---
tags:
  - bash
  - shell
  - scripting
  - POSIX
  - archive
  - tar
  - packing
  - zip
---

# pax - the POSIX archiver

pax can do a lot of fancy stuff, feel free to contribute more awesome
pax tricks!

## Introduction

The POSIX archiver, `pax`, is an attempt at a standardized archiver with
the best features of `tar` and `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.

## Overview

### Operation modes

There are four basic operation modes to *list*, *read*, *write* and
*copy* archives. They're switched with combinations of `-r` and `-w`
command line options:

|Mode|RW-Options|
|----|----------|
|List|*no RW-options*|
|Read|`-r`|
|Write|`-w`|
|Copy|`-r -w`|

#### List

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

*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

*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
`STDIN`.

The write mode is the only mode where you need to specify the archive
type with `-x <TYPE>`, e.g. `-x ustar`.

#### Copy

*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.

### Archive data

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 `-f` This
file will be used as input or output, depending on the operation
(read/write/list).

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, Berkeley `pax`
creates `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|

Berkeley `pax` supports options `-z` and `-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 `extglob`
-   if the specified pattern matches a complete directory, it affects
    all files and subdirectories of the specified directory
-   if you specify the `-c` option, `pax` will invert the matches, i.e.
    it matches all filenames **except** those matching the specified
    patterns
-   if no patterns are given, `pax` will "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[135].txt'
    # should be equivalent to
    pax -r <myarchive.tar 'data/sales/year_2001.txt' 'data/sales/year_2003.txt' 'data/sales/year_2005.txt'

## Using pax

This is a brief description of using `pax` as a normal archiver system,
like you would use `tar`.

### 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
archive:

-   `README.txt` is a normal file, it will be packed
-   `*.png` is a pathname glob **for your shell**, the shell will
    substitute all matching filenames **before** `pax` is executed. The
    result is a list of filenames that will be packed like the
    `README.txt` example above
-   `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 `-v` option, `pax` will write the pathnames of the
files inserted into the archive to `STDERR`.

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` with `pax`:

    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.

**Unfortunately** the `pax` utility delivered with Debian doesn't seem
to support these extended listing formats.

However, `pax` lists archive members in a `ls -l`-like format, when you
give the `-v` option:

    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'

### Copying files

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 data

These commands create a copy of localdir in distandir (distantdir/dir)
on the remote machine.

## Advanced usage

### Backup your daily work

<u>**Note:**</u> `-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
modified today:

    mkdir /n/mybackups/$(date +%A)/
    pax -rw -T 0000 data/ /n/mybackups/$(date +%A)/

This is done using the `-T` switch, 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.

<u>**Note:**</u> The `%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 `.tar` but 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 `holiday_pics/`:

    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 `#/old/path#/new/path#`.

The optional `g` and `p` flags are used to apply substitution
**(g)**lobally to the line or to **(p)**rint the original and rewritten
strings to `STDERR`.

Multiple `-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/.*,,' myapplication

You 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

Like `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

The `-d` option tells pax `not` to recurse into directories it reads
(`cpio`-style). Without `-d`, pax recurses into all directories
(`tar`-style).

**Note**: the `-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 `~/.bashrc` can be useful :-D.

Here is a quick table comparing (GNU) `tar` and `pax` to help you to
make the switch:

|TAR|PAX|Notes|
|---|---|-----|
|`tar xzvf file.tar.gz`|`pax -rvz -f file.tar.gz`|`-z` is an extension, POSIXly: `gunzip <file.tar.gz | pax -rv`|
|`tar czvf archive.tar.gz path ...`|`pax -wvz -f archive.tar.gz path ...`|`-z` is an extension, POSIXly: `pax -wv path | gzip > archive.tar.gz`|
|`tar xjvf file.tar.bz2`|`bunzip2 <file.tar.bz2 | pax -rv`|
|`tar cjvf archive.tar.bz2 path ...`|`pax -wv path | bzip2 > archive.tar.bz2`|
|`tar tzvf file.tar.gz`|`pax -vz -f file.tar.gz`|`-z` is an extension, POSIXly: `gunzip <file.tar.gz | pax -v`|

`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!

## Implementations

-   [AT&T AST toolkit](http://www2.research.att.com/sw/download/) \|
    [manpage](http://www2.research.att.com/~gsf/man/man1/pax.html)
-   [Heirloom toolchest](http://heirloom.sourceforge.net/index.html) \|
    [manpage](http://heirloom.sourceforge.net/man/pax.1.html)
-   [OpenBSD pax](http://www.openbsd.org/cgi-bin/cvsweb/src/bin/pax/) \|
    [manpage](http://www.openbsd.org/cgi-bin/man.cgi?query=pax&apropos=0&sektion=0&manpath=OpenBSD+Current&arch=i386&format=html)
-   [MirBSD pax](https://launchpad.net/paxmirabilis) \|
    [manpage](https://www.mirbsd.org/htman/i386/man1/pax.htm) - Debian
    bases their package upon this.
-   [SUS pax
    specification](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html)