pax - the POSIX archiver
pax can do a lot of fancy stuff, feel free to contribute more awesome pax tricks!
The POSIX archiver,
pax, is an attempt at a standardized archiver with
the best features of
cpio, able to handle all common archive types.
However, this is not a manpage, it will not list all possible options,
it will not you detailed information about
pax. It's only an introduction.
This article is based on the debianized Berkeley implementation of
pax, but implementation-specific things should be tagged as such. Unfortunately, the Debian package doesn't seem to be maintained anymore.
There are four basic operation modes to list, read, write and
copy archives. They're switched with combinations of
command line options:
In list mode,
pax writes the list of archive members to standard
output (a table of contents). If a pattern match is specified on the
command line, only matching filenames are printed.
Read an archive.
pax will read archive data and extract the members to the
current directory. If a pattern match is specified on the command line, only matching
filenames are extracted.
When reading an archive, the archive type is determined from the archive data.
Write an archive, which means create a new one or append to an existing one. All files and directories specified on the command line are inserted into the archive. The archive is written to standard output by default.
If no files are specified on the command line, filenames are read from
The write mode is the only mode where you need to specify the archive type
-x <TYPE>, e.g.
Copy mode is similar to
cpio passthrough mode. It provides a way to replicate a complete or partial file hierarchy
(with all the
pax options, e.g. rewriting groups) to another location.
When you don't specify anything special,
pax will attempt to read archive
data from standard input (read/list modes) and write archive data to
standard output (write mode). This ensures
pax can be easily
used as part of a shell pipe construct, e.g. to read a compressed
archive that's decompressed in the pipe.
The option to specify the pathname of a file to be archived is
This file will be used as input or output, depending on the operation
When pax reads an archive, it tries to guess the archive type.
However, in write mode, you must specify which type of archive
to append using the
-x <TYPE> switch. If you omit this switch,
a default archive will be created (POSIX says it's implementation defined,
ustar if no options are specified).
The following archive formats are supported (Berkeley implementation):
|ustar||POSIX TAR format (default)|
|cpio||POSIX CPIO format|
|tar||classic BSD TAR format|
|bcpio||old binary CPIO format|
|sv4cpio||SVR4 CPIO format|
|sv4crc||SVR4 CPIO format with CRC|
pax supports options
-j, similar to GNU
tar, to filter archive files through GZIP/BZIP2.
Matching archive members
In read and list modes, you can specify patterns to determine which files to list or extract.
- the pattern notation is the one known by a POSIX-shell, i.e. the one known by Bash without
- if the specified pattern matches a complete directory, it affects all files and subdirectories of the specified directory
- if you specify the
paxwill invert the matches, i.e. it matches all filenames except those matching the specified patterns
- if no patterns are given,
paxwill "match" (list or extract) all files from the archive
- To avoid conflicts with shell pathname expansion, it's wise to quote patterns!
Some assorted examples of patterns
pax -r <myarchive.tar 'data/sales/*.txt' 'data/products/*.png'
pax -r <myarchive.tar 'data/sales/year_200.txt' # should be equivalent to pax -r <myarchive.tar 'data/sales/year_2001.txt' 'data/sales/year_2003.txt' 'data/sales/year_2005.txt'
This is a brief description of using
pax as a normal archiver
system, like you would use
Creating an archive
This task is done with basic syntax
# archive contents to stdout pax -w >archive.tar README.txt *.png data/ # equivalent, extract archive contents directly to a file pax -w -x ustar -f archive.tar README.txt *.png data/
pax is in write mode, the given filenames are packed into an
README.txtis a normal file, it will be packed
*.pngis a pathname glob for your shell, the shell will substitute all matching filenames before
paxis executed. The result is a list of filenames that will be packed like the
data/is a directory. Everything in this directory will be packed into the archive, i.e. not just an empty directory
When you specify the
pax will write the pathnames of the
files inserted into the archive to
When, and only when, no filename arguments are specified,
pax attempts to
read filenames from
STDIN, separated by newlines.
This way you can easily combine
find . -name '*.txt' | pax -wf textfiles.tar -x ustar
Listing archive contents
The standard output format to list archive members simply is to print each
filename to a separate line. But the output format can be customized to include
permissions, timestamps, etc. with the
-o listopt=<FORMAT> specification.
