38fc91ca96
Approved by: re
716 lines
24 KiB
Groff
716 lines
24 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 May 20, 2004
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.Dt TAR 5
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.Os
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.Sh NAME
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.Nm tar
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.Nd format of tape archive files
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.Sh DESCRIPTION
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The
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.Nm
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archive format collects any number of files, directories, and other
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file system objects (symbolic links, device nodes, etc.) into a single
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stream of bytes.
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The format was originally designed to be used with
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tape drives that operate with fixed-size blocks, but is widely used as
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a general packaging mechanism.
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.Ss General Format
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A
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.Nm
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archive consists of a series of 512-byte records.
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Each file system object requires a header record which stores basic metadata
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(pathname, owner, permissions, etc.) and zero or more records containing any
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file data.
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The end of the archive is indicated by two records consisting
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entirely of zero bytes.
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.Pp
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For compatibility with tape drives that use fixed block sizes,
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programs that read or write tar files always read or write a fixed
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number of records with each I/O operation.
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These
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.Dq blocks
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are always a multiple of the record size.
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The most common block size\(emand the maximum supported by historic
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implementations\(emis 10240 bytes or 20 records.
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(Note: the terms
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.Dq block
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and
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.Dq record
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here are not entirely standard; this document follows the
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convention established by John Gilmore in documenting
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.Nm pdtar . )
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.Ss Old-Style Archive Format
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The original tar archive format has been extended many times to
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include additional information that various implementors found
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necessary.
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This section describes the variant implemented by the tar command
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included in
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.At v7 ,
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which is one of the earliest widely-used versions of the tar program.
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.Pp
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The header record for an old-style
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.Nm
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archive consists of the following:
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.Bd -literal -offset indent
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struct header_old_tar {
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char name[100];
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char mode[8];
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char uid[8];
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char gid[8];
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char size[12];
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char mtime[12];
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char checksum[8];
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char linkflag[1];
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char linkname[100];
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char pad[255];
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};
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.Ed
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All unused bytes in the header record are filled with nulls.
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.Bl -tag -width indent
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.It Va name
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Pathname, stored as a null-terminated string.
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Early tar implementations only stored regular files (including
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hardlinks to those files).
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One common early convention used a trailing "/" character to indicate
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a directory name, allowing directory permissions and owner information
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to be archived and restored.
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.It Va mode
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File mode, stored as an octal number in ASCII.
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.It Va uid , Va gid
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User id and group id of owner, as octal numbers in ASCII.
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.It Va size
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Size of file, as octal number in ASCII.
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For regular files only, this indicates the amount of data
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that follows the header.
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In particular, this field was ignored by early tar implementations
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when extracting hardlinks.
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Modern writers should always store a zero length for hardlink entries.
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.It Va mtime
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Modification time of file, as an octal number in ASCII.
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This indicates the number of seconds since the start of the epoch,
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00:00:00 UTC January 1, 1970.
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Note that negative values should be avoided
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here, as they are handled inconsistently.
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.It Va checksum
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Header checksum, stored as an octal number in ASCII.
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To compute the checksum, set the checksum field to all spaces,
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then sum all bytes in the header using unsigned arithmetic.
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This field should be stored as six octal digits followed by a null and a space
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character.
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Note that many early implementations of tar used signed arithmetic
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for the checksum field, which can cause interoperability problems
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when transferring archives between systems.
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Modern robust readers compute the checksum both ways and accept the
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header if either computation matches.
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.It Va linkflag , Va linkname
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In order to preserve hardlinks and conserve tape, a file
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with multiple links is only written to the archive the first
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time it is encountered.
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The next time it is encountered, the
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.Va linkflag
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is set to an ASCII
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.Sq 1
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and the
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.Va linkname
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field holds the first name under which this file appears.
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(Note that regular files have a null value in the
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.Va linkflag
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field.)
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.El
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.Pp
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Early tar implementations varied in how they terminated these fields.
