pax is an archiving utility created by POSIX and defined by the POSIX.1-2001[1] standard. By default, it creates archives in ustar format, also defined by the POSIX standard. Rather than sort out the incompatible options that have crept up between tar and cpio, along with their implementations across various versions of UNIX, the IEEE designed a new archive utility. The name "pax" is an acronym for portable archive exchange. Furthermore, "pax" means "peace" in Latin, so its name implies that it shall create peace between the tar and cpio format supporters. The command invocation and command structure is somewhat a unification of both tar and cpio.
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pax has four general modes which are invoked by a combination of the -r ("read") option and -w ("write") option.
This table summarizes the modal behaviour:
| Option | Mode | Description |
|---|---|---|
| (none) | "list" | shows contents of archive, does not modify or extract anything. |
| -r | "read" | reads and extracts contents of an archive |
| -w | "write" | creates archives or appends files to an archive |
| -rw | "copy" | reads and copies files and directory tree to a specified directory |
Examples:
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:
When used in the cpio style, the find command can be used to get a list of files to be archived:
find . -depth -print | pax -wd > archive.tar
Copy current directory tree to another location:
The target directory must exist beforehand!
find . -depth -print | pax -rwd target_dir
pax can be either used in a similar manner as cpio or tar. The cpio syntax takes a list of files from standard input (stdin) when archiving or an already existing archive, when in listing contents or extracting files:
find . -depth -print | pax -wd > archive.tar
and
pax -r < archive.tar
respectively.
It is possible to invoke these commands in a tar-like syntax as well:
pax -wf archive.tar .
and
pax -rf archive.tar
And for clarity and completeness:
Listing files from an archive:
pax -f archive.tar
and "copy" mode:
pax -rw . archive_dir
The -f option specifies which archive to use, instead of writing to stdio or reading from stdin. Also note the -d option when using pax together with find, this keeps pax from traversing directory trees.
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 gziped archive:
pax -rzf archive.tar.gz
Due to the possibility to use pax in a cpio-like fashion, it is possible to use whatever compression program, as an example xz is used here:
pax -w . | xz > archive.tar.xz
and listing an xz-compressed archive:
xzcat archive.tar.xz | pax
As of September 2009, pax supports the following formats, selectable via the -x option:
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 medias, pax asks for the next media in the same fashion, when the end of the media is reached before the end of the archive.
Despite being standardized in 2001 by IEEE, as of 2010, pax enjoys relatively little popularity and penetration rate.
pax is required to be present in all conformant systems by Linux Standard Base since version 3.0 (released on July 6, 2005),[2] but so far few Linux distributions ship and install it by default. However, most distributions include pax as a separately installable package.
Debian popularity contest estimates that, as of 2011, pax is installed on 7.6% of Debian systems that participated in statistics.[3] For comparison, rivalling "tar" archiver has a penetration rate of 99.99%.[4]
Ubuntu popularity contest estimates pax number of installations as 57739 out of 1593595 submissions, that makes even smaller 3.6% penetration rate. "tar" penetration rate in Ubuntu is 1592184 out of 1593595, i.e. 99.91%.[5]
pax has also been present in Windows NT, where it is limited to file archives (tapes not supported).[6] It was later moved to the Interix subsystem. It does not support archiving or restoring Win32 ACLs.[7]
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