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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: tape archiver. 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.
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.
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.
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.
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