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FreeBSD Basics Archiving with Pax

by Dru Lavigne
08/22/2002

In today's article, I'd like to finish up the archiving series with the pax utility. It's unfortunate that this utility never seems to get the coverage that tar and cpio do. I've found that it combines the best qualities of both utilities into one easy-to-use and fairly intuitive utility.

The name of the utility stands for "portable archive exchange," as it was designed specifically to allow portability between different versions of Unix. There's also a bit of wry humor in the name, as pax attempts to bring some "peace" to the long-standing battle over which is better: tar or cpio. The pax utility can be used to create either type of archive, and during a restore, it automagically detects the type of archive for you. And it doesn't matter what type of Unix that archive happened to be created on, meaning you can back up files from your FreeBSD system and restore them to, say, a SCO system.

Let's start with some examples of basic pax usage, then move on to some fancier stuff. To back up the contents of your home directory, invoke write mode using the w switch:

cd
pax -wf home.pax .

In this example, I went to my home directory with cd, then told pax to write (w) to a file (f) named home.pax the contents of the current directory ("."). When you use pax, it's very important to remember to include that f switch to indicate the name of the archive you'd like to create. If you forget the f, weird characters will be sent to your screen, accompanied by horrible, pained noises. Also, if you want to watch as pax does its thing, simply add the v, or verbose, switch to the switch portion of the command.

To see what type of file you've just created, use the file command:

file home.pax
home.pax: POSIX tar archive

To see the contents of that archive, tell pax which archive file you'd like to view, using the f switch:

pax -f home.pax |more

Since my archive file is rather large, I piped this output to the more command so I could read the contents of the archive one page at a time. If you also include the v switch, you'll get an "ls -l"-type output of the archive contents. Again, don't forget to specify the name of the archive with the f switch, or nothing will happen, except that you'll lose your prompt until you press CTRL-C.

The pax utility does support compression, so I could have performed a compressed backup by including the z switch:

pax -wzf home.pax .

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Let's do another example. This time I'll back up the /etc directory to a floppy:

cd /etc
su
Password:
pax -wf /dev/fd0 .

You'll note that I became the superuser in this example. This was necessary for two reasons. First, the files in /etc are owned by root, since they are the configuration files for the system. Second, by default, only the superuser has permission to back up to a floppy drive. Also notice that I specified the floppy as the name of the archive file (/dev/fd0).

A couple of notes about backing up to a floppy: the pax utility is intelligent enough to realize when it fills up a floppy, and will prompt for another one if the archive file is too large to fit onto one floppy. However, pax does not support compression to a floppy; if you try adding the z switch to the above example, you'll receive this error message:

gzip: stdout: Invalid argument

You should also be aware that, by default, when you back up to a floppy, you will lose any previous data stored on that floppy. If you would like to append to a previous archive, use the a switch:

cd
pax -wvf /dev/fd0 jpegs
pax -wavf /dev/fd0 pdfs
pax -f /dev/fd0

The above example will back up the jpegs directory to a floppy, then append the pdfs directory to the backup. When I list the archive, the contents of both the jpegs and pdfs directories will be on the floppy.

To restore (or use read mode on) an archive, first cd into the destination directory, then use the r switch. For example, I'll restore the backup named home.pax into the test subdirectory of my home directory:

cd test
pax -rvf ~/home.pax

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