228 lines
9.1 KiB
Groff
228 lines
9.1 KiB
Groff
.\" Copyright (c) 2003-2004 Tim Kientzle
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd April 27, 2004
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.Dt libarchive-formats 3
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.Os
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.Sh NAME
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.Nm libarchive-formats
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.Nd archive formats supported by the libarchive library
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.Sh DESCRIPTION
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The
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.Xr libarchive 3
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library reads and writes a variety of streaming archive formats.
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Generally speaking, all of these archive formats consist of a series of
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.Dq entries .
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Each entry stores a single filesystem object, such as a file, directory,
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or symbolic link.
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.Pp
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The following provides a brief description of each format supported
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by libarchive, with some information about recognized extensions or
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limitations of the current library support.
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Note that just because a format is supported by libarchive does not
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imply that a program that uses libarchive will support that format.
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Applications that use libarchive specify which formats they wish
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to support.
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.Ss Tar Formats
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The
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.Xr libarchive 3
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library can read most tar archives.
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However, it only writes POSIX-standard
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.Dq ustar
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and
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.Dq pax interchange
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formats.
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.Pp
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All tar formats store each entry in one or more 512-byte records.
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The first record is used for file metadata, including filename,
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timestamp, and mode information, and the file data is stored in
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subsequent records.
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Later variants have extended this by either appropriating undefined
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areas of the header record, extending the header to multiple records,
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or by storing special entries that modify the interpretation of
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subsequent entries.
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.Pp
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.Bl -tag -width indent
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.It Cm gnutar
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The
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.Xr libarchive 3
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library can read GNU-format tar archives.
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It currently supports the most popular GNU extensions, including
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modern long filename and linkname support, as well as atime and ctime data.
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The libarchive library does not support sparse files, multi-volume
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archives, nor the old GNU long filename format.
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.It Cm pax
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The
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.Xr libarchive 3
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library can read and write POSIX-compliant pax interchange format
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archives.
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Pax interchange format archives are an extension of the older ustar
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format that adds a separate entry with additional attributes stored
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as key/value pairs.
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The presence of this additional entry is the only difference between
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pax interchange format and the older ustar format.
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The extended attributes are of unlimited length and are stored
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as UTF-8 Unicode strings.
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Keywords defined in the standard are in all lowercase; vendors are allowed
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to define custom keys by preceding them with the vendor name in all uppercase.
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When writing pax archives, libarchive uses many of the SCHILY keys
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defined by Joerg Schilling's
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.Dq star
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archiver.
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The libarchive library can read most of the SCHILY keys.
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It ignores any keywords that it does not understand.
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.It Cm restricted pax
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The libarchive library can also write pax archives in which it
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attempts to suppress the extended attributes entry whenever
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possible.
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The result will be identical to a ustar archive unless the
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extended attributes entry is required to store a long file
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name, long linkname, extended ACL, file flags, or if any of the standard
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ustar data (user name, group name, UID, GID, etc) cannot be fully
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represented in the ustar header.
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In all cases, the result can be dearchived by any program that
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can read POSIX-compliant pax interchange format archives.
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.It Cm ustar
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The libarchive library can both read and write this format.
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This format has the following limitations:
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.Bl -bullet -compact
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.It
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Device major and minor numbers are limited to 21 bits.
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Nodes with larger numbers will not be added to the archive.
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.It
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Path names in the archive are limited to 255 bytes.
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(Shorter if there is no / character in exactly the right place.)
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.It
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Symbolic links and hard links are stored in the archive with
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the name of the referenced file.
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This name is limited to 100 bytes.
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.It
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Extended attributes, file flags, and other extended
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security information cannot be stored.
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.It
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Archive entries are limited to 2 gigabytes in size.
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.El
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Note that the pax interchange format has none of these restrictions.
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.El
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.Pp
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The libarchive library can also read a variety of commonly-used extensions to
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the basic tar format.
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In particular, it supports base-256 values in certain numeric fields.
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This essentially removes the limitations on file size, modification time,
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and device numbers.
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.Pp
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The first tar program appeared in Sixth Edition Unix (circa 1976).
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This makes the tar format one of the oldest and most widely-supported
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archive formats.
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The first official standard for the tar file format was the
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.Dq ustar
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(Unix Standard Tar) format defined by POSIX in 1988.
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POSIX.1-2001 extended the ustar format to create the
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.Dq pax interchange
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format.
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There have also been many custom variations.
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.Ss Cpio Formats
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The libarchive library can read a number of common cpio variants and can write
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.Dq odc
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format archives.
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A cpio archive stores each entry as a fixed-size header followed
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by a variable-length filename and variable-length data.
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Unlike tar, cpio does only minimal padding of the header or file data.
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There are a variety of cpio formats, which differ primarily in
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how they store the initial header: some store the values as
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octal or hexadecimal numbers in ASCII, others as binary values of
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varying byte order and length.
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.Bl -tag -width indent
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.It Cm binary
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The libarchive library can read both big-endian and little-endian
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variants of the original binary cpio format.
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This format used 32-bit binary values for file size and mtime,
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and 16-bit binary values for the other fields.
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.It Cm odc
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The libarchive library can both read and write this
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POSIX-standard format.
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This format stores the header contents as octal values in ASCII.
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It is standard, portable, and immune from byte-order confusion.
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File sizes and mtime are limited to 33 bits (8GB file size),
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other fields are limited to 18 bits.
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.It Cm SVR4
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The libarchive library can read both CRC and non-CRC variants of
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this format.
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The SVR4 format uses eight-digit hexadecimal values for
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all header fields.
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This limits file size to 4GB, and also limits the mtime and
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other fields to 32 bits.
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The SVR4 format can optionally include a CRC of the file
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contents, although libarchive does not currently verify this CRC.
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.El
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.Pp
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Cpio is an old format that was widely used because of it's simplicity
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and it's support for very long filenames.
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Unfortunately, it has many limitations that make it unsuitable
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for widespread use.
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Only the POSIX format permits files over 4GB, and its 18-bit
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limit for most other fields makes it unsuitable for modern systems.
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In addition, cpio formats only store numeric UID/GID values (not
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usernames and group names), which can make it very difficult to correctly
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transfer archives across systems.
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Finally, there is no good way to extend the format, which means that
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ACLs, file flags, character encoding information, and non-standard file
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types can not be added except by breaking compatibility with existing
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implementations.
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.Ss Shar Formats
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A
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.Dq shell archive
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is a shell script that, when executed on a POSIX-compliant
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system, will recreate a collection of filesystem objects.
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The libarchive library can write two different kinds of shar archives:
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.Bl -tag -width indent
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.It Cm shar
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The traditional shar format uses a limited set of POSIX
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commands, including
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.Xr echo 1 ,
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.Xr mkdir 1 ,
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and
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.Xr sed 1 .
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It is suitable for portably archiving small collections of plain text files.
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However, it is not generally well-suited for large archives
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(many implementations of
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.Xr sh 1
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have limits on the size of a script) nor should it be used with non-text files.
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.It Cm shardump
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This format is similar to shar but encodes files using
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.Xr uuencode 1
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so that the result will be a plain text file regardless of the file contents.
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It also includes additional shell commands that attempt to reproduce as
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many file attributes as possible, including owner, mode, and flags.
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The additional commands used to restore file attributes make
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shardump archives less portable than plain shar archives.
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.El
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.Sh SEE ALSO
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.Xr cpio 1 ,
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.Xr shar 1 ,
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.Xr tar 1 ,
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.Xr tar 5
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