.\" Copyright (c) 2003-2007 Tim Kientzle
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" $FreeBSD$
.\"
.Dd Oct 12, 2009
.Dt BSDTAR 1
.Os
.Sh NAME
.Nm tar
.Nd manipulate tape archives
.Sh SYNOPSIS
.Nm
.Op Ar bundled-flags Ao args Ac
.Op Ao Ar file Ac | Ao Ar pattern Ac ...
.Nm
.Brq Fl c
.Op Ar options
.Op Ar files | Ar directories
.Nm
.Brq Fl r | Fl u
.Fl f Ar archive-file
.Op Ar options
.Op Ar files | Ar directories
.Nm
.Brq Fl t | Fl x
.Op Ar options
.Op Ar patterns
.Sh DESCRIPTION
.Nm
creates and manipulates streaming archive files.
This implementation can extract from tar, pax, cpio, zip, jar, ar,
xar, and ISO 9660 cdrom images and can create tar, pax, cpio, ar, zip,
and shar archives.
.Pp
The first synopsis form shows a
.Dq bundled
option word.
This usage is provided for compatibility with historical implementations.
See COMPATIBILITY below for details.
.Pp
The other synopsis forms show the preferred usage.
The first option to
.Nm
is a mode indicator from the following list:
.Bl -tag -compact -width indent
.It Fl c
Create a new archive containing the specified items.
The long option form is
.Fl Fl create .
.It Fl r
Like
.Fl c ,
but new entries are appended to the archive.
Note that this only works on uncompressed archives stored in regular files.
The
.Fl f
option is required.
The long option form is
.Fl Fl append .
.It Fl t
List archive contents to stdout.
The long option form is
.Fl Fl list .
.It Fl u
Like
.Fl r ,
but new entries are added only if they have a modification date
newer than the corresponding entry in the archive.
Note that this only works on uncompressed archives stored in regular files.
The
.Fl f
option is required.
The long form is
.Fl Fl update .
.It Fl x
Extract to disk from the archive.
If a file with the same name appears more than once in the archive,
each copy will be extracted, with later copies overwriting (replacing)
earlier copies.
The long option form is
.Fl Fl extract .
.El
.Pp
In
.Fl c ,
.Fl r ,
or
.Fl u
mode, each specified file or directory is added to the
archive in the order specified on the command line.
By default, the contents of each directory are also archived.
.Pp
In extract or list mode, the entire command line
is read and parsed before the archive is opened.
The pathnames or patterns on the command line indicate
which items in the archive should be processed.
Patterns are shell-style globbing patterns as
documented in
.Xr tcsh 1 .
.Sh OPTIONS
Unless specifically stated otherwise, options are applicable in
all operating modes.
.Bl -tag -width indent
.It Cm @ Ns Pa archive
(c and r mode only)
The specified archive is opened and the entries
in it will be appended to the current archive.
As a simple example,
.Dl Nm Fl c Fl f Pa - Pa newfile Cm @ Ns Pa original.tar
writes a new archive to standard output containing a file
.Pa newfile
and all of the entries from
.Pa original.tar .
In contrast,
.Dl Nm Fl c Fl f Pa - Pa newfile Pa original.tar
creates a new archive with only two entries.
Similarly,
.Dl Nm Fl czf Pa - Fl Fl format Cm pax Cm @ Ns Pa -
reads an archive from standard input (whose format will be determined
automatically) and converts it into a gzip-compressed
pax-format archive on stdout.
In this way,
.Nm
can be used to convert archives from one format to another.
.It Fl B , Fl Fl read-full-blocks
Ignored for compatibility with other
.Xr tar 1
implementations.
.It Fl b Ar blocksize , Fl Fl block-size Ar blocksize
Specify the block size, in 512-byte records, for tape drive I/O.
As a rule, this argument is only needed when reading from or writing
to tape drives, and usually not even then as the default block size of
20 records (10240 bytes) is very common.
.It Fl C Ar directory
In c and r mode, this changes the directory before adding
the following files.
In x mode, change directories after opening the archive
but before extracting entries from the archive.
