pax (command)

pax has four general modes that are invoked by a combination of the -r ("read") and -w ("write") options.

This table summarizes the modal behaviour:

This model is similar to cpio, which has a similar set of basic operations.

List contents of an archive:

$ pax < archive.tar

Extract contents of an archive into the current directory:

$ pax -r < archive.tar

Create an archive of the current directory:

$ printf '.' | pax -w > archive.tar
$ pax -w . > archive.tar

Copy current directory tree to another location:

$ printf '.' | pax -w > archive.tar
$ pax -rw target_dir .

(The target directory must exist beforehand!)

By default, it uses the standard input/output for archive and listing operations, but this can be overwridden with the "tar-style" option -f that specifies the archive file. Pax differs from cpio by recursively considering the content of a directory; to disable this behavior, POSIX pax has an option -d to disable it.

The pax command is a mish-mash of cpio and tar features. Like tar, pax processes directory entries recursively, a feature that can be disabled with -d for cpio-style behavior. The handling of file input/outputs is also a mix: when a list of file names is specified on the command line, they are taken as shell globs for file input or listing (tar-like); otherwise pax takes the cpio-style behavior of using the standard input for a file list. Finally, pax supports reading/writing to a named archive file using tar's -f FILE option.

For example, if one desires a cpio-style archiving of the current directory, pax can be used with find just like one does using cpio:

$ find . -depth -print | pax -wd > archive.tar

(This construct is pointless without any filters for find, as it becomes identical to the above example.)

The command for extracting the contents for an archive is the same as cpio:

$ pax -r < archive.tar

It is possible to invoke these commands in a tar-like syntax as well:

$ pax -wf archive.tar . # write (archive)
$ pax -rf archive.tar   # read (extract)
$ pax -f archive.tar    # list
$ pax -rw . archive_dir # copy

Most implementations of pax use the -z (gzip) and -j (bzip2) switches for compression; this feature however, is not specified by POSIX. It is important to note that pax cannot append to compressed archives.
Example for extracting a gzipped archive:

$ pax -rzf archive.tar.gz

As in tar and cpio, pax output can be piped to another compressor/decompressor program. As an example xz is used here:

$ pax -w . | xz > archive.tar.xz

and listing an xz-compressed archive as the input:

$ xzcat archive.tar.xz | pax

Almost all extant versions of pax stemmed from the original 4.4BSD implementation. Most of them inherits the formats supported by that version, selectable via the -x option:

• cpio – The extended cpio interchange format specified in the IEEE Std 1003.2 ("POSIX.2") standard.
• bcpio – The old binary cpio format.
• sv4cpio – The System V release 4 cpio.
• sv4crc – The System V release 4 cpio with file crc checksums.
• tar – The old BSD tar format as found in BSD4.3.
• ustar (default) – The tar interchange format specified in the IEEE Std 1003.2 ("POSIX.2") standard.

Notably, the support for the 2001 pax format is not supported by this legacy pax. This is the case on most Linux distributions (which uses the MirBSD branch of MirCPIO-paxmirabilis)[7][8][9] and on FreeBSD.[6]

The Heirloom Project pax has support for the pax support as well as many extra formats.[10]

pax supports archiving on multiple volumes. When the end of a volume is reached, the following message appears:

$ pax -wf /dev/fd0 .
ATTENTION! pax archive volume change required.
/dev/fd0 ready for archive volume: 2
Load the NEXT STORAGE MEDIA (if required) and make sure it is WRITE ENABLED.
Type "y" to continue, "." to quit pax, or "s" to switch to new device.
If you cannot change storage media, type "s"
Is the device ready and online? >

When restoring an archive from multiple media, pax asks for the next media in the same fashion, when the end of the media is reached before the end of the archive.