Operating securely means having your data available reliably. Bugs, accidents, natural disasters, and attacks on your system cannot be predicted. Often, despite your best efforts, they can't be prevented. But if you have backups, you can compare your current system and your backed-up system, and you can restore your system to a stable state. Even if you lose your entire computer?to fire, for instance?with a good set of backups you can restore the information after you have purchased or borrowed a replacement machine. Insurance can cover the cost of a new CPU and disk drive, but your data is something that in many cases can never be replaced.
 This key concept is one reason why most professionals now refer to the field as information security rather than computer security or network security.
Backups can be very simple, such as a Zip disk in your desk drawer, or they can be exceedingly complex, such as a set of redundant drives located on opposite sides of town, connected by fiber channel, with a robotic tape changer that automatically cycles the tapes according to a predefined schedule.
Alas, Unix backup systems are generally less sophisticated than those for Windows systems and somewhat more difficult to use. Many Windows-based systems, for example, will automatically create a special "restore floppy" that you can use to automatically restore all of your computer's files onto a brand new hard drive. Few Unix systems provide such recovery tools. On the other hand, most Unix backup systems operate in a network-based environment, and many of them are free.
This chapter provides basic coverage of principles and programs for backing up Unix systems. An in-depth discussion of backup and restore systems would require another book?for this, we recommend W. Curtis Preston's book, Unix Backup & Recovery (O'Reilly).