Tar (computing)

In computing, tar is a computer software utility for collecting many files into one archive file, often referred to as a tarball, for distribution or backup purposes. The name is derived from "tape archive", as it was originally developed to write data to sequential I/O devices with no file system of their own. The archive data sets created by tar contain various file system parameters, such as name, timestamps, ownership, file access permissions, and directory organization. The command line utility was first introduced in the Version 7 Unix in January 1979, replacing the tp program. The file structure to store this information was standardized in POSIX.1-1988 and later POSIX.1-2001, and became a format supported by most modern file archiving systems.


Many historic tape drives read and write variable-length data blocks, leaving significant wasted space on the tape between blocks. Some tape drives support only fixed-length data blocks. Also, when writing to any medium such as a filesystem or network, it takes less time to write one large block than many small blocks. Therefore, the tar command writes data in blocks of many 512 byte records. The user can specify a blocking factor, which is the number of records per block; the default is 20, producing 10 kibibytes blocks.

File format

A tar archive consists of a series of file objects, hence the popular term tarball, referencing how a tarball collects objects of all kinds that stick to its surface. Each file object includes any file data, and is preceded by a 512-byte header record. The file data is written unaltered except that its length is rounded up to a multiple of 512 bytes. The original tar implementation did not care about the contents of the padding bytes, and left the buffer data unaltered, but most modern tar implementations fill the extra space with zeros. The end of an archive is marked by at least two consecutive zero-filled records. The final block of an archive is padded out to full length with zeros.


The file header record contains metadata about a file. To ensure portability across different architectures with different byte orderings, the information in the header record is encoded in ASCII. Thus if all the files in an archive are ASCII text files, and have ASCII names, then the archive is essentially an ASCII text file.
The fields defined by the original Unix tar format are listed in the table below. The link indicator/file type table includes some modern extensions. When a field is unused it is filled with NUL bytes. The header uses 257 bytes, then is padded with NUL bytes to make it fill a 512 byte record. There is no "magic number" in the header, for file identification.
Pre-POSIX.1-1988 tar header:
Field offsetField sizeField
0100File name
1008File mode
1088Owner's numeric user ID
1168Group's numeric user ID
12412File size in bytes
13612Last modification time in numeric Unix time format
1488Checksum for header record
1561Link indicator
157100Name of linked file

The pre-POSIX.1-1988 Link indicator field can have the following values:
'0' or Normal file
'1'Hard link
'2'Symbolic link

Some pre-POSIX.1-1988 tar implementations indicated a directory by having a trailing slash in the name.
Numeric values are encoded in octal numbers using ASCII digits, with leading zeroes. For historical reasons, a final NUL or space character should also be used. Thus although there are 12 bytes reserved for storing the file size, only 11 octal digits can be stored. This gives a maximum file size of 8 gigabytes on archived files. To overcome this limitation, in 2001 star introduced a base-256 coding that is indicated by setting the high-order bit of the leftmost byte of a numeric field. GNU-tar and BSD-tar followed this idea. Additionally, versions of tar from before the first POSIX standard from 1988 pad the values with spaces instead of zeroes.
The checksum is calculated by taking the sum of the unsigned byte values of the header record with the eight checksum bytes taken to be ascii spaces. It is stored as a six digit octal number with leading zeroes followed by a NUL and then a space. Various implementations do not adhere to this format. For better compatibility, ignore leading and trailing whitespace, and take the first six digits. In addition, some historic tar implementations treated bytes as signed. Implementations typically calculate the checksum both ways, and treat it as good if either the signed or unsigned sum matches the included checksum.
Unix filesystems support multiple links for the same file. If several such files appear in a tar archive, only the first one is archived as a normal file; the rest are archived as hard links, with the "name of linked file" field set to the first one's name. On extraction, such hard links should be recreated in the file system.

