and POSIX tar's might well be fooled by old GNU tar archives. An old GNU format header uses the space used by the prefix field in a POSIX header, and cumulates information normally found in a GNU extra header. With an old GNU tar header, we never see any POSIX header nor GNU extra header. Supplementary sparse headers are allowed, however. */ struct oldgnu_header { /* byte offset */ char unused_pad1[345]; /* 0 */ char atime[12]; /* 345 Incr. archive: atime of the file */ char ctime[12]; /* 357 Incr. archive: ctime of the file */ char offset[12]; /* 369 Multivolume archive: the offset of the start of this volume */ char longnames[4]; /* 381 Not used */ char unused_pad2; /* 385 */ struct sparse sp[SPARSES_IN_OLDGNU_HEADER]; /* 386 */ char isextended; /* 482 Sparse file: Extension sparse header follows */ char realsize[12]; /* 483 Sparse file: Real size*/ /* 495 */ }; /* OLDGNU_MAGIC uses both magic and version fields, which are contiguous. Found in an archive, it indicates an old GNU header format, which will be hopefully become obsolescent. With OLDGNU_MAGIC, uname and gname are valid, though the header is not truly POSIX conforming. */ #define OLDGNU_MAGIC "ustar " /* 7 chars and a null */ /* The standards committee allows only capital A through capital Z for user-defined expansion. Other letters in use include: 'A' Solaris Access Control List 'E' Solaris Extended Attribute File 'I' Inode only, as in 'star' 'X' POSIX 1003.1-2001 eXtended (VU version) */ /* This is a dir entry that contains the names of files that were in the dir at the time the dump was made. */ #define GNUTYPE_DUMPDIR 'D' /* Identifies the *next* file on the tape as having a long linkname. */ #define GNUTYPE_LONGLINK 'K' /* Identifies the *next* file on the tape as having a long name. */ #define GNUTYPE_LONGNAME 'L' /* This is the continuation of a file that began on another volume. */ #define GNUTYPE_MULTIVOL 'M' /* For storing filenames that do not fit into the main header. */ #define GNUTYPE_NAMES 'N' /* This is for sparse files. */ #define GNUTYPE_SPARSE 'S' /* This file is a tape/volume header. Ignore it on extraction. */ #define GNUTYPE_VOLHDR 'V' /* Solaris extended header */ #define SOLARIS_XHDTYPE 'X' /* Jo"rg Schilling star header */ struct star_header { /* byte offset */ char name[100]; /* 0 */ char mode[8]; /* 100 */ char uid[8]; /* 108 */ char gid[8]; /* 116 */ char size[12]; /* 124 */ char mtime[12]; /* 136 */ char chksum[8]; /* 148 */ char typeflag; /* 156 */ char linkname[100]; /* 157 */ char magic[6]; /* 257 */ char version[2]; /* 263 */ char uname[32]; /* 265 */ char gname[32]; /* 297 */ char devmajor[8]; /* 329 */ char devminor[8]; /* 337 */ char prefix[131]; /* 345 */ char atime[12]; /* 476 */ char ctime[12]; /* 488 */ /* 500 */ }; #define SPARSES_IN_STAR_HEADER 4 #define SPARSES_IN_STAR_EXT_HEADER 21 struct star_in_header { char fill[345]; /* 0 Everything that is before t_prefix */ char prefix[1]; /* 345 t_name prefix */ char fill2; /* 346 */ char fill3[8]; /* 347 */ char isextended; /* 355 */ struct sparse sp[SPARSES_IN_STAR_HEADER]; /* 356 */ char realsize[12]; /* 452 Actual size of the file */ char offset[12]; /* 464 Offset of multivolume contents */ char atime[12]; /* 476 */ char ctime[12]; /* 488 */ char mfill[8]; /* 500 */ char xmagic[4]; /* 508 "tar" */ }; struct star_ext_header { struct sparse sp[SPARSES_IN_STAR_EXT_HEADER]; char isextended; }; All characters in header blocks are represented by using 8-bit characters in the local variant of ASCII. Each field within the structure is contiguous; that is, there is no padding used within the structure. Each character on the archive medium is stored contiguously. Bytes representing the contents of files (after the header block of each file) are not translated in any way and are not constrained to represent characters in any character set. The `tar' format does not distinguish text files from binary files, and no translation of file contents is performed. The `name', `linkname', `magic', `uname', and `gname' are null-terminated character strings. All other fields are zero-filled octal numbers in ASCII. Each numeric field of width W contains W minus 1 digits, and a null. The `name' field is the file name of the file, with directory names (if any) preceding the file name, separated by slashes. The `mode' field provides nine bits specifying file permissions and three bits to specify the Set UID, Set GID, and Save Text ("sticky") modes. Values for these bits are defined above. When special permissions are required to create a file with a given mode, and the user restoring files from the archive does not hold such permissions, the mode bit(s) specifying those special permissions are ignored. Modes which are not supported by the operating system restoring files from the archive will be ignored. Unsupported modes should be faked up when creating or updating an archive; e.g., the group permission could be copied from the _other_ permission. The `uid' and `gid' fields are the numeric user and group ID of the file owners, respectively. If the operating system does not support numeric user or group IDs, these fields should be ignored. The `size' field is the size of the file in bytes; linked files are archived with this field specified as zero. The `mtime' field is the data modification time of the file at the time it was archived. It is the ASCII representation of the octal value of the last time the file's contents were modified, represented as an integer number of seconds since January 1, 1970, 00:00 Coordinated Universal Time. The `chksum' field is the ASCII representation of the octal value of the simple sum of all bytes in the header block. Each 8-bit byte in the header is added to an unsigned integer, initialized to zero, the precision of which shall be no less than seventeen bits. When calculating the checksum, the `chksum' field is treated as if it were all blanks. The `typeflag' field specifies the type of file archived. If a particular implementation does not recognize or permit the specified type, the file will be extracted as if it were a regular file. As this action occurs, `tar' issues a warning to the standard error. The `atime' and `ctime' fields are used in making incremental backups; they store, respectively, the particular file's access and status change times. The `offset' is used by the `--multi-volume' (`-M') option, when making a multi-volume archive. The offset is number of bytes into the file that we need to restart at to continue the file on the next tape, i.e., where we store the location that a continued file is continued at. The following fields were added to deal with sparse files. A file is "sparse" if it takes in unallocated blocks which end up being represented as zeros, i.e., no useful data. A test to see if a file is sparse is to look at the number blocks allocated for it versus the number of characters in the file; if there are fewer blocks allocated for the file than would normally be allocated for a file of that size, then the file is sparse. This is the method `tar' uses to detect a sparse file, and once such a file is detected, it is treated differently from non-sparse files. Sparse files are often `dbm' files, or other database-type files which have data at some points and emptiness in the greater part of the file. Such files can appear to be very large when an `ls -l' is done on them, when in truth, there may be a very small amount of important data contained in the file. It is thus undesirable to have `tar' think that it must back up this entire file, as great quantities of room are wasted on empty blocks, which can lead to running out of room on a tape far earlier than is necessary. Thus, sparse files are dealt with so that these empty blocks are not written to the tape. Instead, what is written to the tape is a description, of sorts, of the sparse file: where the holes are, how big the holes are, and how much data is found at the end of the hole. This way, the file takes up potentially far less room on the tape, and when the file is extracted later on, it will look exactly the way it looked beforehand. The following is a description of the fields used to handle a sparse file: The `sp' is an array of `struct sparse'. Each `struct sparse' contains two 12-character strings which represent an offset into the file and a number of bytes to be written at that offset. The offset is absolute, and not relative to the offset in preceding array element. The header can hold four of these `struct sparse' at the moment; if more are needed, they are not stored in the header. The `isextended' flag is set when an `extended_header' is needed to deal with a file. Note that this means that this flag can only be set when dealing with a sparse file, and it is only set in the event that the description of the file will not fit in the allotted room for sparse structures in the header. In other words, an extended_header is needed. The `extended_header' structure is used for sparse files which need more sparse