Shell 脚本：如何在 if else 语句中执行多个条件

Question

How should I phrase an if statement with multiple conditions:

if [ <T/F Condition> ] && [ <T/F Condition> ] && [ <T/F Condition> ]

or

if [ <T/F Condition> && <T/F Condition> && <T/F Condition>]

?

Answer 1

As "man test" would've shown you "-a" stands for "and".

eg.:

if [ <T/F Condition> -a <T/F Condition> -a <T/F Condition> ]

Watch the spacing too.

Answer 2

The && is a shell operator, in cmd1 && cmd2 , it tells the shell to only run the second command if the first succeeds.

So, this works, and does what you want:

if [ "$x" = a ] && [ "$y" = b ]; then ...

However, in this:

if [ "$x" = a && "$y" = b ]; then ...

The commands to run would be [ "$x" = a and "$y" = b ] , the first of which will give an error for the missing ] argument. (Here, it's good to remember that [ is a regular command, similar to echo or so, it just has a funny name.)

Also, you should avoid this:

if [ "$x" = a -a "$y" = b ]; then ...

even though it works in some shells in simple cases. This has to do with parsing issues when the variables contain strings like ! (negation) or others that are operators for [ . Again, since [ is a regular command, and not special shell syntax, it can't tell which arguments come from variables and which are hardcoded operators.