Restoring Files from a Backup

Chapter 17, "Protecting Your System with Backups," describes system backup procedures. That chapter also includes information on emergency recovery procedures—restoring most or all of a working system from a backup. Such procedures are useful after a disk failure, security breach, or a seriously damaging administrative blunder. System backups can also be very useful in restoring deleted files. In this scenario, an accidentally deleted file can be restored from a backup. One drawback to this procedure is that the original file must have existed prior to the last regular system backup. If your backups are infrequent, the file might not exist. Even if you make daily backups, this procedure is unlikely to help if a user creates a file, quickly deletes it, and then wants it back immediately. A trash can utility is the best protection against that sort of damage.

As an example, suppose you create backups to tape using tar. You can recover files from this backup by using the -extract (-x) command. Typically, you also pass the -verbose (-v) option so that you know when the target file has been restored, and you use -file (-f) to point to the tape device file. You must also pass the name of the file to be restored:

# tar -xvf /dev/stO home/al/election.txt

This command recovers the file home/al/election.txt from the /dev/stO tape device. A few points about this command require attention:

Permissions The user who runs the command must have read/write access to the tape device. This user must also have write permission to the restore directory (normally, the current directory). Therefore, root normally runs this command, although other users may have sufficient privileges on some systems. Ownership and permissions on the restored file may change if a user other than root runs the command.

Filename Specification The preceding command omitted the leading slash (/) in the target filename specification (home/al/election.txt). This is because tar normally strips this slash when it writes files, so when you specify files for restoration, the slash must also be missing. A few utilities and methods of creating a backup add a leading ./ to the filename. If your backups include this feature, you must include it in the filename specification to restore the file.

Restore Directory Normally, tar restores files to the current working directory. Thus, if you type the preceding command while in /root, it will create a /root/home/al/election.txt file (assuming it's on the tape). I recommend restoring to an empty subdirectory and then moving the restored file to its intended target area. This practice minimizes the risk that you might mistype the target file specification and overwrite a newer file with an older one, or even overwrite the entire Linux installation with the backup.

Unfortunately, tar requires that you have a complete filename, including its path, ready in order to recover a file. If you don't know the exact filename, you can try taking a directory of the tape by typing tar tvf /dev/stO (substituting another tape device filename, if necessary). You may want to pipe the result through less or grep to help you search for the correct filename, or redirect it to a file you can search.

Tip You can keep a record of files on a tape at backup time to simplify searches at restore time. Using the -verbose option and redirecting the results to a file will do the trick. Some incremental backup methods automatically store information on a backup's contents, too. Some backup tools, such as the commercial Backup/Recover Utility (BRU; http://www.bru.com), store an index of files on the tape. This index enables you to quickly scan the tape and select files for recovery from the index.

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