Making backups of your system is an important way to protect yourself from data corruption or loss in case you have problems with your hardware, or you make a mistake such as deleting important files inadvertently. During your experiences with Linux, you're likely to make quite a few customizations to the system that can't be restored by simply reinstalling from your original installation media. However, if you happen to have your original Linux floppies or CD-ROM handy, it may not be necessary to back up your entire system. Your original installation media already serve as an excellent backup.
Under Linux, as with any Unix-like system, you can make mistakes while logged in as root that would make it impossible to boot the system or log in later. Many newcomers approach such a problem by reinstalling the system entirely from backup, or worse, from scratch. This is seldom, if ever, necessary. In Section 8.6, later in this chapter, we'll talk about what to do in these cases.
If you do experience data loss, it is sometimes possible to recover that data using the filesystem maintenance tools described in Section 6.1.5 in Chapter 6. Unlike some other operating systems, however, it's generally not possible to "undelete" a file that has been removed by rm or overwritten by a careless cp or mv command (for example, copying one file over another destroys the file to which you're copying). In these extreme cases, backups are key to recovering from problems.
Backups are usually made to tape, floppy or CD-R(W). None of these media is 100% reliable, although tape and CD-R(W) are more dependable than floppy in the long term. Many tools are available that help you make backups. In the simplest case, you can use a combination of gzip (or bzip2) and tar to back up files from your hard drive to floppy or tape. This is the best method to use when you make only occasional backups, no more often than, say, once a month.
If you have numerous users on your system or you make frequent changes to the system configuration, it makes more sense to employ an incremental backup scheme. Under such a scheme, you would take a "full backup" of the system only about once a month. Then, every week, you would back up only those files that changed in the last week. Likewise, each night, you could back up just those files that changed over the previous 24 hours. There are several tools to aid you in this type of backup.
The idea behind an incremental backup is that it is more efficient to take backups in small steps; you use fewer floppies, tapes, or CDs, and the weekly and nightly backups are shorter and easier to run. With this method, you have a backup that is at most a day old. If you were to, say, accidentally delete your entire system, you would restore it from backup in the following manner:
1. Restore from the most recent monthly backup. For instance, if you wiped the system on July 17, you would restore the July 1 full backup. Your system now reflects the state of files when the July 1 backup was made.
2. Restore from each weekly backup made so far this month. In our case, we could restore from the two weekly backups from July 7 and 14. Restoring each weekly backup updates all the files that changed during that week.
3. Restore from each daily backup during the last week — that is, since the last weekly backup. In this case, we would restore the daily backups from July 15 and 16. The system now looks as it did when the daily backup was taken on July 16; no more than a day's worth of files have been lost.
Depending on the size of your system, the full monthly backup might require 4 GB or more of backup storage — often not more than one tape using today's tape media, but quite a few ZIP disks. However, the weekly and daily backups would generally require much less storage space. Depending on how your system is used, you might decide to take the weekly backup on Sunday night and not bother with daily backups for the weekend.
One important characteristic that backups should (usually) have is the ability to select individual files from the backup for restoration. This way, if you accidentally delete a single file or group of files, you can simply restore those files without having to do a full system restoration. Depending on how you take backups, however, this task will be either very easy or painfully difficult.
In this section, we're going to talk about the use of tar, gzip, and a few related tools for making backups to floppy and tape. We'll even cover the use of floppy and tape drives in the bargain. These tools allow you to take backups more or less "by hand"; you can automate the process by writing shell scripts and even schedule your backups to run automatically during the night using cron. All you have to do is flip tapes. Other software packages provide a nice menu-driven interface for creating backups, restoring specific files from backup, and so forth. Many of these packages are, in fact, nice frontends to tar and gzip. You can decide for yourself what kind of backup system suits you best.
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