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Compound conditionals

Short-circuit operators && and ||

BASH allows you to combine exit statuses logically, so that you can test more than one thing at a time:

In both cases it's useful to think about them as "short-circuit and" and "short-circuit or," respectively:

In if statement consists of two conditions (which in turn can be statements ;-) connected by && like in following example

if condition1 && condition2

condition1 is always executed and the result is used to decide if condition 2 should be evaluated. If and only if  the first statement returns a return code 0 conditions2 is evaluated (executed). The then clause will be executed only if  both succeeded.

In case of short-circuit or the situation is similar:

if statement1 || statement2

statement1 is always executed. If it returns code zero (success), then statement2 will  not be executed.  Otherwise statement2 will be executed and its return code will be used for the deciding whether to execute then clause of the if statement or else clause. In other words then clause runs either statement1 or statement2  returns a zero code.

Note: These constructs can be used outside if statement as well. They provide an elegant way to implement if statement with only else clause.  

Let's assume that we need to write a script that checks a /etc/passwd file for the presence of two users who left the company.  We can use grep for this: it returns exit status 0 if it found the given string in its input, non-0 if not:

if grep $user /etc/passwd || -e $home/$user
    print "user $user is not fully removed from the server."

You can use the double-ampersand operator in bash to provide conditional execution:

	$ cd mytmp && rm *

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 (( $? )); 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 have to do with bash? Well, if the exit status of the first command (the one to the left of the &&) is non-zero (i.e., failed) then it won't bother to evaluate the second expression—i.e., it won't run the other command at all.

If you want to be thorough about your error checking, but don't want if statements all over the place, you can have bash exit any time it encounters a failure (i.e., a non-zero exit status) from every command in your script (except in whileloops and if statements where it is already capturing and using the exit status) by setting the -e flag.

	set -e
	cd mytmp
	rm *
Setting the -e flag will cause the shell to exit when a command fails. If the cd fails, the script will exit and never even try to execute the rm* command. We don't recommend doing this on an interactive shell, because when the shell exits it will make your shell window go away.



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