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The tar command in
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.At v7
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used the following conventions (this is also documented in early BSD manpages):
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the pathname must be null-terminated;
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the mode, uid, and gid fields must end in a space and a null byte;
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the size and mtime fields must end in a space;
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the checksum is terminated by a null and a space.
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Early implementations filled the numeric fields with leading spaces.
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This seems to have been common practice until the
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.St -p1003.1
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standard was released.
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For best portability, modern implementations should fill the numeric
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fields with leading zeros.
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.Ss Pre-POSIX Archives
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An early draft of
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.St -p1003.1-88
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served as the basis for John Gilmore's
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.Nm pdtar
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program and many system implementations from the late 1980s
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and early 1990s.
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These archives generally follow the POSIX ustar
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format described below with the following variations:
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.Bl -bullet -compact -width indent
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.It
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The magic value is
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.Dq ustar\ \&
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(note the following space).
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The version field contains a space character followed by a null.
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.It
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The numeric fields are generally filled with leading spaces
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(not leading zeros as recommended in the final standard).
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.It
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The prefix field is often not used, limiting pathnames to
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the 100 characters of old-style archives.
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.El
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.Ss POSIX ustar Archives
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.St -p1003.1-88
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defined a standard tar file format to be read and written
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by compliant implementations of
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.Xr tar 1
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and
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.Xr pax 1 .
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This format is often called the
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.Dq ustar
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format, after the magic value used
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in the header.
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(The name is an acronym for
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.Dq Unix Standard TAR . )
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It extends the historic format with new fields:
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.Bd -literal -offset indent
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struct header_posix_ustar {
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char name[100];
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char mode[8];
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char uid[8];
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char gid[8];
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char size[12];
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char mtime[12];
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char checksum[8];
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char typeflag[1];
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char linkname[100];
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char magic[6];
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char version[2];
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char uname[32];
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char gname[32];
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char devmajor[8];
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char devminor[8];
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char prefix[155];
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char pad[12];
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};
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.Ed
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.Bl -tag -width indent
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.It Va typeflag
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Type of entry.
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POSIX extended the earlier
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.Va linkflag
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field with several new type values:
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.Bl -tag -width indent -compact
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.It Dq 0
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Regular file.
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NULL should be treated as a synonym, for compatibility purposes.
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.It Dq 1
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Hard link.
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.It Dq 2
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Symbolic link.
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.It Dq 3
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Character device node.
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.It Dq 4
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Block device node.
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.It Dq 5
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Directory.
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.It Dq 6
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FIFO node.
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.It Dq 7
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Reserved.
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.It Other
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A POSIX-compliant implementation must treat any unrecognized typeflag value
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as a regular file.
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In particular, writers should ensure that all entries
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have a valid filename so that they can be restored by readers that do not
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support the corresponding extension.
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Uppercase letters "A" through "Z" are reserved for custom extensions.
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Note that sockets and whiteout entries are not archivable.
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.El
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It is worth noting that the
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.Va size
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field, in particular, has different meanings depending on the type.
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For regular files, of course, it indicates the amount of data
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following the header.
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For directories, it may be used to indicate the total size of all
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files in the directory, for use by operating systems that pre-allocate
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directory space.
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For all other types, it should be set to zero by writers and ignored
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by readers.
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.It Va magic
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Contains the magic value
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.Dq ustar
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followed by a NULL byte to indicate that this is a POSIX standard archive.
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Full compliance requires the uname and gname fields be properly set.
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.It Va version
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Version.
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This should be
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.Dq 00
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(two copies of the ASCII digit zero) for POSIX standard archives.
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.It Va uname , Va gname
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User and group names, as null-terminated ASCII strings.
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These should be used in preference to the uid/gid values
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when they are set and the corresponding names exist on
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the system.
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.It Va devmajor , Va devminor
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Major and minor numbers for character device or block device entry.
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.It Va prefix
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First part of pathname.
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If the pathname is too long to fit in the 100 bytes provided by the standard
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format, it can be split at any
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.Pa /
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character with the first portion going here.
