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TAR(5)			    BSD	File Formats Manual			TAR(5)

NAME
     tar -- format of tape archive files

DESCRIPTION
     The tar archive format collects any number	of files, directories, and
     other file	system objects (symbolic links,	device nodes, etc.) into a
     single stream of bytes.  The format was originally	designed to be used
     with tape drives that operate with	fixed-size blocks, but is widely used
     as	a general packaging mechanism.

   General Format
     A tar archive consists of a series	of 512-byte records.  Each file	system
     object requires a header record which stores basic	metadata (pathname,
     owner, permissions, etc.) and zero	or more	records	containing any file
     data.  The	end of the archive is indicated	by two records consisting en-
     tirely of zero bytes.

     For compatibility with tape drives	that use fixed block sizes, programs
     that read or write	tar files always read or write a fixed number of
     records with each I/O operation.  These "blocks" are always a multiple of
     the record	size.  The most	common block size--and the maximum supported
     by	historic implementations--is 10240 bytes or 20 records.	 (Note:	the
     terms "block" and "record"	here are not entirely standard;	this document
     follows the convention established	by John	Gilmore	in documenting pdtar.)

   Old-Style Archive Format
     The original tar archive format has been extended many times to include
     additional	information that various implementors found necessary.	This
     section describes the variant implemented by the tar command included in
     Version 7 AT&T UNIX, which	is one of the earliest widely-used versions of
     the tar program.

     The header	record for an old-style	tar archive consists of	the following:

	   struct header_old_tar {
		   char	name[100];
		   char	mode[8];
		   char	uid[8];
		   char	gid[8];
		   char	size[12];
		   char	mtime[12];
		   char	checksum[8];
		   char	linkflag[1];
		   char	linkname[100];
		   char	pad[255];
	   };
     All unused	bytes in the header record are filled with nulls.

     name    Pathname, stored as a null-terminated string.  Early tar imple-
	     mentations	only stored regular files (including hardlinks to
	     those files).  One	common early convention	used a trailing	"/"
	     character to indicate a directory name, allowing directory	per-
	     missions and owner	information to be archived and restored.

     mode    File mode,	stored as an octal number in ASCII.

     uid, gid
	     User id and group id of owner, as octal numbers in	ASCII.

     size    Size of file, as octal number in ASCII.  For regular files	only,
	     this indicates the	amount of data that follows the	header.	 In
	     particular, this field was	ignored	by early tar implementations
	     when extracting hardlinks.	 Modern	writers	should always store a
	     zero length for hardlink entries.

     mtime   Modification time of file,	as an octal number in ASCII.  This in-
	     dicates the number	of seconds since the start of the epoch,
	     00:00:00 UTC January 1, 1970.  Note that negative values should
	     be	avoided	here, as they are handled inconsistently.

     checksum
	     Header checksum, stored as	an octal number	in ASCII.  To compute
	     the checksum, set the checksum field to all spaces, then sum all
	     bytes in the header using unsigned	arithmetic.  This field	should
	     be	stored as six octal digits followed by a null and a space
	     character.	 Note that many	early implementations of tar used
	     signed arithmetic for the checksum	field, which can cause inter-
	     operability problems when transferring archives between systems.
	     Modern robust readers compute the checksum	both ways and accept
	     the header	if either computation matches.

     linkflag, linkname
	     In	order to preserve hardlinks and	conserve tape, a file with
	     multiple links is only written to the archive the first time it
	     is	encountered.  The next time it is encountered, the linkflag is
	     set to an ASCII `1' and the linkname field	holds the first	name
	     under which this file appears.  (Note that	regular	files have a
	     null value	in the linkflag	field.)

     Early tar implementations varied in how they terminated these fields.
     The tar command in	Version	7 AT&T UNIX used the following conventions
     (this is also documented in early BSD manpages): the pathname must	be
     null-terminated; the mode,	uid, and gid fields must end in	a space	and a
     null byte;	the size and mtime fields must end in a	space; the checksum is
     terminated	by a null and a	space.	Early implementations filled the nu-
     meric fields with leading spaces.	This seems to have been	common prac-
     tice until	the IEEE Std 1003.1-1988 ("POSIX.1") standard was released.
     For best portability, modern implementations should fill the numeric
     fields with leading zeros.

