tar (computing)
 | This article includes a list of references, related reading, or external links, but its sources remain unclear because it lacks inline citations. (February 2009) |
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 GNU tar 1.16 showing three common types of Tarballs (shown in red). | |
| Filename extension |
.tar |
|---|---|
| Internet media type |
application/x-tar |
| Uniform Type Identifier (UTI) | public.tar-archive |
| Magic number | ustar at byte 257 |
| Type of format | file archiver |
| Contained by | gzip, bzip2, lzma, lzip, compress |
In computing, tar (derived from tape archive) is both a file format (in the form of a type of archive bitstream) and the name of the program used to handle such files. The format was created in the early days of Unix and standardized by POSIX.1-1988 and later POSIX.1-2001. Initially developed to be written directly to sequential I/O devices for tape backup purposes, it is now commonly used to collect many files into one larger file, for distribution or archiving, while preserving file system information such as user and group permissions, dates, and directory structures.
Compression and naming
Conventionally, uncompressed tar archive files have names ending in ".tar". Unlike more recent archive formats, tar files (somefile.tar) are commonly compressed as a whole rather than piecemeal. Applying a compression utility such as gzip, bzip2, lzma or compress to a tar file produces a compressed tar file, typically named with an extension indicating the type of compression (e.g.: somefile.tar.gz).
Popular tar programs like the BSD and GNU versions of tar support the command line options -z (gzip), and -j (bzip2) to automatically compress or decompress the archive file it is currently working with. GNU tar from version 1.20 onwards also supports --lzma (LZMA). Both will automatically extract compressed gzip and bzip2 archives with or without these options.
MS-DOS's 8.3 filename limitations, and a desire for brevity, resulted in additional popular conventions for naming compressed tar archives:
- ".tgz" is equivalent to ".tar.gz".
- ".tbz" (or ".tbz2" or ".tb2") is equivalent to ".tar.bz2".
- ".taz" is equivalent to ".tar.Z",
A tar file or compressed tar file is commonly referred to as a tarball.
Format details
A tar file is the concatenation of one or more files. Each file 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 and the extra space is zero filled. The end of an archive is marked by at least two consecutive zero-filled records. (The origin of tar's record size appears to be the 512-byte disk sectors used in the Version 7 Unix file system.)
Many historic tape drives read and write variable-length data blocks, leaving significant wasted space on the tape between blocks (for the tape to physically start and stop moving). Some tape drives (and raw disks) only support 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 kilobyte blocks (which was large when UNIX was invented, but now seems rather small). The final block of an archive is padded out to full length with zero bytes.
File header
The file header block contains metadata about a file. To ensure portability across different architectures with different byte orderings, the information in the header block is encoded in ASCII. Thus if all the files in an archive are text files, and have ASCII names, then the archive is essentially an ASCII text file (containing many NUL or all-zero characters). Subsequent extensions have diluted this original attribute (which was never particularly valuable, due to the presence of NULs, which are often poorly handled by text-manipulation programs).
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 is padded with NUL bytes to make it fill a 512 byte block.
| Field Offset | Field Size | Field |
|---|---|---|
| 0 | 100 | File name |
| 100 | 8 | File mode |
| 108 | 8 | Owner's numeric user ID |
| 116 | 8 | Group's numeric user ID |
| 124 | 12 | File size in bytes |
| 136 | 12 | Last modification time in numeric Unix time format |
| 148 | 8 | Checksum for header block |
| 156 | 1 | Link indicator (file type) |
| 157 | 100 | Name of linked file |
| Value | Meaning |
|---|---|
| '0' | Normal file |
| (ASCII NUL) | Normal file (now obsolete) |
| '1' | Hard link |
| '2' | Symbolic link |
| '3' | Character special |
| '4' | Block special |
| '5' | Directory |
| '6' | FIFO |
A directory is also indicated 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 null or space character should 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, some versions of tar, including the GNU implementation, support an extension in which the high-order bit of the leftmost byte of a numeric field indicates that the number is encoded in big-endian binary. Additionally, versions of tar from 1999 and before pad the values with spaces instead of zeroes.
The checksum is calculated by taking the sum of the unsigned byte values of the header block with the eight checksum bytes taken to be ascii spaces (decimal value 32). It is stored as a six digit octal number with leading zeroes followed by a null and then a space. Various implementations do not adhere to this, so relying on the first white space trimmed six digits for checksum yields better compatibility. In addition, some historic tar implementations treated bytes as signed. Readers must 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 (names) 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.
Tarbombs
A tarbomb is derogatory hacker slang used to refer to a tarball that does not follow the usual conventions, i.e. it contains many files that extract into the working directory. Such a tarball can create problems by overwriting files of the same name in the working directory, or mixing one project's files into another. It is almost always an inconvenience to the user, who is obliged to identify and delete a number of files scattered throughout the directory's 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 tarballs. Files extracted from such tarballs will often be created in unusual locations outside the working directory, and like a tarbomb have the potential to overwrite existing files. GNU tar by default refuses to create or extract absolute paths, but is still vulnerable to parent-directory references.
A user can avoid both of these problems by first examining a tar file with a command like tar -tf archive.tar. This does not extract any files, but displays the names of all files in the archive. If any are problematic, the user can create a new empty directory and extract the tarball into it -- or avoid the tarball entirely.
Use in software distribution
The most common publication format for the released source code of free software is a tarball. For example, the GNU Project published a version of Emacs as "emacs-22.3.tar.gz" in September 2008. This use of the common tar format (and publication of the tarball on the Internet via http or ftp) masks the varying details of each project's source code revision control system.
See also
Notes
External links
- The tar Command by The Linux Information Project (LINFO)
- Detailed information on Tar and USTAR file headers
- tar(1) man page via OpenBSD
- C#/.NET/Mono implementation and example
| Archiving only | |
|---|---|
| Compression only | |
| Archiving and compression | |
| Software packaging and distribution | |
| Document packaging and distribution | |