.It Fl Fl chroot
(x mode only)
.Fn chroot
to the current directory after processing any
.Fl C
options and before extracting any files.
.It Fl Fl disable-copyfile
Mac OS X specific.
Disable the use of
.Xr copyfile 3 .
.It Fl Fl exclude Ar pattern
Do not process files or directories that match the
specified pattern.
Note that exclusions take precedence over patterns or filenames
specified on the command line.
.It Fl Fl format Ar format
(c, r, u mode only)
Use the specified format for the created archive.
Supported formats include
.Dq cpio ,
.Dq pax ,
.Dq shar ,
and
.Dq ustar .
Other formats may also be supported; see
.Xr libarchive-formats 5
for more information about currently-supported formats.
In r and u modes, when extending an existing archive, the format specified
here must be compatible with the format of the existing archive on disk.
.It Fl f Ar file , Fl Fl file Ar file
Read the archive from or write the archive to the specified file.
The filename can be
.Pa -
for standard input or standard output.
The default varies by system;
on
.Fx ,
the default is
.Pa /dev/sa0 ;
on Linux, the default is
.Pa /dev/st0 .
.It Fl Fl gid Ar id
Use the provided group id number.
On extract, this overrides the group id in the archive;
the group name in the archive will be ignored.
On create, this overrides the group id read from disk;
if
.Fl Fl gname
is not also specified, the group name will be set to
match the group id.
.It Fl Fl gname Ar name
Use the provided group name.
On extract, this overrides the group name in the archive;
if the provided group name does not exist on the system,
the group id
(from the archive or from the
.Fl Fl gid
option)
will be used instead.
On create, this sets the group name that will be stored
in the archive;
the name will not be verified against the system group database.
.It Fl H
(c and r mode only)
Symbolic links named on the command line will be followed; the
target of the link will be archived, not the link itself.
.It Fl h
(c and r mode only)
Synonym for
.Fl L .
.It Fl I
Synonym for
.Fl T .
.It Fl Fl help
Show usage.
.It Fl Fl include Ar pattern
Process only files or directories that match the specified pattern.
Note that exclusions specified with
.Fl Fl exclude
take precedence over inclusions.
If no inclusions are explicitly specified, all entries are processed by
default.
The
.Fl Fl include
option is especially useful when filtering archives.
For example, the command
.Dl Nm Fl c Fl f Pa new.tar Fl Fl include='*foo*' Cm @ Ns Pa old.tgz
creates a new archive
.Pa new.tar
containing only the entries from
.Pa old.tgz
containing the string
.Sq foo .
.It Fl J , Fl Fl xz
(c mode only)
Compress the resulting archive with
.Xr xz 1 .
In extract or list modes, this option is ignored.
Note that, unlike other
.Nm tar
implementations, this implementation recognizes XZ compression
automatically when reading archives.
.It Fl j , Fl Fl bzip , Fl Fl bzip2 , Fl Fl bunzip2
(c mode only)
Compress the resulting archive with
.Xr bzip2 1 .
In extract or list modes, this option is ignored.
Note that, unlike other
.Nm tar
implementations, this implementation recognizes bzip2 compression
automatically when reading archives.
.It Fl k , Fl Fl keep-old-files
(x mode only)
Do not overwrite existing files.
In particular, if a file appears more than once in an archive,
later copies will not overwrite earlier copies.
.It Fl Fl keep-newer-files
(x mode only)
Do not overwrite existing files that are newer than the
versions appearing in the archive being extracted.
.It Fl L , Fl Fl dereference
(c and r mode only)
All symbolic links will be followed.
Normally, symbolic links are archived as such.
With this option, the target of the link will be archived instead.
.It Fl l , Fl Fl check-links
(c and r modes only)
Issue a warning message unless all links to each file are archived.
.It Fl Fl lzma
(c mode only) Compress the resulting archive with the original LZMA algorithm.
Use of this option is discouraged and new archives should be created with
.Fl Fl xz
instead.
Note that, unlike other
.Nm tar
implementations, this implementation recognizes LZMA compression
automatically when reading archives.
.It Fl m , Fl Fl modification-time
(x mode only)
Do not extract modification time.
