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Tar archives contain detailed information about files stored
in them and full file names are part of that information. When
storing a file to an archive, its file name is recorded in it,
along with the actual file contents. When restoring from an archive,
a file is created on disk with exactly the same name as that stored
in the archive. In the majority of cases this is the desired behavior
of a file archiver. However, there are some cases when it is not.
First of all, it is often unsafe to extract archive members with
absolute file names or those that begin with a `../'. GNU
takes special precautions when extracting such names and provides a
special option for handling them, which is described in
Absolute File Names.
Secondly, you may wish to extract file names without some leading directory components, or with otherwise modified names. In other cases it is desirable to store files under differing names in the archive.
tar provides several options for these needs.
Strip given number of leading components from file names before extraction.
For example, suppose you have archived whole `/usr' hierarchy to a tar archive named `usr.tar'. Among other files, this archive contains `usr/include/stdlib.h', which you wish to extract to the current working directory. To do so, you type:
$ tar -xf usr.tar --strip=2 usr/include/stdlib.h
The option `--strip=2' instructs
tar to strip the
two leading components (`usr/' and `include/') off the file
If you add the `--verbose' (`-v') option to the invocation
above, you will note that the verbose listing still contains the
full file name, with the two removed components still in place. This
can be inconvenient, so
tar provides a special option for
altering this behavior:
Display file or member names with all requested transformations applied.
$ tar -xf usr.tar -v --strip=2 usr/include/stdlib.h usr/include/stdlib.h $ tar -xf usr.tar -v --strip=2 --show-transformed usr/include/stdlib.h stdlib.h
Notice that in both cases the file `stdlib.h' is extracted to the current working directory, `--show-transformed-names' affects only the way its name is displayed.
This option is especially useful for verifying whether the invocation will have the desired effect. Thus, before running
$ tar -x --strip=n
it is often advisable to run
$ tar -t -v --show-transformed --strip=n
to make sure the command will produce the intended results.
In case you need to apply more complex modifications to the file name,
tar provides a general-purpose transformation option:
Modify file names using supplied expression.
The expression is a
sed-like replace expression of the
where regexp is a regular expression, replace is a replacement for each file name part that matches regexp. Both regexp and replace are described in detail in The "s" Command: (sed)The "s" Command section `The `s' Command' in GNU sed.
Any delimiter can be used in lieu of `/', the only requirement being that it be used consistently throughout the expression. For example, the following two expressions are equivalent:
Changing delimiters is often useful when the regex contains
slashes. For example, it is more convenient to write
sed, you can give several replace expressions,
separated by a semicolon.
Supported flags are:
Apply the replacement to all matches to the regexp, not just the first.
Use case-insensitive matching.
regexp is an extended regular expression (see Extended regular expressions: (sed)Extended regexps section `Extended regular expressions' in GNU sed).
Only replace the numberth match of the regexp.
Note: the POSIX standard does not specify what should happen
when you mix the `g' and number modifiers. GNU
follows the GNU
sed implementation in this regard, so
the interaction is defined to be: ignore matches before the
numberth, and then match and replace all matches from the
In addition, several transformation scope flags are supported, that control to what files transformations apply. These are:
Apply transformation to regular archive members.
Do not apply transformation to regular archive members.
Apply transformation to symbolic link targets.
Do not apply transformation to symbolic link targets.
Apply transformation to hard link targets.
Do not apply transformation to hard link targets.
Default is `rsh', which means to apply transformations to both archive members and targets of symbolic and hard links.
Default scope flags can also be changed using `flags=' statement in the transform expression. The flags set this way remain in force until next `flags=' statement or end of expression, whichever occurs first. For example:
Here are several examples of `--transform' usage:
$ tar --transform='s,usr/,usr/local/,' -x -f arch.tar
$ tar --transform='s,/*[^/]*/[^/]*/,,' -x -f arch.tar
$ tar --transform 's/.*/\L&/' -x -f arch.tar
$ tar --transform 's,^,/prefix/,' -x -f arch.tar
$ tar --transform 's,^,/usr/local/,S' -c -f arch.tar /lib
Notice the use of flags in the last example. The `/lib' directory often contains many symbolic links to files within it. It may look, for example, like this:
$ ls -l drwxr-xr-x root/root 0 2008-07-08 16:20 /lib/ -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /lib/libc-2.3.2.so lrwxrwxrwx root/root 0 2008-06-24 17:12 /lib/libc.so.6 -> libc-2.3.2.so ...
Using the expression `s,^,/usr/local/,' would mean adding `/usr/local' to both regular archive members and to link targets. In this case, `/lib/libc.so.6' would become:
/usr/local/lib/libc.so.6 -> /usr/local/libc-2.3.2.so
This is definitely not desired. To avoid this, the `S' flag is used, which excludes symbolic link targets from filename transformations. The result is:
$ tar --transform 's,^,/usr/local/,S', -c -v -f arch.tar \ --show-transformed /lib drwxr-xr-x root/root 0 2008-07-08 16:20 /usr/local/lib/ -rwxr-xr-x root/root 1250840 2008-05-25 07:44 /usr/local/lib/libc-2.3.2.so lrwxrwxrwx root/root 0 2008-06-24 17:12 /usr/local/lib/libc.so.6 \ -> libc-2.3.2.so
Unlike `--strip-components', `--transform' can be used
in any GNU
tar operation mode. For example, the following command
adds files to the archive while replacing the leading `usr/'
component with `var/':
$ tar -cf arch.tar --transform='s,^usr/,var/,' /
To test `--transform' effect we suggest using `--show-transformed-names' option:
$ tar -cf arch.tar --transform='s,^usr/,var/,' \ --verbose --show-transformed-names /
If both `--strip-components' and `--transform' are used together, then `--transform' is applied first, and the required number of components is then stripped from its result.
You can use as many `--transform' options in a single command line as you want. The specified expressions will then be applied in order of their appearance. For example, the following two invocations are equivalent:
$ tar -cf arch.tar --transform='s,/usr/var,/var/' \ --transform='s,/usr/local,/usr/,' $ tar -cf arch.tar \ --transform='s,/usr/var,/var/;s,/usr/local,/usr/,'
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