第96页 | Learning the Bash Shell | 阅读 ‧ 电子书库

if/else

The simplest type of flow control construct is the conditional, embodied in bash's if statement. You use a conditional when you want to choose whether or not to do something, or to choose among a small number of things to do, according to the truth or falsehood of conditions. Conditions test values of shell variables, characteristics of files, whether or not commands run successfully, and other factors. The shell has a large set of built-in tests that are relevant to the task of shell programming.

The if construct has the following syntax:

if condition
then
    statements
[elif condition
    then statements...]
[else
 statements]
fi

The simplest form (without the elif and else parts, or clauses) executes the statements only if the condition is true. If you add an else clause, you get the ability to execute one set of statements if a condition is true or another set of statements if the condition is false. You can use as many elif (a contraction of "else if") clauses as you wish; they introduce more conditions, and thus more choices for which set of statements to execute. If you use one or more elifs, you can think of the else clause as the "if all else fails" part.