structures than can fit in the header. The header can fit 4 such structures; if more are needed, the flag `isextended' gets set and the next block is an `extended_header'. Each `extended_header' structure contains an array of 21 sparse structures, along with a similar `isextended' flag that the header had. There can be an indeterminate number of such `extended_header's to describe a sparse file. `REGTYPE' `AREGTYPE' These flags represent a regular file. In order to be compatible with older versions of `tar', a `typeflag' value of `AREGTYPE' should be silently recognized as a regular file. New archives should be created using `REGTYPE'. Also, for backward compatibility, `tar' treats a regular file whose name ends with a slash as a directory. `LNKTYPE' This flag represents a file linked to another file, of any type, previously archived. Such files are identified in Unix by each file having the same device and inode number. The linked-to name is specified in the `linkname' field with a trailing null. `SYMTYPE' This represents a symbolic link to another file. The linked-to name is specified in the `linkname' field with a trailing null. `CHRTYPE' `BLKTYPE' These represent character special files and block special files respectively. In this case the `devmajor' and `devminor' fields will contain the major and minor device numbers respectively. Operating systems may map the device specifications to their own local specification, or may ignore the entry. `DIRTYPE' This flag specifies a directory or sub-directory. The directory name in the `name' field should end with a slash. On systems where disk allocation is performed on a directory basis, the `size' field will contain the maximum number of bytes (which may be rounded to the nearest disk block allocation unit) which the directory may hold. A `size' field of zero indicates no such limiting. Systems which do not support limiting in this manner should ignore the `size' field. `FIFOTYPE' This specifies a FIFO special file. Note that the archiving of a FIFO file archives the existence of this file and not its contents. `CONTTYPE' This specifies a contiguous file, which is the same as a normal file except that, in operating systems which support it, all its space is allocated contiguously on the disk. Operating systems which do not allow contiguous allocation should silently treat this type as a normal file. `A' ... `Z' These are reserved for custom implementations. Some of these are used in the GNU modified format, as described below. Other values are reserved for specification in future revisions of the P1003 standard, and should not be used by any `tar' program. The `magic' field indicates that this archive was output in the P1003 archive format. If this field contains `TMAGIC', the `uname' and `gname' fields will contain the ASCII representation of the owner and group of the file respectively. If found, the user and group IDs are used rather than the values in the `uid' and `gid' fields. For references, see ISO/IEC 9945-1:1990 or IEEE Std 1003.1-1990, pages 169-173 (section 10.1) for `Archive/Interchange File Format'; and IEEE Std 1003.2-1992, pages 380-388 (section 4.48) and pages 936-940 (section E.4.48) for `pax - Portable archive interchange'.  File: tar.info, Node: Extensions, Next: Sparse Formats, Prev: Standard, Up: Tar Internals GNU Extensions to the Archive Format ==================================== _(This message will disappear, once this node revised.)_ The GNU format uses additional file types to describe new types of files in an archive. These are listed below. `GNUTYPE_DUMPDIR' `'D'' This represents a directory and a list of files created by the `--incremental' (`-G') option. The `size' field gives the total size of the associated list of files. Each file name is preceded by either a `Y' (the file should be in this archive) or an `N'. (The file is a directory, or is not stored in the archive.) Each file name is terminated by a null. There is an additional null after the last file name. `GNUTYPE_MULTIVOL' `'M'' This represents a file continued from another volume of a multi-volume archive created with the `--multi-volume' (`-M') option. The original type of the file is not given here. The `size' field gives the maximum size of this piece of the file (assuming the volume does not end before the file is written out). The `offset' field gives the offset from the beginning of the file where this part of the file begins. Thus `size' plus `offset' should equal the original size of the file. `GNUTYPE_SPARSE' `'S'' This flag indicates that we are dealing with a sparse file. Note that archiving a sparse file requires special operations to find holes in the file, which mark the positions of these holes, along with the number of bytes of data to be found after the hole. `GNUTYPE_VOLHDR' `'V'' This file type is used to mark the volume header that was given with the `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') option when the archive was created. The `name' field contains the `name' given after the `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') option. The `size' field is zero. Only the first file in each volume of an archive should have this type. You may have trouble reading a GNU format archive on a non-GNU system if the options `--incremental' (`-G'), `--multi-volume' (`-M'), `--sparse' (`-S'), or `--label=ARCHIVE-LABEL' (`-V ARCHIVE-LABEL') were used when writing the archive. In general, if `tar' does not use the GNU-added fields of the header, other versions of `tar' should be able to read the archive. Otherwise, the `tar' program will give an error, the most likely one being a checksum error.  File: tar.info, Node: Sparse Formats, Next: Snapshot Files, Prev: Extensions, Up: Tar Internals Storing Sparse Files ==================== The notion of sparse file, and the ways of handling it from the point of view of GNU `tar' user have been described in detail in *note sparse::. This chapter describes the internal format GNU `tar' uses to store such files. The support for sparse files in GNU `tar' has a long history. The earliest version featuring this support that I was able to find was 1.09, released in November, 1990. The format introduced back then is called "old GNU" sparse format and in spite of the fact that its design contained many flaws, it was the only format GNU `tar' supported until version 1.14 (May, 2004), which introduced initial support for sparse archives in PAX archives (*note posix::). This format was not free from design flows, either and it was subsequently improved in versions 1.15.2 (November, 2005) and 1.15.92 (June, 2006). In addition to GNU sparse format, GNU `tar' is able to read and extract sparse files archived by `star'. The following subsections describe each format in detail. * Menu: * Old GNU Format:: * PAX 0:: PAX Format, Versions 0.0 and 0.1 * PAX 1:: PAX Format, Version 1.0  File: tar.info, Node: Old GNU Format, Next: PAX 0, Up: Sparse Formats C.0.1 Old GNU Format -------------------- The format introduced some time around 1990 (v. 1.09). It was designed on top of standard `ustar' headers in such an unfortunate way that some of its fields overwrote fields required by POSIX. An old GNU sparse header is designated by type `S' (`GNUTYPE_SPARSE') and has the following layout: Offset Size Name Data type Contents ---------------------------------------------------------------------------- 0 345 N/A Not used. 345 12 atime Number `atime' of the file. 357 12 ctime Number `ctime' of the file . 369 12 offset Number For multivolume archives: the offset of the start of this volume. 381 4 N/A Not used. 385 1 N/A Not used. 386 96 sp `sparse_header'(4 entries) File map. 482 1 isextended Bool `1' if an extension sparse header follows, `0' otherwise. 483 12 realsize Number Real size of the file. Each of `sparse_header' object at offset 386 describes a single data chunk. It has the following structure: Offset Size Data type Contents --------------------------------------------------------------------------- 0 12 Number Offset of the beginning of the chunk. 12 12 Number Size of the chunk. If the member contains more than four chunks, the `isextended' field of the header has the value `1' and the main header is followed by one or more "extension headers". Each such header has the following structure: Offset Size Name Data type Contents ---------------------------------------------------------------------------- 0 21 sp `sparse_header' (21 entires) File map. 504 1 isextended Bool `1' if an extension sparse header follows, or `0' otherwise. A header with `isextended=0' ends the map.  