UStar format

Most modern tar programs read and write archives in the UStar format, introduced by the POSIX IEEE P1003.1 standard from 1988. It introduced additional header fields. Older tar programs will ignore the extra information, while newer programs will test for the presence of the "ustar" string to determine if the new format is in use. The UStar format allows for longer file names and stores additional information about each file. The maximum filename size is 256, but it is split among a preceding path "filename prefix" and the filename itself, so can be much less.
Field offsetField sizeField
1561Type flag
2576UStar indicator "ustar" then NUL
2632UStar version "00"
26532Owner user name
29732Owner group name
3298Device major number
3378Device minor number
345155Filename prefix

The type flag field can have the following values:
'0' or Normal file
'1'Hard link
'2'Symbolic link
'3'Character special
'4'Block special
'7'Contiguous file
'g'global extended header with meta data
'x'extended header with meta data for the next file in the archive
'A'–'Z'Vendor specific extensions
All other valuesreserved for future standardization

POSIX.1-1988 vendor specific extensions using link flag values 'A'..'Z' partially have a different meaning with different vendors and thus are seen as outdated and replaced by the POSIX.1-2001 extensions that also include a vendor tag.
Type '7' is formally marked as reserved in the POSIX standard, but was meant to indicate files which ought to be contiguously allocated on disk. Few operating systems support creating such files explicitly, and hence most TAR programs do not support them, and will treat type 7 files as if they were type 0. An exception is older versions of GNU tar, when running on the MASSCOMP RTU operating system, which supported an O_CTG flag to the open function to request a contiguous file; however, that support was removed from GNU tar version 1.24 onwards.


In 1997, Sun proposed a method for adding extensions to the tar format. This method was later accepted for the POSIX.1-2001 standard. This format is known as extended tar format or pax format. The new tar format allows users to add any type of vendor-tagged vendor-specific enhancements. The following tags are defined by the POSIX standard:
In 2001, the Star program became the first tar to support the new format. In 2004, GNU tar supported the new format, though it does not write them as its default output from the tar program yet.
It's designed so that all implementations able to read the UStar format will be able to read POSIX.1-2001 as well. The only exceptions are files that make use of extended features, such as longer file names. For compatibility, these are encoded in the tar files as special or type files. typically under a directory. A Pax-supporting implementation would make use of the info, while non supporting ones like 7-Zip would process them as extra files.


Command syntax

Basic options:
  • -c, --create — create a new archive;
  • -a, --auto-compress — additionally compress the archive with a compressor which will be automatically determined by the file name extension of the archive. If the archive's name ends with then use gzip, if then use xz, for Zstandard etc.;
  • -r, --append — append files to the end of an archive;
  • -x, --extract, --get — extract files from an archive;
  • -f, --file — specify the archive's name;
  • -t, --list — show a list of files and folders in the archive;
  • -v, --verbose — show a list of processed files.

    Basic usage

Create an archive file from the file and directory :

$ tar -cvf archive.tar README.txt src

Extract contents for the into the current directory:

$ tar -xvf archive.tar

Create an archive file from the file and directory and compress it with gzip :

$ tar -cavf archive.tar.gz README.txt src

Extract contents for the into the current directory:

$ tar -xvf archive.tar.gz


A tarpipe is the method of creating an archive on the standard output file of the tar utility and piping it to another tar process on its standard input, working in another directory, where it is unpacked. This process copies an entire source directory tree including all special files, for example:

$ tar cf - srcdir | tar x -C destdir

Software distribution

The tar format continues to be used extensively for open-source software distribution. *NIX-distributions use it in various source- and binary-package distribution mechanisms, with most software source code made available in compressed tar archives.


The original tar format was created in the early days of UNIX, and despite current widespread use, many of its design features are considered dated.
Many older tar implementations do not record nor restore extended attributes or Access control lists. In 2001, Star introduced support for ACLs and extended attributes, through its own tags for POSIX.1-2001 pax. Bsdtar uses the star extensions to support ACLs. More recent versions of GNU tar support Linux extended attributes, reimplementing star extensions. A number of extensions are reviewed in the filetype manual for BSD tar, tar.
Other formats have been created to address the shortcomings of tar.

Operating system support

operating systems usually include tools to support tar files, as well as utilities commonly used to compress them, such as gzip and bzip2. BSD-tar has been included in Microsoft Windows since Windows 10 April 2018 Update, and there are otherwise multiple third party tools available to read and write these formats on Windows.