The syntax of the format specification is strongly derived from the
printf(3) format specification.
pax utility delivered with Debian doesn't seem to
support these extended listing formats.
pax lists archive members in a
ls -l-like format, when you
pax -v <myarchive.tar # or, of course pax -vf myarchive.tar
Extracting from an archive
You can extract all files, or files (not) matching specific patterns from an archive using constructs like:
# "normal" extraction pax -rf myarchive.tar '*.txt'
# with inverted pattern pax -rf myarchive.tar -c '*.txt'
To copy directory contents to another directory, similar to a
cp -a command, use:
mkdir destdir pax -rw dir destdir #creates a copy of dir in destdir/, i.e. destdir/dir
Copying files via ssh
To copy directory contents to another directory on a remote system, use:
pax -w localdir | ssh user@host "cd distantdest && pax -r -v" pax -w localdir | gzip | ssh user@host "cd distantdir && gunzip | pax -r -v" #compress the sent dataThese commands create a copy of localdir in distandir (distantdir/dir) on the remote machine.
Backup your daily work
-T is an extension and is not defined by POSIX.
Say you have write-access to a fileserver mounted on your filesystem tree.
In copy mode, you can tell
pax to copy only files that were
mkdir /n/mybackups/$(date +%A)/ pax -rw -T 0000 data/ /n/mybackups/$(date +%A)/This is done using the
-Tswitch, which normally allows you to specify a time window, but in this case, only the start time which means "today at midnight".
When you execute this "very simple backup" after your daily work, you will have a copy of the modified files.
%A format from
date expands to the name of the
current day, localized, e.g. "Friday" (en) or "Mittwoch" (de).
The same, but with an archive, can be accomplished by:
pax -w -T 0000 -f /n/mybackups/$(date +%A)In this case, the day-name is an archive-file (you don't need a filename extension like
.tarbut you can add one, if desired).
Changing filenames while archiving
pax is able to rewrite filenames while archiving or while extracting from an archive. This example creates a tar archive containing the
holiday_2007/ directory, but the directory name inside the archive will be
pax -x ustar -w -f holiday_pictures.tar -s '/^holiday_2007/holiday_pics/' holiday_2007/
The option responsible for the string manipulation is the
-s <REWRITE-SPECIFICATION>. It takes the string rewrite specification as an argument, in the form
/OLD/NEW/[gp], which is an
ed(1)-like regular expression (BRE) for
old and generally can be used like the popular sed construct
s/from/to/. Any non-null character can be used as a delimiter, so to mangle pathnames (containing slashes), you could use
p flags are used to apply substitution (g)lobally to the line or to (p)rint the original and rewritten strings to
-s options can be specified on the command line. They are applied to the pathname strings of the files or archive members. This happens in the order they are specified.
Excluding files from an archive
The -s command seen above can be used to exclude a file. The substitution must result in a null string: For example, let's say that you want to exclude all the CVS directories to create a source code archive. We are going to replace the names containing /CVS/ with nothing, note the .* they are needed because we need to match the entire pathname.
pax -w -x ustar -f release.tar -s',.*/CVS/.*,,' myapplicationYou can use several -s options, for instance, let's say you also want to remove files ending in ~:
pax -w -x ustar -f release.tar -'s,.*/CVS/.*,,' -'s/.*~//' myapplication
This can also be done while reading an archive, for instance, suppose you have an archive containing a "usr" and a "etc" directory but that you want to extract only the "usr" directory:
pax -r -f archive.tar -s',^etc/.*,,' #the etc/ dir is not extracted
Getting archive filenames from STDIN
cpio, pax can read filenames from standard input (
stdin). This provides great flexibility - for example, a
find(1) command may select files/directories in ways pax can't do itself. In write mode (creating an archive) or copy mode, when no filenames are given, pax expects to read filenames from standard input. For example:
# Back up config files changed less than 3 days ago find /etc -type f -mtime -3 | pax -x ustar -w -f /backups/etc.tar # Copy only the directories, not the files mkdir /target find . -type d -print | pax -r -w -d /target # Back up anything that changed since the last backup find . -newer /var/run/mylastbackup -print0 | pax -0 -x ustar -w -d -f /backups/mybackup.tar touch /var/run/mylastbackup
-d option tells pax
not to recurse into directories it reads (
-d, pax recurses into all directories (
-0 option is not standard, but is present in some implementations.
From tar to pax
pax can handle the
tar archive format, if you want to switch to the standard tool an alias like:
alias tar='echo USE PAX, idiot. pax is the standard archiver!; # 'in your
~/.bashrccan be useful .
Here is a quick table comparing (GNU)
pax to help you to make the switch:
| || ||
| || ||
| || |
| || |
| || ||
pax might not create ustar (
tar) archives by default but its own pax format, add
-x ustar if you want to ensure pax creates tar archives!