File: tar.info, Node: PAX 0, Next: PAX 1, Prev: Old GNU Format, Up: Sparse Formats C.0.2 PAX Format, Versions 0.0 and 0.1 -------------------------------------- There are two formats available in this branch. The version `0.0' is the initial version of sparse format used by `tar' versions 1.14-1.15.1. The sparse file map is kept in extended (`x') PAX header variables: `GNU.sparse.size' Real size of the stored file `GNU.sparse.numblocks' Number of blocks in the sparse map `GNU.sparse.offset' Offset of the data block `GNU.sparse.numbytes' Size of the data block The latter two variables repeat for each data block, so the overall structure is like this: GNU.sparse.size=SIZE GNU.sparse.numblocks=NUMBLOCKS repeat NUMBLOCKS times GNU.sparse.offset=OFFSET GNU.sparse.numbytes=NUMBYTES end repeat This format presented the following two problems: 1. Whereas the POSIX specification allows a variable to appear multiple times in a header, it requires that only the last occurrence be meaningful. Thus, multiple occurrences of `GNU.sparse.offset' and `GNU.sparse.numbytes' are conflicting with the POSIX specs. 2. Attempting to extract such archives using a third-party `tar's results in extraction of sparse files in _compressed form_. If the `tar' implementation in question does not support POSIX format, it will also extract a file containing extension header attributes. This file can be used to expand the file to its original state. However, posix-aware `tar's will usually ignore the unknown variables, which makes restoring the file more difficult. *Note Extraction of sparse members in v.0.0 format: extracting sparse v.0.x, for the detailed description of how to restore such members using non-GNU `tar's. GNU `tar' 1.15.2 introduced sparse format version `0.1', which attempted to solve these problems. As its predecessor, this format stores sparse map in the extended POSIX header. It retains `GNU.sparse.size' and `GNU.sparse.numblocks' variables, but instead of `GNU.sparse.offset'/`GNU.sparse.numbytes' pairs it uses a single variable: `GNU.sparse.map' Map of non-null data chunks. It is a string consisting of comma-separated values "OFFSET,SIZE[,OFFSET-1,SIZE-1...]" To address the 2nd problem, the `name' field in `ustar' is replaced with a special name, constructed using the following pattern: %d/GNUSparseFile.%p/%f The real name of the sparse file is stored in the variable `GNU.sparse.name'. Thus, those `tar' implementations that are not aware of GNU extensions will at least extract the files into separate directories, giving the user a possibility to expand it afterwards. *Note Extraction of sparse members in v.0.1 format: extracting sparse v.0.x, for the detailed description of how to restore such members using non-GNU `tar's. The resulting `GNU.sparse.map' string can be _very_ long. Although POSIX does not impose any limit on the length of a `x' header variable, this possibly can confuse some tars.  File: tar.info, Node: PAX 1, Prev: PAX 0, Up: Sparse Formats C.0.3 PAX Format, Version 1.0 ----------------------------- The version `1.0' of sparse format was introduced with GNU `tar' 1.15.92. Its main objective was to make the resulting file extractable with little effort even by non-posix aware `tar' implementations. Starting from this version, the extended header preceding a sparse member always contains the following variables that identify the format being used: `GNU.sparse.major' Major version `GNU.sparse.minor' Minor version The `name' field in `ustar' header contains a special name, constructed using the following pattern: %d/GNUSparseFile.%p/%f The real name of the sparse file is stored in the variable `GNU.sparse.name'. The real size of the file is stored in the variable `GNU.sparse.realsize'. The sparse map itself is stored in the file data block, preceding the actual file data. It consists of a series of octal numbers of arbitrary length, delimited by newlines. The map is padded with nulls to the nearest block boundary. The first number gives the number of entries in the map. Following are map entries, each one consisting of two numbers giving the offset and size of the data block it describes. The format is designed in such a way that non-posix aware tars and tars not supporting `GNU.sparse.*' keywords will extract each sparse file in its condensed form with the file map prepended and will place it into a separate directory. Then, using a simple program it would be possible to expand the file to its original form even without GNU `tar'. *Note Sparse Recovery::, for the detailed information on how to extract sparse members without GNU `tar'.  File: tar.info, Node: Snapshot Files, Next: Dumpdir, Prev: Sparse Formats, Up: Tar Internals Format of the Incremental Snapshot Files ======================================== A "snapshot file" (or "directory file") is created during incremental backups (*note Incremental Dumps::). It contains the status of the file system at the time of the dump and is used to determine which files were modified since the last backup. GNU `tar' version 1.16 supports two snapshot file formats. The first format, called "format 0", is the one used by GNU `tar' versions up to 1.15.1. The second format, called "format 1" is an extended version of this format, that contains more metadata and allows for further extensions. `Format 0' snapshot file begins with a line containing a decimal number that represents the UNIX timestamp of the beginning of the last archivation. This line is followed by directory metadata descriptions, one per line. Each description has the following format: [NFS]DEV INODE NAME where optional NFS is a single plus character (`+') if this directory is located on an NFS-mounted partition, DEV and INODE are the device and inode numbers of the directory, and NAME is the directory name. `Format 1' snapshot file begins with a line specifying the format of the file. This line has the following structure: `GNU tar-'TAR-VERSION`-'INCR-FORMAT-VERSION where TAR-VERSION is the version of GNU `tar' implementation that created this snapshot, and INCR-FORMAT-VERSION is the version number of the snapshot format (in this case `1'). The following line contains two decimal numbers, representing the time of the last backup. First number is the number of seconds, the second one is the number of nanoseconds, since the beginning of the epoch. Following lines contain directory metadata, one line per directory. The line format is: [NFS]MTIME-SEC MTIME-NSEC DEV INODE NAME where MTIME-SEC and MTIME-NSEC represent the last modification time of this directory with nanosecond precision; NFS, DEV, INODE and NAME have the same meaning as with `format 0'.  File: tar.info, Node: Dumpdir, Prev: Snapshot Files, Up: Tar Internals Dumpdir ======= Incremental archives keep information about contents of each dumped directory in special data blocks called "dumpdirs". Dumpdir is a sequence of entries of the following form: C FILENAME \0 where C is one of the "control codes" described below, FILENAME is the name of the file C operates upon, and `\0' represents a nul character (ASCII 0). The white space characters were added for readability, real dumpdirs do not contain them. Each dumpdir ends with a single nul character. The following table describes control codes and their meanings: `Y' FILENAME is contained in the archive. `N' FILENAME was present in the directory at the time the archive was made, yet it was not dumped to the archive, because it had not changed since the last backup. `D' FILENAME is a directory. `R' This code requests renaming of the FILENAME to the name specified with the following `T' command. `T' Specify target file name for `R' command (see below). `X' Specify "temporary directory" name for a rename operation (see below). Codes `Y', `N' and `D' require FILENAME argument to be a relative file name to the directory this dumpdir describes, whereas codes `R', `T' and `X' require their argument to be an absolute file name. The three codes `R', `T' and `X' specify a "renaming operation". In the simplest case it is: R`source'\0T`dest'\0 which means "rename file `source' to file `dest'". However, there are cases that require using a "temporary directory". For example, consider the following scenario: 1. Previous run dumped a directory `foo' which contained the following three directories: a b c 2. They were renamed _cyclically_, so that: `a' became `b' `b' became `c' `c' became `a' 3. New incremental dump was made. This case cannot be handled by three successive renames, since renaming `a' to `b' will destroy existing directory. To handle such case a temporary directory is required. GNU `tar' will create the following dumpdir (newlines have been added for readability): Xfoo\0 Rfoo/a\0T\0 Rfoo/b\0Tfoo/c\0 Rfoo/c\0Tfoo/a\0 R\0Tfoo/a\0 The first command, `Xfoo\0', instructs the extractor to create a temporary directory in the directory `foo'. Second command, `Rfoo/aT\0', says "rename file `foo/a' to the temporary directory that has just been created" (empty file name after a command means use temporary directory). Third and fourth commands work as usual, and, finally, the last command, `R\0Tfoo/a\0' tells tar to rename the temporary directory to `foo/a'. The exact placement of a dumpdir in the archive depends on the archive format (*note Formats::): * PAX archives In PAX archives, dumpdir is stored in the extended header of the corresponding directory, in variable `GNU.dumpdir'. * GNU and old GNU archives These formats implement special header type `D', which is similar to ustar header `5' (directory), except that it precedes a data block containing the dumpdir.  