By default, the modification time is set to the time stored in the archive.
.It Fl n , Fl Fl norecurse , Fl Fl no-recursion
(c, r, u modes only)
Do not recursively archive the contents of directories.
.It Fl Fl newer Ar date
(c, r, u modes only)
Only include files and directories newer than the specified date.
This compares ctime entries.
.It Fl Fl newer-mtime Ar date
(c, r, u modes only)
Like
.Fl Fl newer ,
except it compares mtime entries instead of ctime entries.
.It Fl Fl newer-than Pa file
(c, r, u modes only)
Only include files and directories newer than the specified file.
This compares ctime entries.
.It Fl Fl newer-mtime-than Pa file
(c, r, u modes only)
Like
.Fl Fl newer-than ,
except it compares mtime entries instead of ctime entries.
.It Fl Fl nodump
(c and r modes only)
Honor the nodump file flag by skipping this file.
.It Fl Fl null
(use with
.Fl I
or
.Fl T )
Filenames or patterns are separated by null characters,
not by newlines.
This is often used to read filenames output by the
.Fl print0
option to
.Xr find 1 .
.It Fl Fl no-same-owner
(x mode only)
Do not extract owner and group IDs.
This is the reverse of
.Fl Fl same-owner
and the default behavior if
.Nm
is run as non-root.
.It Fl Fl no-same-permissions
(x mode only)
Do not extract full permissions (SGID, SUID, sticky bit, ACLs,
extended attributes or extended file flags).
This is the reverse of
.Fl p
and the default behavior if
.Nm
is run as non-root.
.It Fl Fl numeric-owner
This is equivalent to
.Fl Fl uname
.Qq
.Fl Fl gname
.Qq .
On extract, it causes user and group names in the archive
to be ignored in favor of the numeric user and group ids.
On create, it causes user and group names to not be stored
in the archive.
.It Fl O , Fl Fl to-stdout
(x, t modes only)
In extract (-x) mode, files will be written to standard out rather than
being extracted to disk.
In list (-t) mode, the file listing will be written to stderr rather than
the usual stdout.
.It Fl o
(x mode)
Use the user and group of the user running the program rather
than those specified in the archive.
Note that this has no significance unless
.Fl p
is specified, and the program is being run by the root user.
In this case, the file modes and flags from
the archive will be restored, but ACLs or owner information in
the archive will be discarded.
.It Fl o
(c, r, u mode)
A synonym for
.Fl Fl format Ar ustar
.It Fl Fl one-file-system
(c, r, and u modes)
Do not cross mount points.
.It Fl Fl options Ar options
Select optional behaviors for particular modules.
The argument is a text string containing comma-separated
keywords and values.
These are passed to the modules that handle particular
formats to control how those formats will behave.
Each option has one of the following forms:
.Bl -tag -compact -width indent
.It Ar key=value
The key will be set to the specified value in every module that supports it.
Modules that do not support this key will ignore it.
.It Ar key
The key will be enabled in every module that supports it.
This is equivalent to
.Ar key Ns Cm =1 .
.It Ar !key
The key will be disabled in every module that supports it.
.It Ar module:key=value , Ar module:key , Ar module:!key
As above, but the corresponding key and value will be provided
only to modules whose name matches
.Ar module .
.El
The currently supported modules and keys are:
.Bl -tag -compact -width indent
.It Cm iso9660:joliet
Support Joliet extensions.
This is enabled by default, use
.Cm !joliet
or
.Cm iso9660:!joliet
to disable.
.It Cm iso9660:rockridge
Support Rock Ridge extensions.
This is enabled by default, use
.Cm !rockridge
or
.Cm iso9660:!rockridge
to disable.
.It Cm gzip:compression-level
A decimal integer from 0 to 9 specifying the gzip compression level.
.It Cm xz:compression-level
A decimal integer from 0 to 9 specifying the xz compression level.
.It Cm mtree: Ns Ar keyword
The mtree writer module allows you to specify which mtree keywords
will be included in the output.
