tar — Format of tar archives

Description

tar reads and writes headers in either the original TAR format from UNIX systems or the USTAR format defined by the POSIX 1003.1 standard.

The pax command reads and writes headers in any of the tar formats.

The tar command supports both the older UNIX-compatible tar formats and the extended USTAR format. The –X option needs to be used to enable extended USTAR format. The extended USTAR format allows more information to be stored and supports longer pathnames. There is also a non-portable OS390 format (–S option) which also allows storing of additional file attributes and longer pathnames.

A tar archive, in either format, consists of one or more blocks, which are used to represent member files. Each block is 512 bytes long; you can use the –b option with tar to indicate how many of these blocks are read or written (or both) at once.

Each member file consists of a header block, followed by zero or more blocks containing the file contents. The end of the archive is indicated by two blocks filled with binary zeros. Unused space in the header is left as binary zeros.

The header information in a block is stored in a printable ASCII form, so that tar archives are easily ported to different environments. If the contents of the files on the archive are all ASCII, the entire archive is ASCII.

Table 1 shows the UNIX format of the header block for a file:
Table 1. Archive file: UNIX-compatible format
Field width Field Name Meaning
100 name Name of file
8 mode File mode
8 uid Owner user ID
8 gid Owner group ID
12 size Length of file in bytes
12 mtime Modify time of file
8 chksum Checksum for header
1 link Indicator for links
100 linkname Name of linked file

tar determines that the USTAR format is being used by the presence of the null-terminated string USTAR in the magic field. All fields before the magic field correspond to those of the UNIX format, except that typeflag replaces the link field.

Table 2. Archive file: USTAR format
Field width Field name Meaning
100 name Name of file
8 mode File mode
8 uid Owner user ID
8 gid Owner group ID
12 size Length of file in bytes
12 mtime Modify time of file
8 chksum Checksum for header
1 typeflag Type of file
100 linkname Name of linked file
6 magic USTAR indicator
2 version USTAR version
32 uname Owner user name
32 gname Owner group name
8 devmajor Device major number
8 devminor Device minor number
155 prefix Prefix for file name

Description of the header files

In the headers:
  • The name field contains the name of the archived file. On USTAR format archives, the value of the prefix field, if non-null, is prefixed to the name field to allow names longer than 100 characters.
  • The magic, uname, and gname fields are null-terminated character strings
  • The name, linkname, and prefix fields are null-terminated unless the full field is used to store a name (that is, the last character is not null).
  • All other fields are zero-filled octal numbers, in ASCII. Trailing nulls are present for these numbers, except for the size, mtime, and version fields.
  • prefix is null unless the file name exceeds 100 characters.
  • The size field is zero if the header describes a link.
  • The chksum field is a checksum of all the bytes in the header, assuming that the chksum field itself is all blanks.
  • For USTAR, the typeflag field is a compatible extension of the link field of the older tar format. The following values are recognized:
    Flag
    File Type
    0 or null
    Regular file
    1
    Link to another file already archived
    2
    Symbolic link
    3
    Character special file
    4
    Block special file (not supported)
    5
    Directory
    6
    FIFO special file
    7
    Reserved
    S
    z/OS extended USTAR special header
    T
    z/OS extended USTAR special header summary (S and T are z/OS extensions. See z/OS-extended USTAR support for more information.)
    A–Z
    Available for custom usage
  • In USTAR format, the uname and gname fields contain the name of the owner and group of the file, respectively.

Compressed tar archives are equivalent to the corresponding archive being passed to a 14-bit compress command.

Related information

The cpio and tar commands

Parent topic: File formats