(tar)What tar Does

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Introduction

Introduction

What `tar' Does
===============

   The `tar' program is used to create and manipulate `tar' archives.
An "archive" is a single file which contains within it the contents of
many files.  In addition, the archive identifies the names of the
files, their owner, and so forth.  (Archives record access permissions,
user and group, size in bytes, and last modification time.  Some
archives also record the file names in each archived directory, as well
as other file and directory information.)

   The files inside an archive are called "members".  Within this
manual, we use the term "file" to refer only to files accessible in the
normal ways (by `ls', `cat', and so forth), and the term "members" to
refer only to the members of an archive.  Similarly, a "file name" is
the name of a file, as it resides in the filesystem, and a "member
name" is the name of an archive member within the archive.

   Initially, `tar' archives were used to store files conveniently on
magnetic tape.  The name `tar' comes from this use; it stands for:
`t'ape `ar'chiver.  Despite the utility's name, `tar' can direct its
output to any available device, as well as store it in a file or direct
it to another program via a pipe.  `tar' may even access, as archives,
remote devices or files.

   You can use `tar' archives in many ways.  We want to stress a few of
them: storage, backup or transportation.

Storage
     Often, `tar' archives are used to store related files for
     convenient file transfer over a network.  For example, the GNU
     Project distributes its software bundled into `tar' archives, so
     that all the files relating to a particular program (or set of
     related programs) can be transferred as a single unit.

     A magnetic tape can store several files in sequence, but has no
     names for them, just relative position on the tape.  A `tar'
     archive or something like it is one way to store several files on
     one tape and retain their names.  Even when the basic transfer
     mechanism can keep track of names, as FTP can, the nuisance of
     handling multiple files, directories, and multiple links, makes
     `tar' archives an attractive method.

     Archive files are also used for long-term storage, which you can
     think of as transportation from one time to another.

Backup
     Because the archive created by `tar' is capable of preserving file
     information and directory structure, `tar' is commonly used for
     performing full and incremental backups of disks, putting all
     together bunch of files possibly pertaining to many users and
     different projects, to secure against accidental destruction of
     those disks.

     The GNU version of `tar' has special features that allow it to be
     used to make incremental and full dumps of all the files in a
     filesystem.

Transportation
     Archive files can be used for transporting a group of files from
     one system to another: put all relevant files into an archive on
     one computer system, transfer the archive to another, and extract
     the contents there.  The basic transfer medium might be magnetic
     tape, Internet FTP, or even electronic mail (though you must
     encode the archive with `uuencode' or some functional equivalent
     in order to transport it properly by mail).  Both machines do not
     have to use the same operating system, as long as they both
     support the `tar' program.

     Piping one `tar' to another is an easy way to copy a directory's
     contents from one disk to another, while preserving the dates,
     modes, owners and link structure of all the files therein.  `tar'
     is also ideal for transferring directories over networks.  We
     sometimes see a copy of `tar' packing many files into one archive
     on one machine, and sending the produced archive over a pipe over
     the network to another copy of `tar' on another machine, reading
     its archive from the pipe and unpacking all files there.

   The `tar' program provides the ability to create `tar' archives, as
well as for various other kinds of manipulation.  For example, you can
use `tar' on previously created archives to extract files, to store
additional files, or to update or list files already stored.  The term
"extraction" is used to refer to the process of copying an archive
member into a file in the filesystem.  One might speak of extracting a
single member.  Extracting all the members of an archive is often
called extracting the archive.  Also, the term "unpack" is used to
refer to the extraction of many or all the members of an archive.

   Conventionally, `tar' archives are given names ending with `.tar'.
This is not necessary for `tar' to operate properly, but this manual
follows the convention in order to get the reader used to seeing it.

   Occasionally, `tar' archives are referred to as `tar' files, archive
members are referred to as files, or entries.  For people familiar with
the operation of `tar', this causes no difficulty.  However, this
manual consistently uses the terminology above in referring to archives
and archive members, to make it easier to learn how to use `tar'.