   Pre-POSIX Archives
     An	early draft of IEEE Std	1003.1-1988 ("POSIX.1")	served as the basis
     for John Gilmore's	pdtar program and many system implementations from the
     late 1980s	and early 1990s.  These	archives generally follow the POSIX
     ustar format described below with the following variations:
     +o	     The magic value is	"ustar " (note the following space).  The ver-
	     sion field	contains a space character followed by a null.
     +o	     The numeric fields	are generally filled with leading spaces (not
	     leading zeros as recommended in the final standard).
     +o	     The prefix	field is often not used, limiting pathnames to the 100
	     characters	of old-style archives.

   POSIX ustar Archives
     IEEE Std 1003.1-1988 ("POSIX.1") defined a	standard tar file format to be
     read and written by compliant implementations of tar(1).  This format is
     often called the "ustar" format, after the	magic value used in the
     header.  (The name	is an acronym for "Unix	Standard TAR".)	 It extends
     the historic format with new fields:

	   struct header_posix_ustar {
		   char	name[100];
		   char	mode[8];
		   char	uid[8];
		   char	gid[8];
		   char	size[12];
		   char	mtime[12];
		   char	checksum[8];
		   char	typeflag[1];
		   char	linkname[100];
		   char	magic[6];
		   char	version[2];
		   char	uname[32];
		   char	gname[32];
		   char	devmajor[8];
		   char	devminor[8];
		   char	prefix[155];
		   char	pad[12];
	   };

     typeflag
	     Type of entry.  POSIX extended the	earlier	linkflag field with
	     several new type values:
	     "0"     Regular file.  NULL should	be treated as a	synonym, for
		     compatibility purposes.
	     "1"     Hard link.
	     "2"     Symbolic link.
	     "3"     Character device node.
	     "4"     Block device node.
	     "5"     Directory.
	     "6"     FIFO node.
	     "7"     Reserved.
	     Other   A POSIX-compliant implementation must treat any unrecog-
		     nized typeflag value as a regular file.  In particular,
		     writers should ensure that	all entries have a valid file-
		     name so that they can be restored by readers that do not
		     support the corresponding extension.  Uppercase letters
		     "A" through "Z" are reserved for custom extensions.  Note
		     that sockets and whiteout entries are not archivable.
	     It	is worth noting	that the size field, in	particular, has	dif-
	     ferent meanings depending on the type.  For regular files,	of
	     course, it	indicates the amount of	data following the header.
	     For directories, it may be	used to	indicate the total size	of all
	     files in the directory, for use by	operating systems that pre-al-
	     locate directory space.  For all other types, it should be	set to
	     zero by writers and ignored by readers.

     magic   Contains the magic	value "ustar" followed by a NULL byte to indi-
	     cate that this is a POSIX standard	archive.  Full compliance re-
	     quires the	uname and gname	fields be properly set.

     version
	     Version.  This should be "00" (two	copies of the ASCII digit
	     zero) for POSIX standard archives.

     uname, gname
	     User and group names, as null-terminated ASCII strings.  These
	     should be used in preference to the uid/gid values	when they are
	     set and the corresponding names exist on the system.

     devmajor, devminor
	     Major and minor numbers for character device or block device en-
	     try.

     prefix  First part	of pathname.  If the pathname is too long to fit in
	     the 100 bytes provided by the standard format, it can be split at
	     any / character with the first portion going here.	 If the	prefix
	     field is not empty, the reader will prepend the prefix value and
	     a / character to the regular name field to	obtain the full	path-
	     name.

     Note that all unused bytes	must be	set to NULL.