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If the prefix field is not empty, the reader will prepend
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the prefix value and a
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.Pa /
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character to the regular name field to obtain the full pathname.
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.El
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.Pp
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Note that all unused bytes must be set to
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.Dv NULL .
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.Pp
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Field termination is specified slightly differently by POSIX
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than by previous implementations.
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The
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.Va magic ,
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.Va uname ,
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and
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.Va gname
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fields must have a trailing
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.Dv NULL .
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The
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.Va pathname ,
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.Va linkname ,
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and
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.Va prefix
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fields must have a trailing
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.Dv NULL
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unless they fill the entire field.
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(In particular, it is possible to store a 256-character pathname if it
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happens to have a
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.Pa /
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as the 156th character.)
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POSIX requires numeric fields to be zero-padded in the front, and allows
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them to be terminated with either space or
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.Dv NULL
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characters.
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.Pp
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Currently, most tar implementations comply with the ustar
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format, occasionally extending it by adding new fields to the
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blank area at the end of the header record.
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.Ss Pax Interchange Format
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There are many attributes that cannot be portably stored in a
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POSIX ustar archive.
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.St -p1003.1-2001
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defined a
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.Dq pax interchange format
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that uses two new types of entries to hold text-formatted
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metadata that applies to following entries.
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Note that a pax interchange format archive is a ustar archive in every
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respect.
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The new data is stored in ustar-compatible archive entries that use the
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.Dq x
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or
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.Dq g
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typeflag.
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In particular, older implementations that do not fully support these
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extensions will extract the metadata into regular files, where the
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metadata can be examined as necessary.
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.Pp
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An entry in a pax interchange format archive consists of one or
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two standard ustar entries, each with its own header and data.
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The first optional entry stores the extended attributes
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for the following entry.
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This optional first entry has an "x" typeflag and a size field that
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indicates the total size of the extended attributes.
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The extended attributes themselves are stored as a series of text-format
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lines encoded in the portable UTF-8 encoding.
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Each line consists of a decimal number, a space, a key string, an equals
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sign, a value string, and a new line.
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The decimal number indicates the length of the entire line, including the
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initial length field and the trailing newline.
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An example of such a field is:
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.Dl 25 ctime=1084839148.1212\en
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Keys in all lowercase are standard keys.
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Vendors can add their own keys by prefixing them with an all uppercase
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vendor name and a period.
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Note that, unlike the historic header, numeric values are stored using
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decimal, not octal.
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A description of some common keys follows:
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.Bl -tag -width indent
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.It Cm atime , Cm ctime , Cm mtime
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File access, inode change, and modification times.
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These fields can be negative or include a decimal point and a fractional value.
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.It Cm uname , Cm uid , Cm gname , Cm gid
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User name, group name, and numeric UID and GID values.
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The user name and group name stored here are encoded in UTF8
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and can thus include non-ASCII characters.
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The UID and GID fields can be of arbitrary length.
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.It Cm linkpath
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The full path of the linked-to file.
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Note that this is encoded in UTF8 and can thus include non-ASCII characters.
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.It Cm path
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The full pathname of the entry.
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Note that this is encoded in UTF8 and can thus include non-ASCII characters.
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.It Cm realtime.* , Cm security.*
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These keys are reserved and may be used for future standardization.
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.It Cm size
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The size of the file.
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Note that there is no length limit on this field, allowing conforming
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archives to store files much larger than the historic 8GB limit.
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.It Cm SCHILY.*
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Vendor-specific attributes used by Joerg Schilling's
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.Nm star
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implementation.
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.It Cm SCHILY.acl.access , Cm SCHILY.acl.default
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Stores the access and default ACLs as textual strings in a format
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that is an extension of the format specified by POSIX.1e draft 17.
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In particular, each user or group access specification can include a fourth
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colon-separated field with the numeric UID or GID.
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This allows ACLs to be restored on systems that may not have complete
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user or group information available (such as when NIS/YP or LDAP services
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are temporarily unavailable).
