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Backup and recovery

tar

tar is a classic UNIX command that has been ported into Linux. tar is short for tape archive, and was originally designed for packaging files onto tape. You have probably already encountered tar files if you have downloaded any source code for Linux. It is a file-based command that essentially serially stacks the files end to end.

Entire directory trees can be packaged with tar, which makes it especially suited to backups. Archives can be restored in their entirety, or files and directories can be expanded individually. Backups can go to file-based devices or tape devices. Files can be redirected upon restoration to replace to a different directory (or system) from where they were originally saved. tar is file system-independent. It can be used on ext2, ext3, jfs, Reiser, and other file systems.

Using tar is very much like using a file utility, such as PKZip. You point it toward a destination, which is a file or a device, and then name the files that you want to package. You can compress archives on the fly with standard compression types, or specify an external compression program of your choice. To compress or uncompress files through bzip2, use tar -z.

To back up the entire file system using tar to a SCSI tape drive, excluding the /proc directory:

tar -cpf /dev/st0 / --exclude=/proc

In the above example, the -c switch indicates that the archive is being created. The -p switch indicates that we want to preserve the file permissions, critical for a good backup. The -f switch points to the filename for the archive. In this case, we are using the raw tape device, /dev/st0. The / indicates what we want to back up. Since we wanted the entire file system, we specified the root. tar automatically recurses when pointed to a directory (ending in a /). Finally, we exclude the /proc directory, since it doesn't contain anything we need to save. If the backup will not fit on a single tape, we will add the -M switch (not shown), for multi-volume.

Just in case   Don't forget that Linux is case sensitive. The tar command should always be executed in lowercase, for example. Switches can be upper, lower, or mixed case. For example -t and -T perform different functions. File or directory names may be mixed case and, like commands and switches, are case sensitive.

To restore a file or files, the tar command is used with the extract switch (-x):

tar -xpf /dev/st0 -C /

The -f switch again points to our file, and -p indicates that we want to restore archived permissions. The -x switch indicates an extraction of the archive. The -C / indicates that we want the restore to occur from /. tar normally restores to the directory from which the command is run. The -C switch makes our current directory irrelevant.

The two other tar commands that you will probably use often are the -t and -d switches. The -t switch lists the contents of an archive. The -d switch compares the contents of the archive to current files on a system.

For ease of operation and editing, you can put the files and directories that you want to archive in a text file, which you reference with the -T switch. These can be combined with other directories listed on the command line. The following line backs up all the files and directories listed in MyFiles, the /root directory, and all of the iso files in the /tmp directory:

tar -cpf /dev/st0 -T MyFiles /root /tmp/*.iso

The file list is simply a text file with the list of files or directories. Here's an example:

/etc
/var
/home
/usr/local
/opt

Please note that the tar -T (or files-from) command cannot accept wildcards. Files must be listed explicitly. The example above shows one way to reference files separately. You could also execute a script to search the system and then build a list. Here is an example of such a script:

#!/bin/sh
cat MyFiles > TempList
find /usr/share -iname *.png >> TempList
find /tmp -iname *.iso >> TempList
tar -cpzMf /dev/st0 -T TempList

The above script first copies all of our existing file list from MyFiles to TempList. Then it executes a couple of find commands to search the file system for files that match a pattern and to append them to the TempList. The first search is for all files in the /usr/share directory tree that end in .png. The second search is for all files in the /tmp directory tree that end in .iso. Once the list is built, then tar is run to create a new archive on the file device /dev/st0 (the first SCSI tape drive), which is compressed using the gzip format and retains all of the file permissions. The archive will span Multiple volumes. The file names to be archived will be Taken from the file TempList.

Scripting can also be used to perform much more elaborate actions such as incremental backups. An excellent script is listed by Gerhard Mourani in his book Securing and Optimizing Linux, which you will find listed in the Resources section at the end of this article.

Scripts can also be written to restore files, though restoration is often done manually. As mentioned above, the -x switch for extract replaces the -c switch. Entire archives can be restored, or individual files or directories can be specified. Wildcards are okay to reference files in the archive. You can also use switches to dump and restore.