Typeflag

Typeflag is a field in the header of a tar archive that specifies the type of the file entry. It is a single byte that guides tar and other archive tools on how to interpret the data and how to recreate the entry when extracting. The typeflag helps parsers distinguish between ordinary files, links, devices, directories, and special entries within the archive.

In the POSIX ustar and similar tar formats, the standard values are: '0' (or the ASCII NUL, '\0') for a regular file; '1' for a hard link; '2' for a symbolic link; '3' for a character device; '4' for a block device; '5' for a directory; '6' for a FIFO; '7' for a contiguous file. Additional values are used for extended metadata in newer formats: 'x' for an extended header and 'g' for a global extended header (as part of the PAX format). These extended typeflags signal that supplementary key-value data follows to describe the entry more fully.

There are also nonstandard or vendor-specific extensions used by some tar implementations. For example, GNU tar and related tools have used 'L' to indicate a long name and 'K' for a long link, enabling the storage of longer path or link names than the standard fields allow. These extensions are not part of the POSIX standard and may not be recognized by all tar utilities.

Overall, typeflag matters for the correct reconstruction of archives: it instructs tar how to treat the entry’s data, what kind of filesystem object to create, and how to interpret any accompanying metadata. Compatibility can vary across implementations, especially for extensions beyond the standard typeflag values.