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As described above, a tar archive keeps, for each member it contains,
its metadata, such as modification time, mode and ownership of
the file. GNU tar allows to replace these data with other values
when adding files to the archive. The options described in this
section affect creation of archives of any type. For POSIX archives,
see also Controlling Extended Header Keywords, for additional ways of controlling
metadata, stored in the archive.
When adding files to an archive, tar will use
permissions for the archive members, rather than the permissions
from the files. permissions can be specified either as an octal
number or as symbolic permissions, like with
chmod (See File permissions in GNU core utilities. This reference
also has useful information for those not being overly familiar with
the UNIX permission system). Using latter syntax allows for
more flexibility. For example, the value ‘a+rw’ adds read and write
permissions for everybody, while retaining executable bits on directories
or on any other file already marked as executable:
$ tar -c -f archive.tar --mode='a+rw' .
When adding files to an archive, tar uses date as
the modification time of members when creating archives, instead of
their actual modification times. The argument date can be
either a textual date representation in almost arbitrary format
(see section Date input formats) or a name of an existing file, starting
with ‘/’ or ‘.’. In the latter case, the modification time
of that file is used.
The following example sets the modification date to 00:00:00 UTC on January 1, 1970:
$ tar -c -f archive.tar --mtime='@0' .
When used with ‘--verbose’ (see section The ‘--verbose’ Option) GNU tar
converts the specified date back to a textual form and compares it
with the one given with ‘--mtime’.
If the two forms differ, tar prints both forms in a message,
to help the user check that the right date is being used.
For example:
$ tar -c -f archive.tar -v --mtime=yesterday . tar: Option --mtime: Treating date 'yesterday' as 2006-06-20 13:06:29.152478 …
When used with ‘--clamp-mtime’ GNU tar sets the
modification date to date only on files whose actual modification
date is later than date. This makes it easier to build
reproducible archives given a common timestamp for generated files
while still retaining the original timestamps of untouched files.
See section Making tar Archives More Reproducible.
$ tar -c -f archive.tar --clamp-mtime --mtime="$SOURCE_EPOCH" .
Specifies that tar should use user as the owner of members
when creating archives, instead of the user associated with the source
file.
If user contains a colon, it is taken to be of the form name:id where a nonempty name specifies the user name and a nonempty id specifies the decimal numeric user ID. If user does not contain a colon, it is taken to be a user number if it is one or more decimal digits; otherwise it is taken to be a user name.
If a name is given but no number, the number is inferred from the current host’s user database if possible, and the file’s user number is used otherwise. If a number is given but no name, the name is inferred from the number if possible, and an empty name is used otherwise. If both name and number are given, the user database is not consulted, and the name and number need not be valid on the current host.
There is no value indicating a missing number, and ‘0’ usually means
root. Some people like to force ‘0’ as the value to offer in
their distributions for the owner of files, because the root user is
anonymous anyway, so that might as well be the owner of anonymous
archives. For example:
$ tar -c -f archive.tar --owner=0 .
or:
$ tar -c -f archive.tar --owner=root .
Files added to the tar archive will have a group ID of group,
rather than the group from the source file. As with ‘--owner’,
the argument group can be an existing group symbolic name, or a
decimal numeric group ID, or name:id.
The ‘--owner’ and ‘--group’ options affect all files
added to the archive. GNU tar provides also two options that allow
for more detailed control over owner translation:
Read UID translation map from file.
When reading, empty lines are ignored. The ‘#’ sign, unless quoted, introduces a comment, which extends to the end of the line. Each nonempty line defines mapping for a single UID. It must consist of two fields separated by any amount of whitespace. The first field defines original username and UID. It can be a valid user name or a valid UID prefixed with a plus sign. In both cases the corresponding UID or user name is inferred from the current host’s user database.
The second field defines the UID and username to map the original one to. Its format can be the same as described above. Otherwise, it can have the form newname:newuid, in which case neither newname nor newuid are required to be valid as per the user database.
For example, consider the following file:
+10 bin smith root:0
Given this file, each input file that is owner by UID 10 will be stored in archive with owner name ‘bin’ and owner UID corresponding to ‘bin’. Each file owned by user ‘smith’ will be stored with owner name ‘root’ and owner ID 0. Other files will remain unchanged.
When used together with ‘--owner-map’, the ‘--owner’ option affects only files whose owner is not listed in the map file.
Read GID translation map from file.
The format of file is the same as for ‘--owner-map’ option:
Each nonempty line defines mapping for a single GID. It must consist of two fields separated by any amount of whitespace. The first field defines original group name and GID. It can be a valid group name or a valid GID prefixed with a plus sign. In both cases the corresponding GID or user name is inferred from the current host’s group database.
The second field defines the GID and group name to map the original one to. Its format can be the same as described above. Otherwise, it can have the form newname:newgid, in which case neither newname nor newgid are required to be valid as per the group database.
When used together with ‘--group-map’, the ‘--group’ option affects only files whose owner group is not rewritten using the map file.
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