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NetBSD 6.1.5 - man page for tar (netbsd section 5)

tar(5)				     BSD File Formats Manual				   tar(5)

     tar -- format of tape archive files

     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 entirely 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 maximum block size supported by
     early implementations was 10240 bytes or 20 records.  This is still the default for most
     implementations although block sizes of 1MiB (2048 records) or larger are commonly used with
     modern high-speed tape drives.  (Note: the terms ``block'' and ``record'' here are not
     entirely standard; this document follows the convention established by John Gilmore in docu-
     menting pdtar.)

   Old-Style Archive Format
     The original tar archive format has been extended many times to include additional informa-
     tion that various implementors found necessary.  This section describes the variant imple-
     mented by the tar command included in Version 7 AT&T UNIX, which seems to be the earliest
     widely-used version 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 implementations 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 indicates 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.

	     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 arith-
	     metic for the checksum field, which can cause interoperability problems when trans-
	     ferring 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 termi-
     nated by a null and a space.  Early implementations filled the numeric fields with leading
     spaces.  This seems to have been common practice 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 varia-
     o	     The magic value is ``ustar '' (note the following space).	The version field con-
	     tains 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];

	     Type of entry.  POSIX extended the earlier linkflag field with several new type val-
	     ``0''   Regular file.  NUL should be treated as a synonym, for compatibility pur-
	     ``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 unrecognized typeflag value
		     as a regular file.  In particular, writers should ensure that all entries
		     have a valid filename 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 different 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-allocate
	     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 NUL byte to indicate that this is a
	     POSIX standard archive.  Full compliance requires the uname and gname fields be
	     properly set.

	     Version.  This should be ``00'' (two copies of the ASCII digit zero) for POSIX stan-
	     dard 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 entry.

     name, prefix
	     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 into the prefix
	     field.  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 pathname.  The stan-
	     dard does not require a trailing / character on directory names, though most imple-
	     mentations still include this for compatibility reasons.

     Note that all unused bytes must be set to NUL.

     Field termination is specified slightly differently by POSIX than by previous implementa-
     tions.  The magic, uname, and gname fields must have a trailing NUL.  The pathname,
     linkname, and prefix fields must have a trailing NUL 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
     requires them to be terminated with either space or NUL characters.

     Currently, most tar implementations comply with the ustar format, occasionally 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 metadata that applies to following entries.  Note that a pax
     interchange format 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 extensions 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 standard ustar entries,
     each with its own header and data.  The first optional entry stores the extended attributes
     for the following entry.  This optional first entry has an "x" typeflag and a size field
     that indicates the total size of the extended attributes.	The extended attributes them-
     selves 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, includ-
     ing 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 decimal, not octal.  A description of some common keys fol-

     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.

	     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 standardization.

     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.

	     Vendor-specific attributes used by Joerg Schilling's star implementation.

     SCHILY.acl.access, SCHILY.acl.default
	     Stores the access and default ACLs as textual strings in a format that is an exten-
	     sion 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.

	     The file flags.

	     The full size of the file on disk.  XXX explain? XXX

     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 stores POSIX.1e-style extended attributes using keys of this form.  The
	     key value is URL-encoded: All non-ASCII characters 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

	     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 ignore the regular fields when they are
     overridden.  This is important, as existing archivers are known to store non-compliant val-
     ues in the standard 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 specifies attributes that serve as defaults
     for all subsequent archive entries.  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 described 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 modify
     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 compatible, although more lenient POSIX-compliant readers can successfully
     extract 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];

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

	     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-standard "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 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.

		     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 following regular entry.

	     L	     The data for this entry is a long pathname for the following 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 vol-
		     ume, 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 size field specifies the size of this
		     entry.  The offset field at bytes 369-380 specifies the offset where this
		     file fragment begins.  The realsize field specifies the total size of the
		     file (which must equal size plus 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

	     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 descrip-
		     tion 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.	Due to security concerns, "N" records are now generally ignored
		     when reading archives.

	     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 entries are required, the header is extended as neces-
		     sary 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.

	     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.

	     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 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.

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

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

	     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.

   GNU tar pax archives
     GNU tar 1.14 (XXX check this XXX) and later will write pax interchange format archives when
     you specify the --posix flag.  This format uses custom keywords to store sparse file infor-
     mation.  There have been three iterations of this support, referred to as ``0.0'', ``0.1'',
     and ``1.0''.

     GNU.sparse.numblocks, GNU.sparse.offset, GNU.sparse.numbytes, GNU.sparse.size
	     The ``0.0'' format used an initial GNU.sparse.numblocks attribute to indicate the
	     number of blocks in the file, a pair of GNU.sparse.offset and GNU.sparse.numbytes to
	     indicate the offset and size of each block, and a single GNU.sparse.size to indicate
	     the full size of the file.  This is not the same as the size in the tar header
	     because the latter value does not include the size of any holes.  This format
	     required that the order of attributes be preserved and relied on readers accepting
	     multiple appearances of the same attribute names, which is not officially permitted
	     by the standards.

	     The ``0.1'' format used a single attribute that stored a comma-separated list of
	     decimal numbers.  Each pair of numbers indicated the offset and size, respectively,
	     of a block of data.  This does not work well if the archive is extracted by an
	     archiver that does not recognize this extension, since many pax implementations sim-
	     ply discard unrecognized attributes.

     GNU.sparse.major, GNU.sparse.minor, GNU.sparse.name, GNU.sparse.realsize
	     The ``1.0'' format stores the sparse block map in one or more 512-byte blocks
	     prepended to the file data in the entry body.  The pax attributes indicate the exis-
	     tence of this map (via the GNU.sparse.major and GNU.sparse.minor fields) and the
	     full size of the file.  The GNU.sparse.name holds the true name of the file.  To
	     avoid confusion, the name stored in the regular tar header is a modified name so
	     that extraction errors will be apparent to users.

   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 entry.
     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 octal number followed by a zero byte, fol-
	     lowed by the textual ACL description.  The octal value is the number of ACL entries
	     plus a constant that indicates the ACL type: 01000000 for POSIX.1e ACLs and 03000000
	     for NFSv4 ACLs.

   AIX Tar
     XXX More details needed XXX

   Mac OS X Tar
     The tar distributed with Apple's Mac OS X stores most regular files as two separate entries
     in the tar archive.  The two entries have the same name except that the first one has ``._''
     added to the beginning of the name.  This first entry stores the ``resource fork'' with
     additional attributes for the file.  The Mac OS X CopyFile() API is used to separate a file
     on disk into separate resource and data streams and to reassemble those separate streams
     when the file is restored to disk.

   Other Extensions
     One obvious extension to increase the size of files is to eliminate the terminating charac-
     ters from the various numeric fields.  For example, the standard only allows the size field
     to contain 11 octal digits, reserving the twelfth byte for a trailing NUL character.  Allow-
     ing 12 octal digits allows file sizes up to 64 GB.

     Another extension, utilized by GNU tar, star, and other newer tar implementations, permits
     binary numbers in the standard numeric fields.  This is flagged by setting the high bit of
     the first byte.  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 is no longer supported by any implementation.

     ar(1), pax(1), tar(1)

     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 subse-
     quent standards by pax(1).  The ustar format is currently part of the specification for the
     pax(1) utility.  The pax interchange file format is new with IEEE Std 1003.1-2001

     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 (circa 1988).  Joerg Shilling's star
     archiver is another open-source (GPL) archiver (originally developed circa 1985) which fea-
     tures complete support for pax interchange format.

     This documentation was written as part of the libarchive and bsdtar project by Tim Kientzle

BSD					December 27, 2009				      BSD

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