Supported keywords include:
.Cm cksum , Cm device , Cm flags , Cm gid , Cm gname , Cm indent ,
.Cm link , Cm md5 , Cm mode , Cm nlink , Cm rmd160 , Cm sha1 , Cm sha256 ,
.Cm sha384 , Cm sha512 , Cm size , Cm time , Cm uid , Cm uname .
The default is equivalent to:
.Dq device, flags, gid, gname, link, mode, nlink, size, time, type, uid, uname .
.It Cm mtree:all
Enables all of the above keywords.
You can also use
.Cm mtree:!all
to disable all keywords.
.It Cm mtree:use-set
Enable generation of
.Cm /set
lines in the output.
.It Cm mtree:indent
Produce human-readable output by indenting options and splitting lines
to fit into 80 columns.
.It Cm zip:compression Ns = Ns Ar type
Use
.Ar type
as compression method.
Supported values are store (uncompressed) and deflate (gzip algorithm).
.El
If a provided option is not supported by any module, that
is a fatal error.
.It Fl P , Fl Fl absolute-paths
Preserve pathnames.
By default, absolute pathnames (those that begin with a /
character) have the leading slash removed both when creating archives
and extracting from them.
Also,
.Nm
will refuse to extract archive entries whose pathnames contain
.Pa ..
or whose target directory would be altered by a symlink.
This option suppresses these behaviors.
.It Fl p , Fl Fl insecure , Fl Fl preserve-permissions
(x mode only)
Preserve file permissions.
Attempt to restore the full permissions, including owner, file modes, file
flags and ACLs, if available, for each item extracted from the archive.
This is the default, if
.Nm
is being run by root and can be overriden by also specifying
.Fl Fl no-same-owner
and
.Fl Fl no-same-permissions .
.It Fl Fl posix
(c, r, u mode only)
Synonym for
.Fl Fl format Ar pax
.It Fl q , Fl Fl fast-read
(x and t mode only)
Extract or list only the first archive entry that matches each pattern
or filename operand.
Exit as soon as each specified pattern or filename has been matched.
By default, the archive is always read to the very end, since
there can be multiple entries with the same name and, by convention,
later entries overwrite earlier entries.
This option is provided as a performance optimization.
.It Fl S
(x mode only)
Extract files as sparse files.
For every block on disk, check first if it contains only NULL bytes and seek
over it otherwise.
This works similiar to the conv=sparse option of dd.
.It Fl Fl same-owner
(x mode only)
Extract owner and group IDs.
This is the reverse of
.Fl Fl no-same-owner
and the default behavior if
.Nm
is run as root.
.It Fl Fl strip-components Ar count
(x mode only)
Remove the specified number of leading path elements.
Pathnames with fewer elements will be silently skipped.
Note that the pathname is edited after checking inclusion/exclusion patterns
but before security checks.
.It Fl s Ar pattern
Modify file or archive member names according to
.Pa pattern .
The pattern has the format
.Ar /old/new/ Ns Op gps
where
.Ar old
is a basic regular expression,
.Ar new
is the replacement string of the matched part,
and the optional trailing letters modify
how the replacement is handled.
If
.Ar old
is not matched, the pattern is skipped.
Within
.Ar new ,
~ is substituted with the match, \e1 to \e9 with the content of
the corresponding captured group.
The optional trailing g specifies that matching should continue
after the matched part and stopped on the first unmatched pattern.
The optional trailing s specifies that the pattern applies to the value
of symbolic links.
The optional trailing p specifies that after a successful substitution
the original path name and the new path name should be printed to
standard error.
.It Fl T Ar filename , Fl Fl files-from Ar filename
In x or t mode,
.Nm
will read the list of names to be extracted from
.Pa filename .
In c mode,
.Nm
will read names to be archived from
.Pa filename .
The special name
.Dq -C
on a line by itself will cause the current directory to be changed to
the directory specified on the following line.
Names are terminated by newlines unless
.Fl Fl null
is specified.
Note that
.Fl Fl null
also disables the special handling of lines containing
.Dq -C .
.It Fl Fl totals
(c, r, u mode only)
After archiving all files, print a summary to stderr.
.It Fl U , Fl Fl unlink , Fl Fl unlink-first
(x mode only)
Unlink files before creating them.