File: tar.info, Node: Genfile, Next: Free Software Needs Free Documentation, Prev: Tar Internals, Up: Top Appendix D Genfile ****************** This appendix describes `genfile', an auxiliary program used in the GNU tar testsuite. If you are not interested in developing GNU tar, skip this appendix. Initially, `genfile' was used to generate data files for the testsuite, hence its name. However, new operation modes were being implemented as the testsuite grew more sophisticated, and now `genfile' is a multi-purpose instrument. There are three basic operation modes: File Generation This is the default mode. In this mode, `genfile' generates data files. File Status In this mode `genfile' displays status of specified files. Synchronous Execution. In this mode `genfile' executes the given program with `--checkpoint' option and executes a set of actions when specified checkpoints are reached. * Menu: * Generate Mode:: File Generation Mode. * Status Mode:: File Status Mode. * Exec Mode:: Synchronous Execution mode.  File: tar.info, Node: Generate Mode, Next: Status Mode, Up: Genfile D.1 Generate Mode ================= In this mode `genfile' creates a data file for the test suite. The size of the file is given with the `--length' (`-l') option. By default the file contents is written to the standard output, this can be changed using `--file' (`-f') command line option. Thus, the following two commands are equivalent: genfile --length 100 > outfile genfile --length 100 --file outfile If `--length' is not given, `genfile' will generate an empty (zero-length) file. You can instruct `genfile' to create several files at one go, by giving it `--files-from' (`-T') option followed by a name of file containing a list of file names. Using dash (`-') instead of the file name causes `genfile' to read file list from the standard input. For example: # Read file names from file `file.list' genfile --files-from file.list # Read file names from standard input genfile --files-from - The list file is supposed to contain one file name per line. To use file lists separated by ASCII NUL character, use `--null' (`-0') command line option: genfile --null --files-from file.list The default data pattern for filling the generated file consists of first 256 letters of ASCII code, repeated enough times to fill the entire file. This behavior can be changed with `--pattern' option. This option takes a mandatory argument, specifying pattern name to use. Currently two patterns are implemented: `--pattern=default' The default pattern as described above. `--pattern=zero' Fills the file with zeroes. If no file name was given, the program exits with the code `0'. Otherwise, it exits with `0' only if it was able to create a file of the specified length. Special option `--sparse' (`-s') instructs `genfile' to create a sparse file. Sparse files consist of "data fragments", separated by "holes" or blocks of zeros. On many operating systems, actual disk storage is not allocated for holes, but they are counted in the length of the file. To create a sparse file, `genfile' should know where to put data fragments, and what data to use to fill them. So, when `--sparse' is given the rest of the command line specifies a so-called "file map". The file map consists of any number of "fragment descriptors". Each descriptor is composed of two values: a number, specifying fragment offset from the end of the previous fragment or, for the very first fragment, from the beginning of the file, and "contents string", i.e., a string of characters, specifying the pattern to fill the fragment with. File offset can be suffixed with the following quantifiers: `k' `K' The number is expressed in kilobytes. `m' `M' The number is expressed in megabytes. `g' `G' The number is expressed in gigabytes. For each letter in contents string `genfile' will generate a "block" of data, filled with this letter and will write it to the fragment. The size of block is given by `--block-size' option. It defaults to 512. Thus, if the string consists of N characters, the resulting file fragment will contain `N*BLOCK-SIZE' of data. Last fragment descriptor can have only file offset part. In this case `genfile' will create a hole at the end of the file up to the given offset. For example, consider the following invocation: genfile --sparse --file sparsefile 0 ABCD 1M EFGHI 2000K It will create 3101184-bytes long file of the following structure: Offset Length Contents 0 4*512=2048 Four 512-byte blocks, filled with letters `A', `B', `C' and `D'. 2048 1046528 Zero bytes 1050624 5*512=2560 Five blocks, filled with letters `E', `F', `G', `H', `I'. 1053184 2048000 Zero bytes The exit code of `genfile --status' command is `0' only if created file is actually sparse.  File: tar.info, Node: Status Mode, Next: Exec Mode, Prev: Generate Mode, Up: Genfile D.2 Status Mode =============== In status mode, `genfile' prints file system status for each file specified in the command line. This mode is toggled by `--stat' (`-S') command line option. An optional argument to this option specifies output "format": a comma-separated list of `struct stat' fields to be displayed. This list can contain following identifiers : name The file name. dev st_dev Device number in decimal. ino st_ino Inode number. mode[.NUMBER] st_mode[.NUMBER] File mode in octal. Optional NUMBER specifies octal mask to be applied to the mode before outputting. For example, `--stat mode.777' will preserve lower nine bits of it. Notice, that you can use any punctuation character in place of `.'. nlink st_nlink Number of hard links. uid st_uid User ID of owner. gid st_gid Group ID of owner. size st_size File size in decimal. blksize st_blksize The size in bytes of each file block. blocks st_blocks Number of blocks allocated. atime st_atime Time of last access. mtime st_mtime Time of last modification ctime st_ctime Time of last status change sparse A boolean value indicating whether the file is `sparse'. Modification times are displayed in UTC as UNIX timestamps, unless suffixed with `H' (for "human-readable"), as in `ctimeH', in which case usual `tar tv' output format is used. The default output format is: `name,dev,ino,mode, nlink,uid,gid,size,blksize,blocks,atime,mtime,ctime'. For example, the following command will display file names and corresponding times of last access for each file in the current working directory: genfile --stat=name,atime *  File: tar.info, Node: Exec Mode, Prev: Status Mode, Up: Genfile D.3 Exec Mode ============= This mode is designed for testing the behavior of `paxutils' commands when some of the files change during archiving. It is an experimental mode. The `Exec Mode' is toggled by `--run' command line option (or its alias `-r'). The argument to this option gives the command line to be executed. The actual command line is constructed by inserting `--checkpoint' option between the command name and its first argument (if any). Due to this, the argument to `--run' may not use traditional `tar' option syntax, i.e., the following is wrong: # Wrong! genfile --run 'tar cf foo bar' Use the following syntax instead: genfile --run 'tar -cf foo bar' The rest of command line after `--run' or its equivalent specifies checkpoint values and actions to be executed upon reaching them. Checkpoint values are introduced with `--checkpoint' command line option. Argument to this option is the number of checkpoint in decimal. Any number of "actions" may be specified after a checkpoint. Available actions are `--cut FILE' `--truncate FILE' Truncate FILE to the size specified by previous `--length' option (or 0, if it is not given). `--append FILE' Append data to FILE. The size of data and its pattern are given by previous `--length' and `pattern' options. `--touch FILE' Update the access and modification times of FILE. These timestamps are changed to the current time, unless `--date' option was given, in which case they are changed to the specified time. Argument to `--date' option is a date specification in an almost arbitrary format (*note Date input formats::). `--exec COMMAND' Execute given shell command. Option `--verbose' instructs `genfile' to print on standard output notifications about checkpoints being executed and to verbosely describe exit status of the command. While the command is being executed its standard output remains connected to descriptor 1. All messages it prints to file descriptor 2, except checkpoint notifications, are forwarded to standard error. `Genfile' exits with the exit status of the executed command.  