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.It Cm SCHILY.devminor , Cm SCHILY.devmajor
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The full minor and major numbers for device nodes.
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.It Cm SCHILY.dev, Cm SCHILY.ino , Cm SCHILY.nlinks
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The device number, inode number, and link count for the entry.
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In particular, note that a pax interchange format archive using Joerg
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Schilling's
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.Cm SCHILY.*
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extensions can store all of the data from
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.Va struct stat .
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.It Cm VENDOR.*
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XXX document other vendor-specific extensions XXX
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.El
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.Pp
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Any values stored in an extended attribute override the corresponding
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values in the regular tar header.
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Note that compliant readers should ignore the regular fields when they
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are overridden.
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This is important, as existing archivers are known to store non-compliant
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values in the standard header fields in this situation.
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There are no limits on length for any of these fields.
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In particular, numeric fields can be arbitrarily large.
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All text fields are encoded in UTF8.
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Compliant writers should store only portable 7-bit ASCII characters in
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the standard ustar header and use extended
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attributes whenever a text value contains non-ASCII characters.
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.Pp
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In addition to the
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.Cm x
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entry described above, the pax interchange format
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also supports a
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.Cm g
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entry.
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The
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.Cm g
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entry is identical in format, but specifies attributes that serve as
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defaults for all subsequent archive entries.
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The
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.Cm g
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entry is not widely used.
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.Pp
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Besides the new
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.Cm x
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and
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.Cm g
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entries, the pax interchange format has a few other minor variations
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from the earlier ustar format.
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The most troubling one is that hardlinks are permitted to have
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data following them.
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This allows readers to restore any hardlink to a file without
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having to rewind the archive to find an earlier entry.
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However, it creates complications for robust readers, as it is no longer
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clear whether or not they should ignore the size field for hardlink entries.
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.Ss GNU Tar Archives
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The GNU tar program started with a pre-POSIX format similar to that
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described earlier and has extended it using several different mechanisms:
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It added new fields to the empty space in the header (some of which was later
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used by POSIX for conflicting purposes);
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it allowed the header to be continued over multiple records;
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and it defined new entries that modify following entries
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(similar in principle to the
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.Cm x
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entry described above, but each GNU special entry is single-purpose,
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unlike the general-purpose
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.Cm x
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entry).
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As a result, GNU tar archives are not POSIX compatible, although
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more lenient POSIX-compliant readers can successfully extract most
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GNU tar archives.
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.Bd -literal -offset indent
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struct header_gnu_tar {
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char name[100];
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char mode[8];
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char uid[8];
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char gid[8];
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char size[12];
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char mtime[12];
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char checksum[8];
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char typeflag[1];
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char linkname[100];
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char magic[6];
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char version[2];
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char uname[32];
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char gname[32];
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char devmajor[8];
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char devminor[8];
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char atime[12];
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char ctime[12];
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char offset[12];
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char longnames[4];
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char unused[1];
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struct {
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char offset[12];
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char numbytes[12];
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} sparse[4];
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char isextended[1];
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char realsize[12];
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char pad[17];
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};
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.Ed
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.Bl -tag -width indent
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.It Va typeflag
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GNU tar uses the following special entry types, in addition to
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those defined by POSIX:
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.Bl -tag -width indent
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|
.It "7"
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GNU tar treats type "7" records identically to type "0" records,
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except on one obscure RTOS where they are used to indicate the
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pre-allocation of a contiguous file on disk.
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.It "D"
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This indicates a directory entry.
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|
Unlike the POSIX-standard "5"
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typeflag, the header is followed by data records listing the names
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of files in this directory.
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Each name is preceded by an ASCII "Y"
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if the file is stored in this archive or "N" if the file is not
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stored in this archive.
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Each name is terminated with a null, and
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an extra null marks the end of the name list.
