8.1.1 Creating and Reading Compressed Archives

GNU tar is able to create and read compressed archives. It supports a wide variety of compression programs, namely: gzip, bzip2, lzip, lzma, lzop, xz and traditional compress. The latter is supported mostly for backward compatibility, and we recommend against using it, because it is by far less effective than the other compression programs(17).

Creating a compressed archive is simple: you just specify a compression option along with the usual archive creation commands. The compression option is ‘-z’ (‘--gzip’) to create a gzip compressed archive, ‘-j’ (‘--bzip2’) to create a bzip2 compressed archive, ‘--lzip’ to create an lzip compressed archive, ‘-J’ (‘--xz’) to create an XZ archive, ‘--lzma’ to create an LZMA compressed archive, ‘--lzop’ to create an LSOP archive, and ‘-Z’ (‘--compress’) to use compress program. For example:

$ tar czf archive.tar.gz .

You can also let GNU tar select the compression program based on the suffix of the archive file name. This is done using ‘--auto-compress’ (‘-a’) command line option. For example, the following invocation will use bzip2 for compression:

$ tar caf archive.tar.bz2 .

whereas the following one will use lzma:

$ tar caf archive.tar.lzma .

For a complete list of file name suffixes recognized by GNU tar, see auto-compress.

Reading compressed archive is even simpler: you don’t need to specify any additional options as GNU tar recognizes its format automatically. Thus, the following commands will list and extract the archive created in previous example:

# List the compressed archive
$ tar tf archive.tar.gz
# Extract the compressed archive
$ tar xf archive.tar.gz

The format recognition algorithm is based on signatures, a special byte sequences in the beginning of file, that are specific for certain compression formats. If this approach fails, tar falls back to using archive name suffix to determine its format (see auto-compress, for a list of recognized suffixes).

Some compression programs are able to handle different compression formats. GNU tar uses this, if the principal decompressor for the given format is not available. For example, if compress is not installed, tar will try to use gzip. As of version 1.27 the following alternatives are tried(18):

The only case when you have to specify a decompression option while reading the archive is when reading from a pipe or from a tape drive that does not support random access. However, in this case GNU tar will indicate which option you should use. For example:

$ cat archive.tar.gz | tar tf -
tar: Archive is compressed.  Use -z option
tar: Error is not recoverable: exiting now

If you see such diagnostics, just add the suggested option to the invocation of GNU tar:

$ cat archive.tar.gz | tar tzf -

Notice also, that there are several restrictions on operations on compressed archives. First of all, compressed archives cannot be modified, i.e., you cannot update (‘--update’, alias ‘-u’) them or delete (‘--delete’) members from them or add (‘--append’, alias ‘-r’) members to them. Likewise, you cannot append another tar archive to a compressed archive using ‘--concatenate’ (‘-A’). Secondly, multi-volume archives cannot be compressed.

The following options allow to select a particular compressor program:

‘-z’

‘--gzip’

‘--ungzip’

Filter the archive through gzip.

‘-J’

‘--xz’

Filter the archive through xz.

‘-j’

‘--bzip2’

Filter the archive through bzip2.

‘--lzip’

Filter the archive through lzip.

‘--lzma’

Filter the archive through lzma.

‘--lzop’

Filter the archive through lzop.

‘-Z’

‘--compress’

‘--uncompress’

Filter the archive through compress.

When any of these options is given, GNU tar searches the compressor binary in the current path and invokes it. The name of the compressor program is specified at compilation time using a corresponding ‘--with-compname’ option to configure, e.g. ‘--with-bzip2’ to select a specific bzip2 binary. See section Using lbzip2 with GNU tar., for a detailed discussion.

The output produced by tar --help shows the actual compressor names along with each of these options.

You can use any of these options on physical devices (tape drives, etc.) and remote files as well as on normal files; data to or from such devices or remote files is reblocked by another copy of the tar program to enforce the specified (or default) record size. The default compression parameters are used. Most compression programs let you override these by setting a program-specific environment variable. For example, with gzip you can set GZIP:

$ GZIP='-9 -n' tar czf archive.tar.gz subdir

Another way would be to use the ‘-I’ option instead (see below), e.g.:

$ tar -cf archive.tar.gz -I 'gzip -9 -n' subdir

Finally, the third, traditional, way to do this is to use a pipe:

$ tar cf - subdir | gzip -9 -n > archive.tar.gz

Compressed archives are easily corrupted, because compressed files have little redundancy. The adaptive nature of the compression scheme means that the compression tables are implicitly spread all over the archive. If you lose a few blocks, the dynamic construction of the compression tables becomes unsynchronized, and there is little chance that you could recover later in the archive.

Other compression options provide better control over creating compressed archives. These are:

‘--auto-compress’

‘-a’

Select a compression program to use by the archive file name suffix. The following suffixes are recognized:

Suffix	Compression program
‘.gz’	gzip
‘.tgz’	gzip
‘.taz’	gzip
‘.Z’	compress
‘.taZ’	compress
‘.bz2’	bzip2
‘.tz2’	bzip2
‘.tbz2’	bzip2
‘.tbz’	bzip2
‘.lz’	lzip
‘.lzma’	lzma
‘.tlz’	lzma
‘.lzo’	lzop
‘.xz’	xz

‘--use-compress-program=command’

‘-I=command’

Use external compression program command. Use this option if you are not happy with the compression program associated with the suffix at compile time or if you have a compression program that GNU tar does not support. The command argument is a valid command invocation, as you would type it at the command line prompt, with any additional options as needed. Enclose it in quotes if it contains white space (see Running External Commands, for more detail).

The command should follow two conventions:

First, when invoked without additional options, it should read data from standard input, compress it and output it on standard output.

Secondly, if invoked with the additional ‘-d’ option, it should do exactly the opposite, i.e., read the compressed data from the standard input and produce uncompressed data on the standard output.

The latter requirement means that you must not use the ‘-d’ option as a part of the command itself.

The ‘--use-compress-program’ option, in particular, lets you implement your own filters, not necessarily dealing with compression/decompression. For example, suppose you wish to implement PGP encryption on top of compression, using gpg (see gpg —- encryption and signing tool in GNU Privacy Guard Manual). The following script does that:

#! /bin/sh
case $1 in
-d) gpg --decrypt - | gzip -d -c;;
'') gzip -c | gpg -s;;
*)  echo "Unknown option $1">&2; exit 1;;
esac

Suppose you name it ‘gpgz’ and save it somewhere in your PATH. Then the following command will create a compressed archive signed with your private key:

$ tar -cf foo.tar.gpgz -Igpgz .

Likewise, the command below will list its contents:

$ tar -tf foo.tar.gpgz -Igpgz .

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