File: tar.info, Node: Free Software Needs Free Documentation, Next: Copying This Manual, Prev: Genfile, Up: Top Appendix E Free Software Needs Free Documentation ************************************************* The biggest deficiency in the free software community today is not in the software--it is the lack of good free documentation that we can include with the free software. Many of our most important programs do not come with free reference manuals and free introductory texts. Documentation is an essential part of any software package; when an important free software package does not come with a free manual and a free tutorial, that is a major gap. We have many such gaps today. Consider Perl, for instance. The tutorial manuals that people normally use are non-free. How did this come about? Because the authors of those manuals published them with restrictive terms--no copying, no modification, source files not available--which exclude them from the free software world. That wasn't the first time this sort of thing happened, and it was far from the last. Many times we have heard a GNU user eagerly describe a manual that he is writing, his intended contribution to the community, only to learn that he had ruined everything by signing a publication contract to make it non-free. Free documentation, like free software, is a matter of freedom, not price. The problem with the non-free manual is not that publishers charge a price for printed copies--that in itself is fine. (The Free Software Foundation sells printed copies of manuals, too.) The problem is the restrictions on the use of the manual. Free manuals are available in source code form, and give you permission to copy and modify. Non-free manuals do not allow this. The criteria of freedom for a free manual are roughly the same as for free software. Redistribution (including the normal kinds of commercial redistribution) must be permitted, so that the manual can accompany every copy of the program, both on-line and on paper. Permission for modification of the technical content is crucial too. When people modify the software, adding or changing features, if they are conscientious they will change the manual too--so they can provide accurate and clear documentation for the modified program. A manual that leaves you no choice but to write a new manual to document a changed version of the program is not really available to our community. Some kinds of limits on the way modification is handled are acceptable. For example, requirements to preserve the original author's copyright notice, the distribution terms, or the list of authors, are ok. It is also no problem to require modified versions to include notice that they were modified. Even entire sections that may not be deleted or changed are acceptable, as long as they deal with nontechnical topics (like this one). These kinds of restrictions are acceptable because they don't obstruct the community's normal use of the manual. However, it must be possible to modify all the _technical_ content of the manual, and then distribute the result in all the usual media, through all the usual channels. Otherwise, the restrictions obstruct the use of the manual, it is not free, and we need another manual to replace it. Please spread the word about this issue. Our community continues to lose manuals to proprietary publishing. If we spread the word that free software needs free reference manuals and free tutorials, perhaps the next person who wants to contribute by writing documentation will realize, before it is too late, that only free manuals contribute to the free software community. If you are writing documentation, please insist on publishing it under the GNU Free Documentation License or another free documentation license. Remember that this decision requires your approval--you don't have to let the publisher decide. Some commercial publishers will use a free license if you insist, but they will not propose the option; it is up to you to raise the issue and say firmly that this is what you want. If the publisher you are dealing with refuses, please try other publishers. If you're not sure whether a proposed license is free, write to . You can encourage commercial publishers to sell more free, copylefted manuals and tutorials by buying them, and particularly by buying copies from the publishers that paid for their writing or for major improvements. Meanwhile, try to avoid buying non-free documentation at all. Check the distribution terms of a manual before you buy it, and insist that whoever seeks your business must respect your freedom. Check the history of the book, and try reward the publishers that have paid or pay the authors to work on it. The Free Software Foundation maintains a list of free documentation published by other publishers, at `http://www.fsf.org/doc/other-free-books.html'.  File: tar.info, Node: Copying This Manual, Next: Index of Command Line Options, Prev: Free Software Needs Free Documentation, Up: Top Appendix F Copying This Manual ****************************** * Menu: * GNU Free Documentation License:: License for copying this manual  File: tar.info, Node: GNU Free Documentation License, Up: Copying This Manual F.1 GNU Free Documentation License ================================== Version 1.2, November 2002 Copyright (C) 2000,2001,2002 Free Software Foundation, Inc. 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. 0. PREAMBLE The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others. This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software. We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. 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If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. F.1.1 ADDENDUM: How to use this License for your documents ---------------------------------------------------------- To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page: Copyright (C) YEAR YOUR NAME. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''. If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the "with...Texts." line with this: with the Invariant Sections being LIST THEIR TITLES, with the Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST. If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation. If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.  File: tar.info, Node: Index of Command Line Options, Next: Index, Prev: Copying This Manual, Up: Top Appendix G Index of Command Line Options **************************************** This appendix contains an index of all GNU `tar' long command line options. The options are listed without the preceding double-dash. For a cross-reference of short command line options, *note Short Option Summary::. [index] * Menu: * absolute-names: absolute. (line 8) * absolute-names, summary: Option Summary. (line 6) * add-file: files. (line 84) * after-date: after. (line 26) * after-date, summary: Option Summary. (line 12) * anchored: controlling pattern-matching. (line 79) * anchored, summary: Option Summary. (line 15) * append: append. (line 8) * append, summary: Operation Summary. (line 6) * atime-preserve: Attributes. (line 14) * atime-preserve, summary: Option Summary. (line 19) * backup: backup. (line 41) * backup, summary: Option Summary. (line 65) * block-number: verbose. (line 111) * block-number, summary: Option Summary. (line 70) * blocking-factor: Blocking Factor. (line 8) * blocking-factor, summary: Option Summary. (line 76) * bzip2: gzip. (line 88) * bzip2, summary: Option Summary. (line 81) * catenate: concatenate. (line 6) * catenate, summary: Operation Summary. (line 10) * check-links, summary: Option Summary. (line 93) * checkpoint: verbose. (line 83) * checkpoint, summary: Option Summary. (line 86) * compare: compare. (line 8) * compare, summary: Operation Summary. (line 14) * compress: gzip. (line 92) * compress, summary: Option Summary. (line 100) * concatenate: concatenate. (line 6) * concatenate, summary: Operation Summary. (line 20) * confirmation, summary: Option Summary. (line 107) * create, additional options: create options. (line 6) * create, complementary notes: Basic tar. (line 11) * create, introduced: Creating the archive. (line 6) * create, summary: Operation Summary. (line 25) * create, using with --verbose: create verbose. (line 6) * create, using with --verify: verify. (line 24) * delay-directory-restore: Directory Modification Times and Permissions. (line 62) * delay-directory-restore, summary: Option Summary. (line 110) * delete: delete. (line 8) * delete, summary: Operation Summary. (line 29) * dereference: dereference. (line 6) * dereference, summary: Option Summary. (line 115) * diff, summary: Operation Summary. (line 33) * directory: directory. (line 11) * directory, summary: Option Summary. (line 121) * directory, using in --files-from argument: files. (line 60) * exclude: exclude. (line 11) * exclude, potential problems with: problems with exclude. (line 6) * exclude, summary: Option Summary. (line 128) * exclude-caches: exclude. (line 33) * exclude-caches, summary: Option Summary. (line 137) * exclude-from: exclude. (line 22) * exclude-from, summary: Option Summary. (line 132) * extract: extract. (line 8) * extract, additional options: extract options. (line 8) * extract, complementary