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|
The purpose of this
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entry is to support incremental backups; a program restoring from
|
|
such an archive may wish to delete files on disk that did not exist
|
|
in the directory when the archive was made.
|
|
.Pp
|
|
Note that the "D" typeflag specifically violates POSIX, which requires
|
|
that unrecognized typeflags be restored as normal files.
|
|
In this case, restoring the "D" entry as a file could interfere
|
|
with subsequent creation of the like-named directory.
|
|
.It "K"
|
|
The data for this entry is a long linkname for the following regular entry.
|
|
.It "L"
|
|
The data for this entry is a long pathname for the following regular entry.
|
|
.It "M"
|
|
This is a continuation of the last file on the previous volume.
|
|
GNU multi-volume archives guarantee that each volume begins with a valid
|
|
entry header.
|
|
To ensure this, a file may be split, with part stored at the end of one volume,
|
|
and part stored at the beginning of the next volume.
|
|
The "M" typeflag indicates that this entry continues an existing file.
|
|
Such entries can only occur as the first or second entry
|
|
in an archive (the latter only if the first entry is a volume label).
|
|
The
|
|
.Va size
|
|
field specifies the size of this entry.
|
|
The
|
|
.Va offset
|
|
field at bytes 369-380 specifies the offset where this file fragment
|
|
begins.
|
|
The
|
|
.Va realsize
|
|
field specifies the total size of the file (which must equal
|
|
.Va size
|
|
plus
|
|
.Va offset ) .
|
|
When extracting, GNU tar checks that the header file name is the one it is
|
|
expecting, that the header offset is in the correct sequence, and that
|
|
the sum of offset and size is equal to realsize.
|
|
FreeBSD's version of GNU tar does not handle the corner case of an
|
|
archive's being continued in the middle of a long name or other
|
|
extension header.
|
|
.It "N"
|
|
Type "N" records are no longer generated by GNU tar.
|
|
They contained a
|
|
list of files to be renamed or symlinked after extraction; this was
|
|
originally used to support long names.
|
|
The contents of this record
|
|
are a text description of the operations to be done, in the form
|
|
.Dq Rename %s to %s\en
|
|
or
|
|
.Dq Symlink %s to %s\en ;
|
|
in either case, both
|
|
filenames are escaped using K&R C syntax.
|
|
.It "S"
|
|
This is a
|
|
.Dq sparse
|
|
regular file.
|
|
Sparse files are stored as a series of fragments.
|
|
The header contains a list of fragment offset/length pairs.
|
|
If more than four such entries are required, the header is
|
|
extended as necessary with
|
|
.Dq extra
|
|
header extensions (an older format that is no longer used), or
|
|
.Dq sparse
|
|
extensions.
|
|
.It "V"
|
|
The
|
|
.Va name
|
|
field should be interpreted as a tape/volume header name.
|
|
This entry should generally be ignored on extraction.
|
|
.El
|
|
.It Va magic
|
|
The magic field holds the five characters
|
|
.Dq ustar
|
|
followed by a space.
|
|
Note that POSIX ustar archives have a trailing null.
|
|
.It Va version
|
|
The version field holds a space character followed by a null.
|
|
Note that POSIX ustar archives use two copies of the ASCII digit
|
|
.Dq 0 .
|
|
.It Va atime , Va ctime
|
|
The time the file was last accessed and the time of
|
|
last change of file information, stored in octal as with
|
|
.Va mtime.
|
|
.It Va longnames
|
|
This field is apparently no longer used.
|
|
.It Sparse Va offset / Va numbytes
|
|
Each such structure specifies a single fragment of a sparse
|
|
file.
|
|
The two fields store values as octal numbers.
|
|
The fragments are each padded to a multiple of 512 bytes
|
|
in the archive.
|
|
On extraction, the list of fragments is collected from the
|
|
header (including any extension headers), and the data
|
|
is then read and written to the file at appropriate offsets.
|
|
.It Va isextended
|
|
If this is set to non-zero, the header will be followed by additional
|
|
.Dq sparse header
|
|
records.
