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tar(4)									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  pro‐
       duced  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 regu‐
       lar 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 man‐
       ually 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 file‐
       name 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

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

       Files: magic(4)

									tar(4)
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