notes: Basic tar. (line 48) * extract, summary: Operation Summary. (line 37) * extract, using with --listed-incremental: Incremental Dumps. (line 93) * file, short description: file. (line 17) * file, summary: Option Summary. (line 141) * file, tutorial: file tutorial. (line 6) * files-from: files. (line 14) * files-from, summary: Option Summary. (line 147) * force-local, short description: Device. (line 70) * force-local, summary: Option Summary. (line 153) * format, summary: Option Summary. (line 158) * get, summary: Operation Summary. (line 42) * group: override. (line 73) * group, summary: Option Summary. (line 183) * gunzip, summary: Option Summary. (line 191) * gzip: gzip. (line 54) * gzip, summary: Option Summary. (line 191) * help: help tutorial. (line 6) * help, introduction: help. (line 26) * help, summary: Option Summary. (line 199) * ignore-case: controlling pattern-matching. (line 86) * ignore-case, summary: Option Summary. (line 204) * ignore-command-error: Writing to an External Program. (line 82) * ignore-command-error, summary: Option Summary. (line 208) * ignore-failed-read: Ignore Failed Read. (line 7) * ignore-failed-read, summary: Option Summary. (line 212) * ignore-zeros: Ignore Zeros. (line 6) * ignore-zeros, short description: Blocking Factor. (line 156) * ignore-zeros, summary: Option Summary. (line 216) * incremental, summary: Option Summary. (line 221) * incremental, using with --list: Incremental Dumps. (line 158) * index-file, summary: Option Summary. (line 228) * info-script: Multi-Volume Archives. (line 80) * info-script, short description: Device. (line 104) * info-script, summary: Option Summary. (line 231) * interactive: interactive. (line 14) * interactive, summary: Option Summary. (line 239) * keep-newer-files: Keep Newer Files. (line 6) * keep-newer-files, summary: Option Summary. (line 246) * keep-old-files: Keep Old Files. (line 6) * keep-old-files, introduced: Dealing with Old Files. (line 16) * keep-old-files, summary: Option Summary. (line 250) * label: label. (line 8) * label, summary: Option Summary. (line 255) * list: list. (line 6) * list, summary: Operation Summary. (line 46) * list, using with --incremental: Incremental Dumps. (line 158) * list, using with --listed-incremental: Incremental Dumps. (line 158) * list, using with --verbose: list. (line 30) * list, using with file name arguments: list. (line 68) * listed-incremental: Incremental Dumps. (line 14) * listed-incremental, summary: Option Summary. (line 262) * listed-incremental, using with --extract: Incremental Dumps. (line 93) * listed-incremental, using with --list: Incremental Dumps. (line 158) * mode: override. (line 14) * mode, summary: Option Summary. (line 270) * mtime: override. (line 29) * mtime, summary: Option Summary. (line 276) * multi-volume: Multi-Volume Archives. (line 6) * multi-volume, short description: Device. (line 88) * multi-volume, summary: Option Summary. (line 285) * new-volume-script: Multi-Volume Archives. (line 80) * new-volume-script, short description: Device. (line 104) * new-volume-script, summary: Option Summary. (line 231) * newer: after. (line 26) * newer, summary: Option Summary. (line 300) * newer-mtime: after. (line 37) * newer-mtime, summary: Option Summary. (line 308) * no-anchored: controlling pattern-matching. (line 79) * no-anchored, summary: Option Summary. (line 313) * no-delay-directory-restore: Directory Modification Times and Permissions. (line 68) * no-delay-directory-restore, summary: Option Summary. (line 317) * no-ignore-case: controlling pattern-matching. (line 86) * no-ignore-case, summary: Option Summary. (line 323) * no-ignore-command-error: Writing to an External Program. (line 87) * no-ignore-command-error, summary: Option Summary. (line 326) * no-overwrite-dir, summary: Option Summary. (line 330) * no-quote-chars, summary: Option Summary. (line 334) * no-recursion: recurse. (line 13) * no-recursion, summary: Option Summary. (line 339) * no-same-owner: Attributes. (line 67) * no-same-owner, summary: Option Summary. (line 343) * no-same-permissions, summary: Option Summary. (line 349) * no-unquote: Selecting Archive Members. (line 42) * no-unquote, summary: Option Summary. (line 354) * no-wildcards: controlling pattern-matching. (line 41) * no-wildcards, summary: Option Summary. (line 358) * no-wildcards-match-slash: controlling pattern-matching. (line 92) * no-wildcards-match-slash, summary: Option Summary. (line 361) * null: nul. (line 11) * null, summary: Option Summary. (line 364) * numeric-owner: Attributes. (line 73) * numeric-owner, summary: Option Summary. (line 370) * occurrence, summary: Option Summary. (line 387) * old-archive, summary: Option Summary. (line 401) * one-file-system: one. (line 16) * one-file-system, summary: Option Summary. (line 404) * overwrite: Overwrite Old Files. (line 6) * overwrite, introduced: Dealing with Old Files. (line 22) * overwrite, summary: Option Summary. (line 409) * overwrite-dir: Overwrite Old Files. (line 28) * overwrite-dir, introduced: Dealing with Old Files. (line 6) * overwrite-dir, summary: Option Summary. (line 413) * owner: override. (line 57) * owner, summary: Option Summary. (line 417) * pax-option: PAX keywords. (line 6) * pax-option, summary: Option Summary. (line 450) * portability, summary: Option Summary. (line 456) * posix, summary: Option Summary. (line 460) * preserve: Attributes. (line 126) * preserve, summary: Option Summary. (line 463) * preserve-order: Same Order. (line 6) * preserve-order, summary: Option Summary. (line 467) * preserve-permissions: Setting Access Permissions. (line 10) * preserve-permissions, short description: Attributes. (line 113) * preserve-permissions, summary: Option Summary. (line 470) * quote-chars, summary: Option Summary. (line 439) * quoting-style: quoting styles. (line 39) * quoting-style, summary: Option Summary. (line 443) * read-full-records <1>: read full records. (line 6) * read-full-records: Reading. (line 8) * read-full-records, short description: Blocking Factor. (line 172) * read-full-records, summary: Option Summary. (line 480) * record-size, summary: Option Summary. (line 485) * recursion: recurse. (line 24) * recursion, summary: Option Summary. (line 489) * recursive-unlink: Recursive Unlink. (line 6) * recursive-unlink, summary: Option Summary. (line 493) * remove-files: remove files. (line 6) * remove-files, summary: Option Summary. (line 498) * restrict, summary: Option Summary. (line 502) * rmt-command, summary: Option Summary. (line 507) * rsh-command: Device. (line 73) * rsh-command, summary: Option Summary. (line 511) * same-order: Same Order. (line 6) * same-order, summary: Option Summary. (line 515) * same-owner: Attributes. (line 48) * same-owner, summary: Option Summary. (line 523) * same-permissions: Setting Access Permissions. (line 10) * same-permissions, short description: Attributes. (line 113) * same-permissions, summary: Option Summary. (line 470) * seek, summary: Option Summary. (line 293) * show-defaults: defaults. (line 6) * show-defaults, summary: Option Summary. (line 532) * show-omitted-dirs: verbose. (line 103) * show-omitted-dirs, summary: Option Summary. (line 541) * show-stored-names: list. (line 60) * show-stored-names, summary: Option Summary. (line 545) * show-transformed-names: transform. (line 45) * show-transformed-names, summary: Option Summary. (line 545) * sparse: sparse. (line 22) * sparse, summary: Option Summary. (line 553) * sparse-version: sparse. (line 57) * sparse-version, summary: Option Summary. (line 558) * starting-file: Starting File. (line 6) * starting-file, summary: Option Summary. (line 563) * strip-components: transform. (line 25) * strip-components, summary: Option Summary. (line 569) * suffix: backup. (line 68) * suffix, summary: Option Summary. (line 578) * tape-length: Multi-Volume Archives. (line 33) * tape-length, short description: Device. (line 96) * tape-length, summary: Option Summary. (line 584) * test-label: label. (line 37) * test-label, summary: Option Summary. (line 589) * to-command: Writing to an External Program. (line 9) * to-command, summary: Option Summary. (line 593) * to-stdout: Writing to Standard Output. (line 14) * to-stdout, summary: Option Summary. (line 597) * totals: verbose. (line 46) * totals, summary: Option Summary. (line 602) * touch <1>: Attributes. (line 37) * touch: Data Modification Times. (line 15) * touch, summary: Option Summary. (line 607) * transform: transform. (line 74) * transform, summary: Option Summary. (line 426) * uncompress: gzip. (line 92) * uncompress, summary: Option Summary. (line 100) * ungzip: gzip. (line 54) * ungzip, summary: Option Summary. (line 191) * unlink-first: Unlink First. (line 6) * unlink-first, introduced: Dealing with Old Files. (line 42) * unlink-first, summary: Option Summary. (line 619) * unquote: Selecting Archive Members. (line 39) * unquote, summary: Option Summary. (line 625) * update: update. (line 8) * update, summary: Operation Summary. (line 50) * usage: help. (line 53) * use-compress-program: gzip. (line 101) * use-compress-program, summary: Option Summary. (line 629) * utc, summary: Option Summary. (line 633) * verbose: verbose. (line 18) * verbose, introduced: verbose tutorial. (line 6) * verbose, summary: Option Summary. (line 637) * verbose, using with --create: create verbose. (line 6) * verbose, using with --list: list. (line 30) * verify, short description: verify. (line 8) * verify, summary: Option Summary. (line 644) * verify, using with --create: verify. (line 24) * version: help. (line 6) * version, summary: Option Summary. (line 649) * volno-file: Multi-Volume Archives. (line 71) * volno-file, summary: Option Summary. (line 654) * wildcards: controlling pattern-matching. (line 38) * wildcards, summary: Option Summary. (line 659) * wildcards-match-slash: controlling pattern-matching. (line 92) * wildcards-match-slash, summary: Option Summary. (line 663)  File: tar.info, Node: Index, Prev: Index of Command Line Options, Up: Top Appendix H Index **************** [index] * Menu: * abbreviations for months: Calendar date items. (line 38) * absolute file names: Remote Tape Server. (line 17) * Adding archives to an archive: concatenate. (line 6) * Adding files to an Archive: appending files. (line 8) * ADMINISTRATOR: General-Purpose Variables. (line 7) * Age, excluding files by: after. (line 8) * ago in date strings: Relative items in date strings. (line 23) * am in date strings: Time of day items. (line 22) * Appending files to an Archive: appending files. (line 8) * archive: Definitions. (line 6) * Archive creation: file. (line 36) * archive member: Definitions. (line 15) * Archive Name: file. (line 8) * Archive, creation of: create. (line 8) * Archives, Appending files to: appending files. (line 8) * Archiving Directories: create dir. (line 6) * archiving files: Top. (line 24) * ARGP_HELP_FMT, environment variable: Configuring Help Summary. (line 22) * authors of get_date: Authors of get_date. (line 6) * Avoiding recursion in directories: recurse. (line 8) * backup options: backup. (line 6) * backup suffix: backup. (line 68) * BACKUP_DIRS: General-Purpose Variables. (line 29) * BACKUP_FILES: General-Purpose Variables. (line 55) * BACKUP_HOUR: General-Purpose Variables. (line 11) * backups: backup. (line 41) * beginning of time, for POSIX: Seconds since the Epoch. (line 13) * Bellovin, Steven M.: Authors of get_date. (line 6) * Berets, Jim: Authors of get_date. (line 6) * Berry, K.: Authors of get_date. (line 14) * Block number where error occurred: verbose. (line 111) * BLOCKING: General-Purpose Variables. (line 25) * blocking factor: Blocking Factor. (line 194) * Blocking Factor: Blocking Factor. (line 6) * Blocks per record: Blocking Factor. (line 6) * bug reports: Reports. (line 6) * Bytes per record: Blocking Factor. (line 6) * calendar date item: Calendar date items. (line 6) * case, ignored in dates: General date syntax. (line 64) * cat vs concatenate: concatenate. (line 63) * Changing directory mid-stream: directory. (line 6) * Character class, excluding characters from: wildcards. (line 34) * Choosing an archive file: file. (line 8) * comments, in dates: General date syntax. (line 64) * Compressed archives: gzip. (line 6) * concatenate vs cat: concatenate. (line 63) * Concatenating Archives: concatenate. (line 6) * corrupted archives <1>: gzip. (line 73) * corrupted archives: Full Dumps. (line 8) * Creation of the archive: create. (line 8) * DAT blocking: Blocking Factor. (line 204) * Data Modification time, excluding files by: after. (line 8) * Data modification times of extracted files: Data Modification Times. (line 6) * date format, ISO 8601: Calendar date items. (line 30) * date input formats: Date input formats. (line 6) * day in date strings: Relative items in date strings. (line 15) * day of week item: Day of week items. (line 6) * Deleting files from an archive: delete. (line 8) * Deleting from tape archives: delete. (line 19) * Descending directories, avoiding: recurse. (line 8) * Directories, Archiving: create dir. (line 6) * Directories, avoiding recursion: recurse. (line 8) * Directory, changing mid-stream: directory. (line 6) * DIRLIST: General-Purpose Variables. (line 51) * displacement of dates: Relative items in date strings. (line 6) * doc-opt-col: Configuring Help Summary. (line 95) * Double-checking a write operation: verify. (line 6) * DUMP_BEGIN: User Hooks. (line 32) * DUMP_END: User Hooks. (line 36) * DUMP_REMIND_SCRIPT: General-Purpose Variables. (line 102) * dumps, full: Full Dumps. (line 8) * dup-args: Configuring Help Summary. (line 52) * dup-args-note: Configuring Help Summary. (line 69) * Eggert, Paul: Authors of get_date. (line 6) * End-of-archive blocks, ignoring: Ignore Zeros. (line 6) * End-of-archive info script: Multi-Volume Archives. (line 80) * entry: Naming tar Archives. (line 11) * epoch, for POSIX: Seconds since the Epoch. (line 13) * Error message, block number of: verbose. (line 121) * Exabyte blocking: Blocking Factor. (line 204) * exclude: exclude. (line 14) * exclude-caches: exclude. (line 37) * exclude-from: exclude. (line 27) * Excluding characters from a character class: wildcards. (line 34) * Excluding file by age: after. (line 8) * Excluding files by file system: exclude. (line 8) * Excluding files by name and pattern: exclude. (line 8) * Exec Mode, genfile: Exec Mode. (line 6) * existing backup method: backup. (line 59) * exit status: Synopsis. (line 67) * Extraction: extract. (line 8) * extraction: Definitions. (line 22) * FDL, GNU Free Documentation License: GNU Free Documentation License. (line 6) * file archival: Top. (line 24) * File lists separated by NUL characters: Generate Mode. (line 29) * file name: Definitions. (line 15) * File Name arguments, alternatives: files. (line 6) * File name arguments, using --list with: list. (line 68) * File names, excluding files by: exclude. (line 8) * File names, terminated by NUL: nul. (line 6) * File names, using symbolic links: dereference. (line 6) * File system boundaries, not crossing: one. (line 6) * FILELIST: General-Purpose Variables. (line 65) * first in date strings: General date syntax. (line 26) * Format Options: Format Variations. (line 6) * Format Parameters: Format Variations. (line 6) * Format, old style: old. (line 6) * fortnight in date strings: Relative items in date strings. (line 15) * free documentation: Free Software Needs Free Documentation. (line 6) * full dumps: Full Dumps. (line 8) * future time stamps: Large or Negative Values. (line 6) * general date syntax: General date syntax. (line 6) * Generate Mode, genfile: Generate Mode. (line 6) * genfile: Genfile. (line 6) * genfile, create file: Generate Mode. (line 6) * genfile, creating sparse files: Generate Mode. (line 51) * genfile, generate mode: Generate Mode. (line 6) * genfile, reading a list of file names: Generate Mode. (line 18) * get_date: Date input formats. (line 6) * Getting program version number: help. (line 6) * GNU archive format: gnu. (line 6) * GNU.sparse.major, extended header variable: PAX 1. (line 14) * GNU.sparse.map, extended header variable: PAX 0. (line 60) * GNU.sparse.minor, extended header variable: PAX 1. (line 17) * GNU.sparse.name, extended header variable: PAX 0. (line 68) * GNU.sparse.name, extended header variable, in v.1.0: PAX 1. (line 24) * GNU.sparse.numblocks, extended header variable: PAX 0. (line 15) * GNU.sparse.numbytes, extended header variable: PAX 0. (line 21) * GNU.sparse.offset, extended header variable: PAX 0. (line 18) * GNU.sparse.realsize, extended header variable: PAX 1. (line 24) * GNU.sparse.size, extended header variable: PAX 0. (line 11) * gnupg, using with tar: gzip. (line 113) * gpg, using with tar: gzip. (line 113) * header-col: Configuring Help Summary. (line 141) * hook: User Hooks. (line 13) * hour in date strings: Relative items in date strings. (line 15) * Ignoring end-of-archive blocks: Ignore Zeros. (line 6) * Info script: Multi-Volume Archives. (line 80) * Interactive operation: interactive. (line 6) * ISO 8601 date format: Calendar date items. (line 30) * items in date strings: General date syntax. (line 6) * Labeling an archive: label. (line 6) * Labeling multi-volume archives: label. (line 6) * Labels on the archive media: label. (line 6) * language, in dates: General date syntax. (line 40) * Large lists of file names on small machines: Same Order. (line 6) * large values: Large or Negative Values. (line 6) * last DAY: Day of week items. (line 15) * last in date strings: General date syntax. (line 26) * Listing all tar options: help. (line 26) * listing member and file names: list. (line 41) * Listing volume label: label. (line 29) * Lists of file names: files. (line 6) * Local and remote archives: file. (line 73) * long-opt-col: Configuring Help Summary. (line 87) * MacKenzie, David: Authors of get_date. (line 6) * member: Definitions. (line 15) * member name: Definitions. (line 15) * Members, replacing with other members: append. (line 49) * Meyering, Jim: Authors of get_date. (line 6) * Middle of the archive, starting in the: Starting File. (line 11) * midnight in date strings: Time of day items. (line 22) * minute in date strings: Relative items in date strings. (line 15) * minutes, time zone correction by: Time of day items. (line 30) * Modes of extracted files: Setting Access Permissions. (line 6) * Modification time, excluding files by: after. (line 8) * Modification times of extracted files: Data Modification Times. (line 6) * month in date strings: Relative items in date strings. (line 15) * month names in date strings: Calendar date items. (line 38) * months, written-out: General date syntax. (line 36) * MT: General-Purpose Variables. (line 69) * MT_BEGIN: Magnetic Tape Control. (line 11) * MT_OFFLINE: Magnetic Tape Control. (line 32) * MT_REWIND: Magnetic Tape Control. (line 21) * MT_STATUS: Magnetic Tape Control. (line 42) * Multi-volume archives: Multi-Volume Archives. (line 6) * Mutli-volume archives in PAX format, extracting using non-GNU tars: Split Recovery. (line 17) * Mutli-volume archives, extracting using non-GNU tars: Split Recovery. (line 6) * Naming an archive: file. (line 8) * negative time stamps: Large or Negative Values. (line 6) * next DAY: Day of week items. (line 15) * next in date strings: General date syntax. (line 26) * noon in date strings: Time of day items. (line 22) * now in date strings: Relative items in date strings. (line 33) * ntape device: Many. (line 6) * NUL terminated file names: nul. (line 6) * Number of blocks per record: Blocking Factor. (line 6) * Number of bytes per record: Blocking Factor. (line 6) * numbered backup method: backup. (line 55) * numbers, written-out: General date syntax. (line 26) * Obtaining help: help. (line 26) * Obtaining total status information: verbose. (line 46) * Old GNU archive format: gnu. (line 6) * Old GNU sparse format: Old GNU Format. (line 6) * Old style archives: old. (line 6) * Old style format: old. (line 6) * opt-doc-col: Configuring Help Summary. (line 127) * option syntax, traditional: Old Options. (line 60) * Options when reading archives: Reading. (line 6) * Options, archive format specifying: Format Variations. (line 6) * Options, format specifying: Format Variations. (line 6) * ordinal numbers: General date syntax. (line 26) * Overwriting old files, prevention: Dealing with Old Files. (line 16) * pattern, genfile: Generate Mode. (line 35) * PAX archive format: posix. (line 6) * Permissions of extracted files: Setting Access Permissions. (line 6) * Pinard, F.: Authors of get_date. (line 14) * pm in date strings: Time of day items. (line 22) * POSIX archive format: posix. (line 6) * Progress information: verbose. (line 83) * Protecting old files: Dealing with Old Files. (line 26) * pure numbers in date strings: Pure numbers in date strings. (line 6) * Reading file names from a file: files. (line 6) * Reading incomplete records: Reading. (line 8) * Record Size: Blocking Factor. (line 6) * Records, incomplete: Reading. (line 8) * Recursion in directories, avoiding: recurse. (line 8) * relative items in date strings: Relative items in date strings. (line 6) * Remote devices: file. (line 62) * remote tape drive: Remote Tape Server. (line 6) * Removing files from an archive: delete. (line 8) * Replacing members with other members: append. (line 49) * reporting bugs: Reports. (line 6) * RESTORE_BEGIN: User Hooks. (line 39) * RESTORE_END: User Hooks. (line 42) * Resurrecting files from an archive: extract. (line 8) * Retrieving files from an archive: extract. (line 8) * return status: Synopsis. (line 67) * rmargin: Configuring Help Summary. (line 160) * rmt: Remote Tape Server. (line 6) * RSH: General-Purpose Variables. (line 72) * RSH_COMMAND: General-Purpose Variables. (line 77) * Running out of space: Scarce. (line 8) * Salz, Rich: Authors of get_date. (line 6) * short-opt-col: Configuring Help Summary. (line 79) * simple backup method: backup. (line 64) * SIMPLE_BACKUP_SUFFIX: backup. (line 68) * SLEEP_MESSAGE: General-Purpose Variables. (line 111) * SLEEP_TIME: General-Purpose Variables. (line 97) * Small memory: Scarce. (line 8) * Sparse Files: sparse. (line 6) * sparse files v.0.0, extracting with non-GNU tars: Sparse Recovery. (line 92) * sparse files v.0.1, extracting with non-GNU tars: Sparse Recovery. (line 92) * sparse files v.1.0, extracting with non-GNU tars: Sparse Recovery. (line 17) * Sparse files, creating using genfile: Generate Mode. (line 51) * sparse files, extracting with non-GNU tars: Sparse Recovery. (line 6) * sparse formats: Sparse Formats. (line 6) * sparse formats, defined: sparse. (line 50) * sparse formats, Old GNU: Old GNU Format. (line 6) * sparse formats, v.0.0: PAX 0. (line 6) * sparse formats, v.0.1: PAX 0. (line 52) * sparse formats, v.1.0: PAX 1. (line 6) * sparse versions: Sparse Formats. (line 6) * Specifying archive members: Selecting Archive Members. (line 6) * Specifying files to act on: Selecting Archive Members. (line 6) * Standard input and output: file. (line 41) * Standard output, writing extracted files to: Writing to Standard Output. (line 6) * Storing archives in compressed format: gzip. (line 6) * Symbolic link as file name: dereference. (line 6) * TAPE: file tutorial. (line 14) * tape blocking: Blocking Factor. (line 194) * tape marks: Many. (line 44) * tape positioning: Many. (line 26) * TAPE_FILE: General-Purpose Variables. (line 19) * Tapes, using --delete and: delete. (line 19) * TAR: General-Purpose Variables. (line 115) * tar: What tar Does. (line 6) * tar archive: Definitions. (line 6) * Tar archive formats: Formats. (line 6) * tar entry: Naming tar Archives. (line 11) * tar file: Naming tar Archives. (line 11) * tar to a remote device: file. (line 62) * tar to standard input and output: file. (line 41) * TAR_ARCHIVE, info script environment variable: Multi-Volume Archives. (line 100) * TAR_ATIME, to-command environment: Writing to an External Program. (line 49) * TAR_CTIME, to-command environment: Writing to an External Program. (line 58) * TAR_FILENAME, to-command environment: Writing to an External Program. (line 37) * TAR_FILETYPE, to-command environment: Writing to an External Program. (line 22) * TAR_FORMAT, info script environment variable: Multi-Volume Archives. (line 110) * TAR_GID, to-command environment: Writing to an External Program. (line 67) * TAR_GNAME, to-command environment: Writing to an External Program. (line 46) * TAR_MODE, to-command environment: Writing to an External Program. (line 34) * TAR_MTIME, to-command environment: Writing to an External Program. (line 55) * TAR_OPTIONS, environment variable: using tar options. (line 30) * TAR_REALNAME, to-command environment: Writing to an External Program. (line 40) * TAR_SIZE, to-command environment: Writing to an External Program. (line 61) * TAR_SUBCOMMAND, info script environment variable: Multi-Volume Archives. (line 106) * TAR_UID, to-command environment: Writing to an External Program. (line 64) * TAR_UNAME, to-command environment: Writing to an External Program. (line 43) * TAR_VERSION, info script environment variable: Multi-Volume Archives. (line 97) * TAR_VOLUME, info script environment variable: Multi-Volume Archives. (line 103) * tarcat: Tarcat. (line 6) * this in date strings: Relative items in date strings. (line 33) * time of day item: Time of day items. (line 6) * time zone correction: Time of day items. (line 30) * time zone item <1>: Time zone items. (line 6) * time zone item: General date syntax. (line 44) * today in date strings: Relative items in date strings. (line 33) * tomorrow in date strings: Relative items in date strings. (line 29) * TZ: Specifying time zone rules. (line 6) * Ultrix 3.1 and write failure: Remote Tape Server. (line 40) * unpacking: Definitions. (line 22) * Updating an archive: update. (line 8) * usage-indent: Configuring Help Summary. (line 156) * Using encrypted archives: gzip. (line 113) * ustar archive format: ustar. (line 6) * uuencode: Applications. (line 8) * v7 archive format: old. (line 6) * Verbose operation: verbose. (line 18) * Verifying a write operation: verify. (line 6) * Verifying the currency of an archive: compare. (line 6) * Version of the tar program: help. (line 6) * version-control Emacs variable: backup. (line 49) * VERSION_CONTROL: backup. (line 41) * volno file: Multi-Volume Archives. (line 71) * VOLNO_FILE: General-Purpose Variables. (line 82) * Volume label, listing: label. (line 29) * Volume number file: Multi-Volume Archives. (line 71) * week in date strings: Relative items in date strings. (line 15) * Where is the archive?: file. (line 8) * Working directory, specifying: directory. (line 6) * Writing extracted files to standard output: Writing to Standard Output. (line 6) * Writing new archives: file. (line 36) * XLIST: General-Purpose Variables. (line 87) * xsparse: Sparse Recovery. (line 13) * year in date strings: Relative items in date strings. (line 15) * yesterday in date strings: Relative items in date strings. (line 29)