     Field termination is specified slightly differently by POSIX than by pre-
     vious implementations.  The magic,	uname, and gname fields	must have a
     trailing NULL.  The pathname, linkname, and prefix	fields must have a
     trailing NULL unless they fill the	entire field.  (In particular, it is
     possible to store a 256-character pathname	if it happens to have a	/ as
     the 156th character.)  POSIX requires numeric fields to be	zero-padded in
     the front,	and allows them	to be terminated with either space or NULL
     characters.

     Currently,	most tar implementations comply	with the ustar format, occa-
     sionally extending	it by adding new fields	to the blank area at the end
     of	the header record.

   Pax Interchange Format
     There are many attributes that cannot be portably stored in a POSIX ustar
     archive.  IEEE Std	1003.1-2001 ("POSIX.1")	defined	a "pax interchange
     format" that uses two new types of	entries	to hold	text-formatted meta-
     data that applies to following entries.  Note that	a pax interchange for-
     mat archive is a ustar archive in every respect.  The new data is stored
     in	ustar-compatible archive entries that use the "x" or "g" typeflag.  In
     particular, older implementations that do not fully support these exten-
     sions will	extract	the metadata into regular files, where the metadata
     can be examined as	necessary.

     An	entry in a pax interchange format archive consists of one or two stan-
     dard ustar	entries, each with its own header and data.  The first op-
     tional entry stores the extended attributes for the following entry.
     This optional first entry has an "x" typeflag and a size field that indi-
     cates the total size of the extended attributes.  The extended attributes
     themselves	are stored as a	series of text-format lines encoded in the
     portable UTF-8 encoding.  Each line consists of a decimal number, a
     space, a key string, an equals sign, a value string, and a	new line.  The
     decimal number indicates the length of the	entire line, including the
     initial length field and the trailing newline.  An	example	of such	a
     field is:
	   25 ctime=1084839148.1212\n
     Keys in all lowercase are standard	keys.  Vendors can add their own keys
     by	prefixing them with an all uppercase vendor name and a period.	Note
     that, unlike the historic header, numeric values are stored using deci-
     mal, not octal.  A	description of some common keys	follows:

     atime, ctime, mtime
	     File access, inode	change,	and modification times.	 These fields
	     can be negative or	include	a decimal point	and a fractional
	     value.

     uname, uid, gname,	gid
	     User name,	group name, and	numeric	UID and	GID values.  The user
	     name and group name stored	here are encoded in UTF8 and can thus
	     include non-ASCII characters.  The	UID and	GID fields can be of
	     arbitrary length.

     linkpath
	     The full path of the linked-to file.  Note	that this is encoded
	     in	UTF8 and can thus include non-ASCII characters.

     path    The full pathname of the entry.  Note that	this is	encoded	in
	     UTF8 and can thus include non-ASCII characters.

     realtime.*, security.*
	     These keys	are reserved and may be	used for future	standardiza-
	     tion.

     size    The size of the file.  Note that there is no length limit on this
	     field, allowing conforming	archives to store files	much larger
	     than the historic 8GB limit.

     SCHILY.*
	     Vendor-specific attributes	used by	Joerg Schilling's star imple-
	     mentation.

     SCHILY.acl.access,	SCHILY.acl.default
	     Stores the	access and default ACLs	as textual strings in a	format
	     that is an	extension of the format	specified by POSIX.1e draft
	     17.  In particular, each user or group access specification can
	     include a fourth colon-separated field with the numeric UID or
	     GID.  This	allows ACLs to be restored on systems that may not
	     have complete user	or group information available (such as	when
	     NIS/YP or LDAP services are temporarily unavailable).

     SCHILY.devminor, SCHILY.devmajor
	     The full minor and	major numbers for device nodes.

     SCHILY.dev, SCHILY.ino, SCHILY.nlinks
	     The device	number,	inode number, and link count for the entry.
	     In	particular, note that a	pax interchange	format archive using
	     Joerg Schilling's SCHILY.*	extensions can store all of the	data
	     from struct stat.

