tar, pax - Archive file format
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 command 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 nonnull
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).
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.
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
Commands: cpio(1), mknod(8), pax(1), tar(1)
Files: magic(4)
tar(4)
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