This can be a minor performance optimization if most files
already exist, but can make things slower if most files
do not already exist.
This flag also causes
.Nm
to remove intervening directory symlinks instead of
reporting an error.
See the SECURITY section below for more details.
.It Fl Fl uid Ar id
Use the provided user id number and ignore the user
name from the archive.
On create, if
.Fl Fl uname
is not also specified, the user name will be set to
match the user id.
.It Fl Fl uname Ar name
Use the provided user name.
On extract, this overrides the user name in the archive;
if the provided user name does not exist on the system,
it will be ignored and the user id
(from the archive or from the
.Fl Fl uid
option)
will be used instead.
On create, this sets the user name that will be stored
in the archive;
the name is not verified against the system user database.
.It Fl Fl use-compress-program Ar program
Pipe the input (in x or t mode) or the output (in c mode) through
.Pa program
instead of using the builtin compression support.
.It Fl v , Fl Fl verbose
Produce verbose output.
In create and extract modes,
.Nm
will list each file name as it is read from or written to
the archive.
In list mode,
.Nm
will produce output similar to that of
.Xr ls 1 .
Additional
.Fl v
options will provide additional detail.
.It Fl Fl version
Print version of
.Nm
and
.Nm libarchive ,
and exit.
.It Fl w , Fl Fl confirmation , Fl Fl interactive
Ask for confirmation for every action.
.It Fl X Ar filename , Fl Fl exclude-from Ar filename
Read a list of exclusion patterns from the specified file.
See
.Fl Fl exclude
for more information about the handling of exclusions.
.It Fl y
(c mode only)
Compress the resulting archive with
.Xr bzip2 1 .
In extract or list modes, this option is ignored.
Note that, unlike other
.Nm tar
implementations, this implementation recognizes bzip2 compression
automatically when reading archives.
.It Fl Z , Fl Fl compress , Fl Fl uncompress
(c mode only)
Compress the resulting archive with
.Xr compress 1 .
In extract or list modes, this option is ignored.
Note that, unlike other
.Nm tar
implementations, this implementation recognizes compress compression
automatically when reading archives.
.It Fl z , Fl Fl gunzip , Fl Fl gzip
(c mode only)
Compress the resulting archive with
.Xr gzip 1 .
In extract or list modes, this option is ignored.
Note that, unlike other
.Nm tar
implementations, this implementation recognizes gzip compression
automatically when reading archives.
.El
.Sh ENVIRONMENT
The following environment variables affect the execution of
.Nm :
.Bl -tag -width ".Ev BLOCKSIZE"
.It Ev LANG
The locale to use.
See
.Xr environ 7
for more information.
.It Ev TAPE
The default device.
The
.Fl f
option overrides this.
Please see the description of the
.Fl f
option above for more details.
.It Ev TZ
The timezone to use when displaying dates.
See
.Xr environ 7
for more information.
.El
.Sh EXIT STATUS
.Ex -std
.Sh EXAMPLES
The following creates a new archive
called
.Ar file.tar.gz
that contains two files
.Ar source.c
and
.Ar source.h :
.Dl Nm Fl czf Pa file.tar.gz Pa source.c Pa source.h
.Pp
To view a detailed table of contents for this
archive:
.Dl Nm Fl tvf Pa file.tar.gz
.Pp
To extract all entries from the archive on
the default tape drive:
.Dl Nm Fl x
.Pp
To examine the contents of an ISO 9660 cdrom image:
.Dl Nm Fl tf Pa image.iso
.Pp
To move file hierarchies, invoke
.Nm
as
.Dl Nm Fl cf Pa - Fl C Pa srcdir\ . | Nm Fl xpf Pa - Fl C Pa destdir
or more traditionally
.Dl cd srcdir \&; Nm Fl cf Pa -\ . | ( cd destdir \&; Nm Fl xpf Pa - )
.Pp
In create mode, the list of files and directories to be archived
can also include directory change instructions of the form
.Cm -C Ns Pa foo/baz
and archive inclusions of the form
.Cm @ Ns Pa archive-file .
