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8 Controlling the Archive Format

Due to historical reasons, there are several formats of tar archives. All of them are based on the same principles, but have some subtle differences that often make them incompatible with each other.

GNU tar is able to create and handle archives in a variety of formats. The most frequently used formats are (in alphabetical order):

gnu

Format used by GNU tar versions starting with 1.13. This format derived from an early POSIX standard, adding some improvements such as sparse file handling and incremental archives. Unfortunately these features were implemented in a way incompatible with other archive formats.

Archives in ‘gnu’ format are able to hold file names of unlimited length. However, file timestamps in this format are integers, so the timestamps have only one-second resolution; any subsecond information is lost. Also, UIDs and GIDs are limited to 64 bits, and device numbers to 63 bits.

oldgnu

Format used by GNU tar of versions prior to 1.12.

v7

Archive format, compatible with the V7 implementation of tar. This format imposes a number of limitations. The most important of them are:

1. File names and symbolic links can contain at most 100 bytes.
2. File sizes must be less than 8 GiB (2^33 bytes = 8,589,934,592 bytes).
3. File timestamps must be nonnegative integers less than 2^33 seconds since the Epoch, corresponding to the range from 1970年01月01日 00:00:00 (inclusive) to 2242年03月16日 12:56:32 (exclusive) UTC, ignoring leap seconds.
4. It is impossible to store special files (block and character devices, fifos etc.)
5. UIDs and GIDs must be less than 2^21 (2,097,152).
6. V7 archives do not contain symbolic ownership information (user and group name of the file owner).

This format has traditionally been used by Automake when producing Makefiles. This practice will change in the future, in the meantime, however this means that projects containing file names more than 100 bytes long will not be able to use GNU tar 1.35.90 and Automake prior to 1.9.

ustar

Archive format defined by POSIX.1-1988 and later. It stores symbolic ownership information. It is also able to store special files. However, it imposes several restrictions as well:

1. File names can contain at most 255 bytes.
2. File names longer than 100 bytes must be split at a directory separator in two parts, the first being at most 155 bytes long. So, in most cases file names must be a bit shorter than 255 bytes.
3. Symbolic links can contain at most 100 bytes.
4. File sizes must be less than 8 GiB (2^33 bytes = 8,589,934,592 bytes).
5. File timestamps must nonnegative integers less than 2^33 seconds since the Epoch, corresponding to the range from 1970年01月01日 00:00:00 (inclusive) to 2242年03月16日 12:56:32 (exclusive) UTC, ignoring leap seconds.
6. UIDs, GIDs, device major numbers, and device minor numbers must be less than 2^21 (2,097,152).

star

The format used by the late Jörg Schilling’s star implementation. GNU tar can read ‘star’ archives but does not produce them.

posix

The format defined by POSIX.1-2001 and later. This is the most flexible and feature-rich format. It does not impose arbitrary restrictions on file sizes, file timestamp range or resolution, file name lengths, and number of bits in UIDs or device numbers. This format is more recent, so some tar implementations cannot handle it properly. However, any tar implementation able to read ‘ustar’ archives should be able to read most ‘posix’ archives as well, except that it will extract any additional information (such as long file names) as extra plain text files and their parent directories.

This archive format will be the default format for future versions of GNU tar.

The following table summarizes the limits of each of these formats. “Size” and “Date” stands for file size and date, “Res” for file timestamp resolution (where “1 s” means one second), “Name” for length of file names, “Sym” for length of symbolic links, “Un” for the number of bits in a UID or GID, and “Dn” for the number of bits in a device number. A “-” means the archive format imposes no practical limits, though the operating system may have limits of its own.

Format	Size	Date	Res	Name	Sym	Un	Dn
v7	< 8 GiB	1970–2242	1 s	100	100	21	n/a
ustar	< 8 GiB	1970–2242	1 s	255	100	21	21
oldgnu	-	-	1 s	-	-	64	63
gnu	-	-	1 s	-	-	64	63
posix	-	-	-	-	-	-	-

The default format for GNU tar is defined at compilation time. You may check it by running tar --help, and examining the last lines of its output. Usually, GNU tar is configured to create archives in ‘gnu’ format, however, a future version will switch to ‘posix’.

• Using Less Space through Compression
• Handling File Attributes
• Making tar Archives More Portable
• Making tar Archives More Reproducible
• Comparison of tar and cpio

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