9.1.1.1. Archiving with tar

In most cases, we will first collect all the data to back up in a single archive file, which we will compress later on. The process of archiving involves concatenating all listed files and taking out unnecessary blanks. In Linux, this is commonly done with the tar command. tar was originally designed to archive data on tapes, but it can also make archives, known as tarballs.

tar has many options, the most important ones are cited below:

• -v: verbose

• -t: test, shows content of a tarball

• -x: extract archive

• -c: create archive

• -f archivedevice: use archivedevice as source/destination for the tarball, the device defaults to the first tape device (usually /dev/st0 or something similar)

• -j: filter through bzip2, see Section 9.1.1.2

It is common to leave out the dash-prefix with tar options, as you can see from the examples below.

Use GNU tar for compatibility

The archives made with a proprietary tar version on one system, may be incompatible with tar on another proprietary system. This may cause much headaches, such as if the archive needs to be recovered on a system that doesn't exist anymore. Use the GNU tar version on all systems to prevent your system admin from bursting into tears. Linux always uses GNU tar. When working on other UNIX machines, enter tar --help to find out which version you are using. Contact your system admin if you don't see the word GNU somewhere.

In the example below, an archive is created and unpacked.

gaby:~> ls images/
me+tux.jpg  nimf.jpg

gaby:~> tar cvf images-in-a-dir.tar images/
images/
images/nimf.jpg
images/me+tux.jpg

gaby:~> cd images

gaby:~/images> tar cvf images-without-a-dir.tar *.jpg
me+tux.jpg
nimf.jpg

gaby:~/images> cd

gaby:~> ls */*.tar
images/images-without-a-dir.tar

gaby:~> ls *.tar
images-in-a-dir.tar 

gaby:~> tar xvf images-in-a-dir.tar 
images/
images/nimf.jpg
images/me+tux.jpg

gaby:~> tar tvf images/images-without-dir.tar 
-rw-r--r-- gaby/gaby  42888 1999-06-30 20:52:25 me+tux.jpg
-rw-r--r-- gaby/gaby   7578 2000-01-26 12:58:46 nimf.jpg

gaby:~> tar xvf images/images-without-a-dir.tar 
me+tux.jpg
nimf.jpg

gaby:~> ls *.jpg
me+tux.jpg  nimf.jpg

This example also illustrates the difference between a tarred directory and a bunch of tarred files. It is advisable to only compress directories, so files don't get spread all over when unpacking the tarball (which may be on another system, where you may not know which files were already there and which are the ones from the archive).

When a tape drive is connected to your machine and configured by your system administrator, the file names ending in .tar are replaced with the tape device name, for example:

tar cvf /dev/tape mail/

The directory mail and all the files it contains are compressed into a file that is written on the tape immediately. A content listing is displayed because we used the verbose option.