For example, the command line
.Dl Nm Fl c Fl f Pa new.tar Pa foo1 Cm @ Ns Pa old.tgz Cm -C Ns Pa /tmp Pa foo2
will create a new archive
.Pa new.tar .
.Nm
will read the file
.Pa foo1
from the current directory and add it to the output archive.
It will then read each entry from
.Pa old.tgz
and add those entries to the output archive.
Finally, it will switch to the
.Pa /tmp
directory and add
.Pa foo2
to the output archive.
.Pp
An input file in
.Xr mtree 5
format can be used to create an output archive with arbitrary ownership,
permissions, or names that differ from existing data on disk:
.Pp
.Dl $ cat input.mtree
.Dl #mtree
.Dl usr/bin uid=0 gid=0 mode=0755 type=dir
.Dl usr/bin/ls uid=0 gid=0 mode=0755 type=file content=myls
.Dl $ tar -cvf output.tar @input.mtree
.Pp
The
.Fl Fl newer
and
.Fl Fl newer-mtime
switches accept a variety of common date and time specifications, including
.Dq 12 Mar 2005 7:14:29pm ,
.Dq 2005-03-12 19:14 ,
.Dq 5 minutes ago ,
and
.Dq 19:14 PST May 1 .
.Pp
The
.Fl Fl options
argument can be used to control various details of archive generation
or reading.
For example, you can generate mtree output which only contains
.Cm type , Cm time ,
and
.Cm uid
keywords:
.Dl Nm Fl cf Pa file.tar Fl Fl format=mtree Fl Fl options='!all,type,time,uid' Pa dir
or you can set the compression level used by gzip or xz compression:
.Dl Nm Fl czf Pa file.tar Fl Fl options='compression-level=9' .
For more details, see the explanation of the
.Fn archive_read_set_options
and
.Fn archive_write_set_options
API calls that are described in
.Xr archive_read 3
and
.Xr archive_write 3 .
.Sh COMPATIBILITY
The bundled-arguments format is supported for compatibility
with historic implementations.
It consists of an initial word (with no leading - character) in which
each character indicates an option.
Arguments follow as separate words.
The order of the arguments must match the order
of the corresponding characters in the bundled command word.
For example,
.Dl Nm Cm tbf 32 Pa file.tar
specifies three flags
.Cm t ,
.Cm b ,
and
.Cm f .
The
.Cm b
and
.Cm f
flags both require arguments,
so there must be two additional items
on the command line.
The
.Ar 32
is the argument to the
.Cm b
flag, and
.Ar file.tar
is the argument to the
.Cm f
flag.
.Pp
The mode options c, r, t, u, and x and the options
b, f, l, m, o, v, and w comply with SUSv2.
.Pp
For maximum portability, scripts that invoke
.Nm tar
should use the bundled-argument format above, should limit
themselves to the
.Cm c ,
.Cm t ,
and
.Cm x
modes, and the
.Cm b ,
.Cm f ,
.Cm m ,
.Cm v ,
and
.Cm w
options.
.Pp
Additional long options are provided to improve compatibility with other
tar implementations.
.Sh SECURITY
Certain security issues are common to many archiving programs, including
.Nm .
In particular, carefully-crafted archives can request that
.Nm
extract files to locations outside of the target directory.
This can potentially be used to cause unwitting users to overwrite
files they did not intend to overwrite.
If the archive is being extracted by the superuser, any file
on the system can potentially be overwritten.
There are three ways this can happen.
Although
.Nm
has mechanisms to protect against each one,
savvy users should be aware of the implications:
.Bl -bullet -width indent
.It
Archive entries can have absolute pathnames.
By default,
.Nm
removes the leading
.Pa /
character from filenames before restoring them to guard against this problem.
.It
Archive entries can have pathnames that include
.Pa ..
components.
By default,
.Nm
will not extract files containing
.Pa ..
components in their pathname.
.It
Archive entries can exploit symbolic links to restore
files to other directories.
An archive can restore a symbolic link to another directory,
then use that link to restore a file into that directory.
To guard against this,
.Nm
checks each extracted path for symlinks.
If the final path element is a symlink, it will be removed
and replaced with the archive entry.
