bash Cookbook

Using Fewer if Statements

Problem

As a conscientious programmer, you took to heart what we described in the previous recipe, Deciding Whether a Command Succeeds. You applied the concept to your latest shell script, and now you find that the shell script is unreadable, if with all those if statements checking the return code of every command. Isn’t there an alternative?

Solution

Use the double-ampersand operator in bash to provide conditional execution:

$ cd mytmp && rm *

Discussion

Two commands separated by the double ampersands tells bash to run the first command and then to run the second command only if the first command succeeds (i.e., its exit status is 0). This is very much like using an if statement to check the exit status of the first command in order to protect the running of the second command:

cd mytmp
if (( $? == 0 )); then rm * ; fi

The double ampersand syntax is meant to be reminiscent of the logical and operator in C Language. If you know your logic (and your C) then you’ll recall that if you are evaluating the logical expression A AND B, then the entire expression can only be true if both (sub)expression A and (sub)expression B evaluate to true. If either one is false, the whole expression is false. C Language makes use of this fact, and when you code an expression like if (A && B) { … }, it will evaluate expression A first. If it is false, it won’t even bother to evaluate B since the overall outcome (false) has already been determined (by A being false).

So what does this ...