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Tar (file format)

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In computing, the tar file format is a type of archive file format: the Tape ARchive format. These files are produced by the Unix command tar and were standardized by POSIX.1-1998 and later POSIX.1-2001. It is used to collate collections of files into one larger file, while preserving file system information such as user and group permissions, dates, and directory structures.

tar was originally developed for use with sequential access devices such as tape drives, specifically for backup purposes. However, it is now more frequently used as a general archive utility. tar's linear roots can still be seen in its ability to work on any data stream and its slow partial extraction performance, as it has to read through the whole archive to extract only the final file. Commonly a tar file is referred to as a tarball.

As is common with Unix utilities, tar is a single specialist program. It follows the Unix philosophy in that it can "do only one thing" (archive), "but do it well". tar is most commonly used in tandem with an external compression utility such as gzip, bzip2 or, formerly, compress, since it has no built-in data compression facilities. These compression utilities generally only compress a single file, hence the pairing with tar, which can produce a single file from many files. To speed this, the BSD and GNU versions of tar support the command line options -z (gzip), -j (bzip2), and -Z (compress), which will compress or decompress the archive file it is currently working with, although even in this case the (de)compression is still actually performed by an external program. Compression is sometimes avoided because of the greatly amplified potential for damage to data in long term storage.

Contents

Usage

To use bzip2 instead of gzip, simply replace the commands above with bzip2 where gzip is used and bunzip2 where gunzip is used.

BSD & GNU tar only

BSD and GNU tar have a compression flag feature making it easier to archive and compress gzipped, bzipped or compressed tarballs in one go. The following commands can be used to take advantage of this:

It should be noted that some versions of tar use the -y switch to invoke bzip2 rather than -j.

Filename extensions

The following is a list of common filename extensions for uncompressed and compressed tar archives:

MIME-Type

File format details

A limitation of early tape drives was that data could only be written to them in 512 byte blocks. As a result data in tar files is arranged in 512 byte blocks.

A tar file is the concatenation of one or more files. Each file is preceded by a header block. 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 blocks.

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, then the archive is essentially an ASCII file.

The fields defined by the original Unix tar format are listed in the table below. When a field is unused it is zero filled. The header is padded with zero bytes to make it up to a 512 byte block.

Field Offset Field Size Field
0 100 File name
100 8 File mode
108 8 Owner user ID
116 8 Group user ID
124 12 File size in bytes
136 12 Last modification time
148 8 Check sum for header block
156 1 Link indicator
157 100 Name of linked file

The Link indicator field can have the following values:

Value Meaning
0 Normal file
(ASCII NUL)This is probably a workaround for buggy tar implementations (the byte 0x00 is ASCII NUL). Normal file
1 Hard link
2 Symbolic linkGNU tar's headers mark this field as "Reserved"[link]
3 Character special
4 Block special
5 Directory
6 FIFO
7 Contiguous fileApparently relevant on an OS called RTU, this would be a normal file written in one contiguous section on-disc. GNU tar's headers mark this field as 'Reserved', and such items will probably be extracted as normal files on other operating systems.

A directory is also indicated by having a trailing slash(/) in the name.

For historical reasons numerical values are encoded in octal with leading zeroes. The final character is either a null or a space. 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 file size is encoded in binary.

The checksum is calculated by taking the sum of the byte values of the header block with the eight checksum bytes taken to be ascii spaces (value 32). It is stored as a six digit octal number with leading zeroes followed by a nul and then a space.

USTAR format

Most modern tar programs read and write archives in the new USTAR format, which has an extended header definition. 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 extra information about each file.

Field Offset Field Size Field
0 156 (as in old format)
156 1 Type flag
157 100 (as in old format)
257 6 USTAR indicator
263 2 USTAR version
265 32 Owner user name
297 32 Owner group name
329 8 Device major number
337 8 Device minor number
345 155 Filename prefix

Example

The example below shows the hex dump of a header block from a tar file created using the GNU tar program. It was dumped with the od program. The "ustar" magic string can be seen, meaning that the tar file is in USTAR format.

0000000   e   t   c   /   p   a   s   s   w   d nul nul nul nul nul nul
0000020 nul nul nul nul nul nul nul nul nul nul nul nul nul nul nul nul
*
0000140 nul nul nul nul   0   1   0   0   6   4   4 nul   0   0   0   0
0000160   0   0   0 nul   0   0   0   0   0   0   0 nul   0   0   0   0
0000200   0   0   4   1   3   5   5 nul   1   0   1   5   5   0   6   1
0000220   1   0   5 nul   0   1   1   5   5   6 nul  sp   0 nul nul nul
0000240 nul nul nul nul nul nul nul nul nul nul nul nul nul nul nul nul
*
0000400 nul   u   s   t   a   r  sp  sp nul   r   o   o   t nul nul nul
0000420 nul nul nul nul nul nul nul nul nul nul nul nul nul nul nul nul
0000440 nul nul nul nul nul nul nul nul nul   r   o   o   t nul nul nul
0000460 nul nul nul nul nul nul nul nul nul nul nul nul nul nul nul nul
*
0001000
Note, the OpenBSD 3.7 tar does not have the 2 space characters after ustar. They are nul characters.

UNIX tricks with tar

Use tar to copy directories precisely: tar -cf - one_directory | (cd another_directory && tar -xpf - )

This syntax was used almost universally before the -R and -p options were added to the cp command.

Tarbombs

Tarbomb is the slang term used to refer to a tarball containing files that untar to the current directory instead of untarring into a directory of its own.

For example:

$ tar zxvf urchin5703_redhat_ent3.tar.gz
gunzip
inspector
install.sh
install.txt
license.txt
README
urchin.tar.gz
$ pwd
/home/suso
In the above example, all the files are untarred into the directory that the user is currently in. It should have untarred into a directory called 'urchin' or something similar. This can be a potential problem if it overwrites files using the same name in the current directory. It can also be a pain for the user who then needs to delete all the files that are scattered over the directory amongst other files. Often times this ends up happening in the user's home directory.

To protect yourself against tarbombs, you should always use the 't' action/option first to list the files contained in the archive before using the 'x' option to extract. Here is an example:

tar ztvf tarfile.tar.gz
To prevent making tarbombs when you create a tar file, create a working directory for all the files and directories that you want in the tar file and then move into the parent directory and run the following:

tar zcvf tarfile.tar.gz directory_name_you_want_to_tar
For programs, a common convention is to call the directory by the name of the program followed by a hyphen and then the version of the program in the directory. For example.

mkdir randomsig-1.9
mv workingdir/* randomsig-1.9/
tar zcvf randomsig-1.9.tar.gz randomsig-1.9
This way when a user downloads your program and untars it, it will not overwrite older versions of the same program or other important files in her current or other directories.

Notes

See also

External links

 

From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
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