If
.Fl U
is specified, any intermediate symlink will also be unconditionally removed.
If neither
.Fl U
nor
.Fl P
is specified,
.Nm
will refuse to extract the entry.
.El
To protect yourself, you should be wary of any archives that
come from untrusted sources.
You should examine the contents of an archive with
.Dl Nm Fl tf Pa filename
before extraction.
You should use the
.Fl k
option to ensure that
.Nm
will not overwrite any existing files or the
.Fl U
option to remove any pre-existing files.
You should generally not extract archives while running with super-user
privileges.
Note that the
.Fl P
option to
.Nm
disables the security checks above and allows you to extract
an archive while preserving any absolute pathnames,
.Pa ..
components, or symlinks to other directories.
.Sh SEE ALSO
.Xr bzip2 1 ,
.Xr compress 1 ,
.Xr cpio 1 ,
.Xr gzip 1 ,
.Xr mt 1 ,
.Xr pax 1 ,
.Xr shar 1 ,
.Xr xz 1 ,
.Xr libarchive 3 ,
.Xr libarchive-formats 5 ,
.Xr tar 5
.Sh STANDARDS
There is no current POSIX standard for the tar command; it appeared
in
.St -p1003.1-96
but was dropped from
.St -p1003.1-2001 .
The options supported by this implementation were developed by surveying a
number of existing tar implementations as well as the old POSIX specification
for tar and the current POSIX specification for pax.
.Pp
The ustar and pax interchange file formats are defined by
.St -p1003.1-2001
for the pax command.
.Sh HISTORY
A
.Nm tar
command appeared in Seventh Edition Unix, which was released in January, 1979.
There have been numerous other implementations,
many of which extended the file format.
John Gilmore's
.Nm pdtar
public-domain implementation (circa November, 1987)
was quite influential, and formed the basis of GNU tar.
GNU tar was included as the standard system tar
in
.Fx
beginning with
.Fx 1.0 .
.Pp
This is a complete re-implementation based on the
.Xr libarchive 3
library.
It was first released with
.Fx 5.4
in May, 2005.
.Sh BUGS
This program follows
.St -p1003.1-96
for the definition of the
.Fl l
option.
Note that GNU tar prior to version 1.15 treated
.Fl l
as a synonym for the
.Fl Fl one-file-system
option.
.Pp
The
.Fl C Pa dir
option may differ from historic implementations.
.Pp
All archive output is written in correctly-sized blocks, even
if the output is being compressed.
Whether or not the last output block is padded to a full
block size varies depending on the format and the
output device.
For tar and cpio formats, the last block of output is padded
to a full block size if the output is being
written to standard output or to a character or block device such as
a tape drive.
If the output is being written to a regular file, the last block
will not be padded.
Many compressors, including
.Xr gzip 1
and
.Xr bzip2 1 ,
complain about the null padding when decompressing an archive created by
.Nm ,
although they still extract it correctly.
.Pp
The compression and decompression is implemented internally, so
there may be insignificant differences between the compressed output
generated by
.Dl Nm Fl czf Pa - file
and that generated by
.Dl Nm Fl cf Pa - file | Nm gzip
.Pp
The default should be to read and write archives to the standard I/O paths,
but tradition (and POSIX) dictates otherwise.
.Pp
The
.Cm r
and
.Cm u
modes require that the archive be uncompressed
and located in a regular file on disk.
Other archives can be modified using
.Cm c
mode with the
.Pa @archive-file
extension.
.Pp
To archive a file called
.Pa @foo
or
.Pa -foo
you must specify it as
.Pa ./@foo
or
.Pa ./-foo ,
respectively.
.Pp
In create mode, a leading
.Pa ./
is always removed.
A leading
.Pa /
is stripped unless the
.Fl P
option is specified.
.Pp
There needs to be better support for file selection on both create
and extract.
.Pp
There is not yet any support for multi-volume archives or for archiving
sparse files.
.Pp
Converting between dissimilar archive formats (such as tar and cpio) using the
.Cm @ Ns Pa -
convention can cause hard link information to be lost.
(This is a consequence of the incompatible ways that different archive
formats store hardlink information.)