tar(4)							     Kernel Interfaces Manual							    tar(4)

NAME

       tar, pax - Archive file format

DESCRIPTION

       The  cpio, pax, and tar commands dump several files into one, in a medium suitable for transportation.  This file is often referred to as a
       tar file, or tar tape, since it was originally designed to be on an archive tape.

       An archive file is a series of blocks, with each block of size TBLOCK.  A file on the archive  is  represented  by  a  header  block  which
       describes the file, followed by zero or more blocks which give the contents of the file.  At the end of the tape are two blocks filled with
       binary zeros, as an end-of-file indicator.

       The blocks are grouped for physical I/O operations.  Each group of n blocks (where n is set by the b keyletter on  the  tar  command  line,
       with  a default of 20 blocks) is written with a single system call.  On nine-track tapes, the result of this write is a single tape record.
       The last group is always written at the full size, so blocks after the two zero blocks contain random data.  On reading, the  specified	or
       default	group  size  is  used for the first read, but if that read returns less than a full tape block, the reduced block size is used for
       further reads.

       The header block looks like:

       -------------------------------------
       Field Name     Offset	   Length
       -------------------------------------
       name		 0	    100
       mode	       100	      8
       uid	       108	      8
       gid	       116	      8
       size	       124	     12
       mtime	       136	     12
       chksum	       148	      8
       typeflag        156	      1
       linkname        157	    100
       magic	       257	      6
       version	       263	      2
       uname	       265	     32
       gname	       297	     32
       devmajor        329	      8
       devminor        337	      8
       prefix	       345	    155
       -------------------------------------

       The name field is the name of the file, as specified on the command line.  Files dumped because they were in a directory that was named	on
       the command line have the directory name as prefix and /filename as suffix.

       The mode field is the file mode, with the top bit masked off.

       The uid and gid fields are the user id and group id numbers that own the file.

       The size field is the size of the file in bytes.  Links and symbolic links are dumped with this field specified as zero.

       The mtime field is the modification time of the file at the time it was dumped.

       The chksum field is an octal ASCII value which represents the sum of all the bytes in the header block.	When calculating the checksum, the
       chksum field is treated as if it were all blanks.

       The typeflag field identifies the type of data following the header.  Valid values are: A Global  Extended  Header  follows.   An  Extended
       Header follows.	Archive data follows.

       The  Global  Extended Header and Extended Header formats are only produced when the tar command is used with the -E option, or the pax com-
       mand is used with the -x xtar option.

       The linkname field is the name of the file that this file is linked to, if any.	If this field is empty, the file is not linked.

       The first time a given i-node number is dumped, it is dumped as a regular file.	Subsequently, it  is  dumped  as  a  link  instead.   Upon
       retrieval,  if  a  link entry is retrieved but the file it was linked to is not, an error message is printed and the tape must be rescanned
       manually to retrieve the file that it is linked to.

       The magic field is the magic number for the file type as described in the magic(4) reference page.

       The version field is always 00 (zero-zero).

       The uname and gname fields are the user name and group names that own the file.

       The devmajor and devminor fields are the device major and minor numbers as described in the mknod(8) reference page.

       The name, linkname, and prefix fields are null-terminated strings, except when all characters in the field are  non-null  characters.   The
       typeflag field is a single character.  The other fields are zero-filled octal numbers in ASCII format terminated by a null character.

       Unused fields of the header are binary zeros (and are included in the checksum).

NOTES

       The  encoding  of  the header is designed to be portable across platforms.  If filenames are chosen that use characters not in the portable
       filename character set, results are unpredictable.

EXAMPLES

       The layout of an archive with a Global Extended Header, two data files, and the end of archive is shown here.

       -----------------------------------
	 ustar Header (typeflag=g)
	Global Extended Header Data
       -----------------------------------
	 ustar Header (typeflag=x)
	    Extended Header Data
	 ustar Header (typeflag=0)
	      Data for File 1
       -----------------------------------
	 ustar Header (typeflag=0)
	      Data for File 2
       -----------------------------------
	 ustar Header (typeflag=0)
	      Data for File n
       -----------------------------------
	   Block of binary zeroes
       -----------------------------------
	   Block of binary zeroes
       -----------------------------------

RELATED INFORMATION

       Commands: cpio(1), mknod(8), pax(1), tar(1)

       Files: magic(4) delim off

																	    tar(4)