     LIBARCHIVE.xattr.namespace.key
	     Libarchive	stores POSIX.1e-style extended attributes using	keys
	     of	this form.  The	key value is URL-encoded: All non-ASCII	char-
	     acters and	the two	special	characters "=" and "%" are encoded as
	     "%" followed by two uppercase hexadecimal digits.	The value of
	     this key is the extended attribute	value encoded in base 64.  XXX
	     Detail the	base-64	format here XXX

     VENDOR.*
	     XXX document other	vendor-specific	extensions XXX

     Any values	stored in an extended attribute	override the corresponding
     values in the regular tar header.	Note that compliant readers should ig-
     nore the regular fields when they are overridden.	This is	important, as
     existing archivers	are known to store non-compliant values	in the stan-
     dard header fields	in this	situation.  There are no limits	on length for
     any of these fields.  In particular, numeric fields can be	arbitrarily
     large.  All text fields are encoded in UTF8.  Compliant writers should
     store only	portable 7-bit ASCII characters	in the standard	ustar header
     and use extended attributes whenever a text value contains	non-ASCII
     characters.

     In	addition to the	x entry	described above, the pax interchange format
     also supports a g entry.  The g entry is identical	in format, but speci-
     fies attributes that serve	as defaults for	all subsequent archive en-
     tries.  The g entry is not	widely used.

     Besides the new x and g entries, the pax interchange format has a few
     other minor variations from the earlier ustar format.  The	most troubling
     one is that hardlinks are permitted to have data following	them.  This
     allows readers to restore any hardlink to a file without having to	rewind
     the archive to find an earlier entry.  However, it	creates	complications
     for robust	readers, as it is no longer clear whether or not they should
     ignore the	size field for hardlink	entries.

   GNU Tar Archives
     The GNU tar program started with a	pre-POSIX format similar to that de-
     scribed earlier and has extended it using several different mechanisms:
     It	added new fields to the	empty space in the header (some	of which was
     later used	by POSIX for conflicting purposes); it allowed the header to
     be	continued over multiple	records; and it	defined	new entries that mod-
     ify following entries (similar in principle to the	x entry	described
     above, but	each GNU special entry is single-purpose, unlike the general-
     purpose x entry).	As a result, GNU tar archives are not POSIX compati-
     ble, although more	lenient	POSIX-compliant	readers	can successfully ex-
     tract most	GNU tar	archives.

	   struct header_gnu_tar {
		   char	name[100];
		   char	mode[8];
		   char	uid[8];
		   char	gid[8];
		   char	size[12];
		   char	mtime[12];
		   char	checksum[8];
		   char	typeflag[1];
		   char	linkname[100];
		   char	magic[6];
		   char	version[2];
		   char	uname[32];
		   char	gname[32];
		   char	devmajor[8];
		   char	devminor[8];
		   char	atime[12];
		   char	ctime[12];
		   char	offset[12];
		   char	longnames[4];
		   char	unused[1];
		   struct {
			   char	offset[12];
			   char	numbytes[12];
		   } sparse[4];
		   char	isextended[1];
		   char	realsize[12];
		   char	pad[17];
	   };

     typeflag
	     GNU tar uses the following	special	entry types, in	addition to
	     those defined by POSIX:

	     7	     GNU tar treats type "7" records identically to type "0"
		     records, except on	one obscure RTOS where they are	used
		     to	indicate the pre-allocation of a contiguous file on
		     disk.

	     D	     This indicates a directory	entry.	Unlike the POSIX-stan-
		     dard "5" typeflag,	the header is followed by data records
		     listing the names of files	in this	directory.  Each name
		     is	preceded by an ASCII "Y" if the	file is	stored in this
		     archive or	"N" if the file	is not stored in this archive.
		     Each name is terminated with a null, and an extra null
		     marks the end of the name list.  The purpose of this en-
		     try is to support incremental backups; a program restor-
		     ing from such an archive may wish to delete files on disk
		     that did not exist	in the directory when the archive was
		     made.

		     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.

	     K	     The data for this entry is	a long linkname	for the	fol-
		     lowing regular entry.

	     L	     The data for this entry is	a long pathname	for the	fol-
		     lowing regular entry.

