UNIX® Shells by Example, Third Edition

12.5. Conditional Constructs and Flow Control

12.5.1. Exit Status

Conditional commands allow you to perform some task(s) based on whether a condition succeeds or fails. The if command is the simplest form of decision-making; the if/else commands allow a two-way decision; and the if/elif/else commands allow a multiway decision.

Bash allows you to test two types of conditions: the success or failure of commands or whether an expression is true or false. In either case, the exit status is always used. An exit status of zero indicates success or true, and an exit status that is nonzero indicates failure or false. The ? status variable contains a numeric value representing the exit status. To refresh your memory on how exit status works, look at ...