A tarbomb, in hacker slang, is a tar file that contains many files that extract into the working directory. Such a tar file can create problems by overwriting files of the same name in the working directory, or mixing one project's files into another. It is at best an inconvenience to the user, who is obliged to identify and delete a number of files interspersed with the directory's other contents. Such behavior is considered bad etiquette on the part of the archive's creator.
A related problem is the use of absolute paths or parent directory references when creating tar files. Files extracted from such archives will often be created in unusual locations outside the working directory and, like a tarbomb, have the potential to overwrite existing files. However, modern versions of FreeBSD and GNU tar do not create or extract absolute paths and parent-directory references by default, unless it is explicitly allowed with the flag or the option. The bsdtar program, which is also available on many operating systems and is the default tar utility on Mac OS X v10.6, also does not follow parent-directory references or symbolic links.
If a user has only a very old tar available, which does not feature those security measures, these problems can be mitigated by first examining a tar file using the command tar tf archive.tar, which lists the contents and allows to exclude problematic files afterwards.
These commands do not extract any files, but display the names of all files in the archive. If any are problematic, the user can create a new empty directory and extract the archive into it—or avoid the tar file entirely. Most graphical tools can display the contents of the archive before extracting them. Vim can open tar archives and display their contents. GNU Emacs is also able to open a tar archive and display its contents in a dired buffer.

Random access

The tar format was designed without a centralized index or table of content for files and their properties for streaming to tape backup devices. The archive must be read sequentially to list or extract files. For large tar archives, this causes a performance penalty, making tar archives unsuitable for situations that often require random access to individual files.


Another issue with tar format is that it allows several files in archive to have identical path and filename. When extracting such archive, usually the latter version of a file overwrites the former.
This can create a non-explicit tarbomb, which technically does not contain files with absolute paths or referring parent directories, but still causes overwriting files outside current directory.

Key implementations

Historically, many systems have implemented tar, and many general file archivers have at least partial support for tar. The history of tar is a story of incompatibilities, known as the "tar wars". Most tar implementations can also read and create cpio and pax.
Key implementations in order of origin:
  • Solaris tar, based on the original UNIX V7 tar and comes as the default on the Solaris operating system
  • GNU tar is the default on most Linux distributions. It is based on the public domain implementation pdtar which started in 1987. Recent versions can use various formats, including ustar, pax, GNU and v7 formats.
  • FreeBSD tar has become the default tar on most Berkeley Software Distribution-based operating systems including Mac OS X and Microsoft Windows. The core functionality is available as libarchive for inclusion in other applications. This implementation automatically detects the format of the file and can extract from tar, pax, cpio, zip, jar, ar, xar, rpm and ISO 9660 cdrom images. It also comes with a functionally equivalent cpio command-line interface.
  • Schily tar, better known as star, is historically significant as some of its extensions were quite popular. It is the oldest free tar from 1982. It is still maintained.
Additionally, most pax and cpio implementations can read and create many types of tar files.

Suffixes for compressed files

tar archive files usually have the file suffix .tar, e.g., somefile.tar.
A tar archive file contains uncompressed byte streams of the files which it contains. To achieve archive compression, a variety of compression programs are available, such as gzip, bzip2, xz, lzip, lzma, zstd, or compress, which compress the entire tar archive. Typically, the compressed form of the archive receives a filename by appending the format-specific compressor suffix to the archive file name. For example, a tar archive archive.tar, is named archive.tar.gz, when it is compressed by gzip.
Popular tar programs like the BSD and GNU versions of tar support the command line options Z, z, and j to compress or decompress the archive file upon creation or unpacking. GNU tar from version 1.20 onwards also supports the option --lzma. 1.21 added support for lzop with --lzop. 1.22 added support for xz with --xz or -J. 1.23 added support for lzip with --lzip. 1.31 added support for zstd with --zstd. The decompression of these formats is handled automatically if supported filename extensions are used, and compression is handled automatically using the same filename extensions if the option --auto-compress is passed to an applicable version of GNU tar.
MS-DOS's 8.3 filename limitations resulted in additional conventions for naming compressed tar archives.
, to create a compressed archive. As shown, the combination of the files in the archive are compressed as one unit.