	     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 vol-
		     ume.  The "M" typeflag indicates that this	entry contin-
		     ues 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 size field spec-
		     ifies the size of this entry.  The	offset field at	bytes
		     369-380 specifies the offset where	this file fragment be-
		     gins.  The	realsize field specifies the total size	of the
		     file (which must equal size plus offset).	When extract-
		     ing, 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.

	     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 de-
		     scription of the operations to be done, in	the form
		     "Rename %s	to %s\n" or "Symlink %s	to %s\n"; in either
		     case, both	filenames are escaped using K&R	C syntax.

	     S	     This is a "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 en-
		     tries are required, the header is extended	as necessary
		     with "extra" header extensions (an	older format that is
		     no	longer used), or "sparse" extensions.

	     V	     The name field should be interpreted as a tape/volume
		     header name.  This	entry should generally be ignored on
		     extraction.

     magic   The magic field holds the five characters "ustar" followed	by a
	     space.  Note that POSIX ustar archives have a trailing null.

     version
	     The version field holds a space character followed	by a null.
	     Note that POSIX ustar archives use	two copies of the ASCII	digit
	     "0".

     atime, ctime
	     The time the file was last	accessed and the time of last change
	     of	file information, stored in octal as with mtime.

     longnames
	     This field	is apparently no longer	used.

     Sparse offset / 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	ex-
	     traction, the list	of fragments is	collected from the header (in-
	     cluding any extension headers), and the data is then read and
	     written to	the file at appropriate	offsets.

     isextended
	     If	this is	set to non-zero, the header will be followed by	addi-
	     tional "sparse header" records.  Each such	record contains	infor-
	     mation about as many as 21	additional sparse blocks as shown
	     here:

		   struct gnu_sparse_header {
			   struct {
				   char	offset[12];
				   char	numbytes[12];
			   } sparse[21];
			   char	   isextended[1];
			   char	   padding[7];
		   };

     realsize
	     A binary representation of	the file's complete size, with a much
	     larger range than the POSIX file size.  In	particular, with 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	realsize field will indicate the total size of the
	     file.

   Solaris Tar
     XXX More Details Needed XXX

     Solaris tar (beginning with SunOS XXX 5.7 ?? XXX) supports	an "extended"
     format that is fundamentally similar to pax interchange format, with the
     following differences:
     +o	     Extended attributes are stored in an entry	whose type is X, not
	     x,	as used	by pax interchange format.  The	detailed format	of
	     this entry	appears	to be the same as detailed above for the x en-
	     try.
     +o	     An	additional A entry is used to store an ACL for the following
	     regular entry.  The body of this entry contains a seven-digit oc-
	     tal number	(whose value is	01000000 plus the number of ACL	en-
	     tries) followed by	a zero byte, followed by the textual ACL de-
	     scription.

   Other Extensions
     One common	extension, utilized by GNU tar,	star, and other	newer tar im-
     plementations, 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.

     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 re-
     sponsible for a very cryptic warning message that is sometimes seen when
     GNU tar encounters	a damaged archive.

SEE ALSO
     ar(1), pax(1), tar(1)

STANDARDS
     The tar utility is	no longer a part of POSIX or the Single	Unix Standard.
     It	last appeared in Version 2 of the Single UNIX Specification ("SUSv2").
     It	has been supplanted in subsequent standards by pax(1).	The ustar for-
     mat is currently part of the specification	for the	pax(1) utility.	 The
     pax interchange file format is new	with IEEE Std 1003.1-2001 ("POSIX.1").

HISTORY
     A tar command appeared in Seventh Edition Unix, which was released	in
     January, 1979.  It	replaced the tp	program	from Fourth Edition Unix which
     in	turn replaced the tap program from First Edition Unix.	John Gilmore's
     pdtar public-domain implementation	(circa 1987) was highly	influential
     and formed	the basis of GNU tar.  Joerg Shilling's	star archiver is an-
     other open-source (GPL) archiver (originally developed circa 1985)	which
     features complete support for pax interchange format.

BSD				 May 20, 2004				   BSD

NAME | DESCRIPTION | SEE ALSO | STANDARDS | HISTORY

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