Make Peace with pax

James K. Lowden — Mon, 27 Jan 2014 20:36:44 +0000

pax is one of the lesser known utilities in a typical Linux installation. That's too bad, because pax has a very good feature set, and its command-line options are easy to understand and remember. pax is an archiver, like tar(1), but it's also a better version of cp(1) in some ways, not least because you can use pax with SSH to copy sets of files over a network. Once you learn pax, you may wonder how you lived without it all these years.

pax has four modes: list, read, write and copy. Reading and writing are controlled by the -r and -w options, repectively. In combination, -rw, pax acts a little bit like cp -R. If neither is used, pax lists the contents of the archive, which may be a file, device or a pipe.

By default, pax operates as a filter: it reads from standard input and writes to standard output, a feature that turns out to be very useful. But usually these days, the target is an archive file, the familiar tarball. Let's start by creating one:


$ cd /tmp
$ mkdir paxample
$ touch paxample/foo
$ pax -wf paxample.tar paxample

The -w option means write—that is, create an archive. The -f option provides the name of a file to which to write the archive. If desired, pax can gzip or bzip the file at the same time:


$ pax -wzf paxample.tar.gz paxample

Like most tar implementations, pax, by default, uses the Posix ustar file format. Because pax was born of a desire to unify archive file formats, many other formats also are supported, but in practice, they're seldom used. Likely as not, any .tar.gz file you download from the Internet actually will be a ustar archive:


$ pax -wzf paxample.tar.gz paxample
$ file paxample.tar*
paxample.tar:    POSIX tar archive
paxample.tar.gz: gzip compressed data

The first thing you nearly always want to know about any archive is what's in it. Listing the contents is the default action in the absence of either a -r or -w option:


$ pax -f paxample.tar
paxample
paxample/foo

Note that the archive retains the directory name you specified on the command line. That comes into play later when you read it.

To read an archive, use -r:


$ mkdir t
$ cd t
$ pax -rf ../paxample.tar

What did that do? Let's look at the source and target directories:


$ cd /tmp
$ find paxample t # traverse both trees
paxample
paxample/foo
t
t/paxample
t/paxample/foo

When pax read the paxample.tar archive, it created files in the current directory, t. Because the archive included a directory name, paxample, that directory was re-created in the output.

Copying Sets of Files

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