.TAR

Introduction

Tar bundles files and folders into one file, keeping each file’s name, permissions, owner and modification time. It began in Version 7 Unix in 1979 as a way to write files to magnetic tape, which is why everything in it comes in 512-byte blocks. POSIX.1-1988 standardised it as the ustar format, and POSIX.1-2001 extended it as the pax format.

Tar does not compress. A tar file is usually compressed as a whole afterwards, giving .tar.gz or .tgz (gzip), .tar.xz (xz), .tar.bz2 (bzip2) and .tar.zst (Zstandard). Tar is also found inside other formats: Debian packages hold their files in tar archives, and container images are made of tar layers.

Structure

A tar file is a sequence of 512-byte blocks. Each file in the archive is a header block describing it, followed by its contents, padded with zeros to a whole number of blocks. A folder, a link or a device is a header with no contents; the header’s type flag says which it is. Two blocks of zeros end the archive.

There is no index. To list an archive, a reader reads each header, uses the size in it to skip over the file’s data, and reads the next header. Long names, large files and other details the original header cannot hold are carried in extra headers before the file’s own: the pax extensions of POSIX, or the older GNU extensions.

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