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.
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
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.
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
here are not entirely standard; this document follows the
convention established by John Gilmore in documenting
Old-Style Archive Format
The original tar archive format has been extended many times to
include additional information that various implementors found
This section describes the variant implemented by the tar command
which seems to be the earliest widely-used version of the tar program.
The header record for an old-style
archive consists of the following:
All unused bytes in the header record are filled with nulls.
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.
File mode, stored as an octal number in ASCII.
uid , gid
User id and group id of owner, as octal numbers in ASCII.
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.
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
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
is set to an ASCII
field holds the first name under which this file appears.
(Note that regular files have a null value in the
Early tar implementations varied in how they terminated these fields.
The tar command in
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
standard was released.
For best portability, modern implementations should fill the numeric
fields with leading zeros.
An early draft of
served as the basis for John Gilmore's
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:
The magic value is
(note the following space).
The version field contains a space character followed by a null.
The numeric fields are generally filled with leading spaces
(not leading zeros as recommended in the final standard).
The prefix field is often not used, limiting pathnames to
the 100 characters of old-style archives.
POSIX ustar Archives
defined a standard tar file format to be read and written
by compliant implementations of
This format is often called the
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:
Type of entry.
POSIX extended the earlier
field with several new type values:
NUL should be treated as a synonym, for compatibility purposes.
Character device node.
Block device node.
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
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
For all other types, it should be set to zero by writers and ignored
Contains the magic value
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.
This should be
(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
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
Field termination is specified slightly differently by POSIX
than by previous implementations.
fields must have a trailing
fields must have a trailing
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
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.
``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
The new data is stored in ustar-compatible archive entries that use the
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:
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.
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.
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.
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
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.
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
extensions can store all of the data from
LIBARCHIVE.xattr. namespace . key
Libarchive stores POSIX.1e-style extended attributes using
keys of this form.
value is URL-encoded:
All non-ASCII characters and the two special characters
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
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
entry described above, the pax interchange format
also supports a
entry is identical in format, but specifies attributes that serve as
defaults for all subsequent archive entries.
entry is not widely used.
Besides the new
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
entry described above, but each GNU special entry is single-purpose,
unlike the general-purpose
As a result, GNU tar archives are not POSIX compatible, although
more lenient POSIX-compliant readers can successfully extract most
GNU tar archives.
GNU tar uses the following special entry types, in addition to
those defined by POSIX:
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.
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.
The data for this entry is a long linkname for the following regular entry.
The data for this entry is a long pathname for the following regular entry.
This is a continuation of the last file on the previous volume.
GNU multi-volume archives guarantee that each volume begins with a valid
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).
field specifies the size of this entry.
field at bytes 369-380 specifies the offset where this file fragment
field specifies the total size of the file (which must equal
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.
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''
``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.
This is a
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
header extensions (an older format that is no longer used), or
field should be interpreted as a tape/volume header name.
This entry should generally be ignored on extraction.
The magic field holds the five characters
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
atime , ctime
The time the file was last accessed and the time of
last change of file information, stored in octal as with
This field is apparently no longer used.
Sparse offset / numbytes
Each such structure specifies a single fragment of a sparse
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
Each such record contains information about as many as 21 additional
sparse blocks as shown here:
A binary representation of the file's complete size, with a much larger range
than the POSIX file size.
In particular, with
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
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
This format uses custom keywords to store sparse file information.
There have been three iterations of this support, referred to
format used an initial
attribute to indicate the number of blocks in the file, a pair of
to indicate the offset and size of each block,
and a single
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.
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.
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
and the full size of the file.
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
XXX More Details Needed XXX
Solaris tar (beginning with SunOS XXX 5.7 ?? XXX) supports an
format that is fundamentally similar to pax interchange format,
with the following differences:
Extended attributes are stored in an entry whose type is
as used by pax interchange format.
The detailed format of this entry appears to be the same
as detailed above for the
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, 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.
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
added to the beginning of the name.
This first entry stores the
with additional attributes for the file.
The Mac OS X
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.
One obvious extension to increase the size of files is to
eliminate the terminating characters from the various
For example, the standard only allows the size field to contain
11 octal digits, reserving the twelfth byte for a trailing
Allowing 12 octal digits allows file sizes up to 64 GB.
Another extension, utilized by GNU tar, star, and other newer
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
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
The ustar format is currently part of the specification for the
The pax interchange file format is new with
St -p1003.1-2001 .
command appeared in Seventh Edition Unix, which was released in January, 1979.
It replaced the
program from Fourth Edition Unix which in turn replaced the
program from First Edition Unix.
public-domain implementation (circa 1987) was highly influential
and formed the basis of
archiver is another open-source (GPL) archiver (originally developed
circa 1985) which features complete support for pax interchange
This documentation was written as part of the
An Tim Kientzle Aq kientzle@FreeBSD.org .