TAR(5) File Formats and Configurations 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 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 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 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
permissions 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.

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 interoperability 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 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 variations:
+o The magic value consists of the five characters "ustar"
followed by a space. The version 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. NUL 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
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
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
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 standard does not require a trailing / character
on directory names, though most implementations 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 implementations. 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.

Numeric Extensions

There have been several attempts to extend the range of sizes or times
supported by modifying how numbers are stored in the header.

One obvious extension to increase the size of files is to eliminate the
terminating characters 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. Allowing 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. The
remainder of the field is treated as a signed twos-complement value.
This permits 95-bit values for the length and time fields and 63-bit
values for the uid, gid, and device numbers. In particular, this
provides a consistent way to handle negative time values. GNU tar
supports this extension for the length, mtime, ctime, and atime fields.
Joerg Schilling's star program and the libarchive library support 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.

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 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
decimal, 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.

hdrcharset
The character set used by the pax extension values. By
default, all textual values in the pax extended attributes are
assumed to be in UTF-8, including pathnames, user names, and
group names. In some cases, it is not possible to translate
local conventions into UTF-8. If this key is present and the
value is the six-character ASCII string "BINARY", then all
textual values are assumed to be in a platform-dependent multi-
byte encoding. Note that there are only two valid values for
this key: "BINARY" or "ISO-IR 10646 2000 UTF-8". No other
values are permitted by the standard, and the latter value
should generally not be used as it is the default when this key
is not specified. In particular, this flag should not be used
as a general mechanism to allow filenames to be stored in
arbitrary encodings.

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
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.

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

SCHILY.acl.access, SCHILY.acl.default, SCHILY.acl.ace
Stores the access, default and NFSv4 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 an additional 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.fflags
The file flags.

SCHILY.realsize
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.*
Vendor-specific attributes used by the libarchive library and
programs that use it.

LIBARCHIVE.creationtime
The time when the file was created. (This should not be
confused with the POSIX "ctime" attribute, which refers to the
time when the file metadata was last changed.)

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
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

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
ignore the regular fields when they are overridden. This is important,
as existing archivers are known to store non-compliant values 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];
};

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-
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 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 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 realsize.

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 "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 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 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.

isextended
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 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.

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 follows
the pax interchange format closely, using some SCHILY tags and
introducing new keywords to store sparse file information. There have
been three iterations of the sparse file 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.

GNU.sparse.map
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 simply 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 existence 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 header 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, followed 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

AIX Tar uses a ustar-formatted header with the type A for storing coded
ACL information. Unlike the Solaris format, AIX tar writes this header
after the regular file body to which it applies. The pathname in this
header is either NFS4 or AIXC to indicate the type of ACL stored. The
actual ACL is stored in platform-specific binary format.

Mac OS X Tar

The tar distributed with Apple's Mac OS X stores most regular files as
two separate files in the tar archive. The two files have the same
name except that the first one has "._" prepended to the last path
element. This special file stores an AppleDouble-encoded binary blob
with additional metadata about the second file, including ACL, extended
attributes, and resources. To recreate the original file on disk, each
separate file can be extracted and the Mac OS X copyfile() function can
be used to unpack the separate metadata file and apply it to th regular
file. Conversely, the same function provides a "pack" option to encode
the extended metadata from a file into a separate file whose contents
can then be put into a tar archive.

Note that the Apple extended attributes interact badly with long
filenames. Since each file is stored with the full name, a separate
set of extensions needs to be included in the archive for each one,
doubling the overhead required for files with long names.

Summary of tar type codes

The following list is a condensed summary of the type codes used in tar
header records generated by different tar implementations. More
details about specific implementations can be found above:
NUL Early tar programs stored a zero byte for regular files.
0 POSIX standard type code for a regular file.
1 POSIX standard type code for a hard link description.
2 POSIX standard type code for a symbolic link description.
3 POSIX standard type code for a character device node.
4 POSIX standard type code for a block device node.
5 POSIX standard type code for a directory.
6 POSIX standard type code for a FIFO.
7 POSIX reserved.
7 GNU tar used for pre-allocated files on some systems.
A Solaris tar ACL description stored prior to a regular file header.
A AIX tar ACL description stored after the file body.
D GNU tar directory dump.
K GNU tar long linkname for the following header.
L GNU tar long pathname for the following header.
M GNU tar multivolume marker, indicating the file is a continuation
of a file from the previous volume.
N GNU tar long filename support. Deprecated.
S GNU tar sparse regular file.
V GNU tar tape/volume header name.
X Solaris tar general-purpose extension header.
g POSIX pax interchange format global extensions.
x POSIX pax interchange format per-file extensions.

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

This documentation was written as part of the libarchive and bsdtar
project by Tim Kientzle <kientzle@FreeBSD.org>.

illumos December 27, 2016 illumos