File extension

.tar

TAR Archive File Format

.tar files are Unix tape archives: an uncompressed container that concatenates many files and directories into one stream, preserving names, permissions, ownership, timestamps, and links.

Known meanings

Formats and reference material for .tar

TAR Archive File Format

TAR Archive File Format

  • archive
  • Max Maischein
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Details

Format notes

TAR (Tape Archive)

The .tar format (“tape archive”) originated as a way to write files to sequential tape on Unix, and it remains the standard way to bundle a directory tree into a single stream on Unix-like systems. A tar file simply concatenates its members: each file is preceded by a 512-byte header block recording its name, mode, owner and group, size, modification time, and type, followed by the file contents padded to a multiple of 512 bytes.

Crucially, tar does not compress. It packages files, and compression is applied separately by piping the archive through a compressor, which is why archives are commonly seen as .tar.gz (gzip), .tar.bz2 (bzip2), or .tar.xz (xz). This separation of concerns — archiving versus compression — is a defining trait of the format.

Format Variants

Several header conventions coexist. The historic format was standardized as POSIX ustar, which added fields such as user and group names and a magic string. Long path names, large files, sparse files, and extended metadata are handled by the GNU tar extensions and by the POSIX pax extended headers. Because these variants differ, a robust extractor needs to recognize which convention an archive uses.

Preservation And Security Notes

Tar is well suited to preservation of file trees because the format is simple, openly documented, and preserves POSIX metadata and symbolic/hard links. Tools extracting untrusted tar files should reject absolute paths and .. traversal entries, be cautious with device nodes, symlinks, and hard links that could escape the target directory, and limit resource use when the archive is compressed.

Further Reading

  • GNU tar manual: https://www.gnu.org/software/tar/manual/
  • POSIX pax/tar format (IEEE Std 1003.1): https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html