|
|
Each such record contains information about as many as 21 additional
|
|
sparse blocks as shown here:
|
|
.Bd -literal -offset indent
|
|
struct gnu_sparse_header {
|
|
struct {
|
|
char offset[12];
|
|
char numbytes[12];
|
|
} sparse[21];
|
|
char isextended[1];
|
|
char padding[7];
|
|
};
|
|
.Ed
|
|
.It Va realsize
|
|
A binary representation of the file's complete size, with a much larger range
|
|
than the POSIX file size.
|
|
In particular, with
|
|
.Cm M
|
|
type files, the current entry is only a portion of the file.
|
|
In that case, the POSIX size field will indicate the size of this
|
|
entry; the
|
|
.Va realsize
|
|
field will indicate the total size of the file.
|
|
.El
|
|
.Ss Solaris Tar
|
|
XXX More Details Needed XXX
|
|
.Pp
|
|
Solaris tar (beginning with SunOS XXX 5.7 ?? XXX) supports an
|
|
.Dq extended
|
|
format that is fundamentally similar to pax interchange format,
|
|
with the following differences:
|
|
.Bl -bullet -compact -width indent
|
|
.It
|
|
Extended attributes are stored in an entry whose type is
|
|
.Cm X ,
|
|
not
|
|
.Cm x ,
|
|
as used by pax interchange format.
|
|
The detailed format of this entry appears to be the same
|
|
as detailed above for the
|
|
.Cm x
|
|
entry.
|
|
.It
|
|
An additional
|
|
.Cm A
|
|
entry is used to store an ACL for the following regular entry.
|
|
The body of this entry contains a seven-digit octal number
|
|
(whose value is 01000000 plus the number of ACL entries)
|
|
followed by a zero byte, followed by the
|
|
textual ACL description.
|
|
.El
|
|
.Ss Other Extensions
|
|
One common extension, utilized by GNU tar, star, and other newer
|
|
.Nm
|
|
implementations, permits binary numbers in the standard numeric
|
|
fields.
|
|
This is flagged by setting the high bit of the first character.
|
|
This permits 95-bit values for the length and time fields
|
|
and 63-bit values for the uid, gid, and device numbers.
|
|
GNU tar supports this extension for the
|
|
length, mtime, ctime, and atime fields.
|
|
Joerg Schilling's star program supports this extension for
|
|
all numeric fields.
|
|
Note that this extension is largely obsoleted by the extended attribute
|
|
record provided by the pax interchange format.
|
|
.Pp
|
|
Another early GNU extension allowed base-64 values rather
|
|
than octal.
|
|
This extension was short-lived and such archives are almost never seen.
|
|
However, there is still code in GNU tar to support them; this code is
|
|
responsible for a very cryptic warning message that is sometimes seen when
|
|
GNU tar encounters a damaged archive.
|
|
.Sh SEE ALSO
|
|
.Xr ar 1 ,
|
|
.Xr pax 1 ,
|
|
.Xr tar 1
|
|
.Sh STANDARDS
|
|
The
|
|
.Nm tar
|
|
utility is no longer a part of POSIX or the Single Unix Standard.
|
|
It last appeared in
|
|
.St -susv2 .
|
|
It has been supplanted in subsequent standards by
|
|
.Xr pax 1 .
|
|
The ustar format is currently part of the specification for the
|
|
.Xr pax 1
|
|
utility.
|
|
The pax interchange file format is new with
|
|
.St -p1003.1-2001 .
|
|
.Sh HISTORY
|
|
A
|
|
.Nm tar
|
|
command appeared in Seventh Edition Unix, which was released in January, 1979.
|
|
It replaced the
|
|
.Nm tp
|
|
program from Fourth Edition Unix which in turn replaced the
|
|
.Nm tap
|
|
program from First Edition Unix.
|
|
John Gilmore's
|
|
.Nm pdtar
|
|
public-domain implementation (circa 1987) was highly influential
|
|
and formed the basis of GNU tar.
|
|
Joerg Shilling's
|
|
.Nm star
|
|
archiver is another open-source (GPL) archiver (originally developed
|
|
circa 1985) which features complete support for pax interchange
|
|
format.
|