Combinations of Exit Statuses

One of the more obscure parts of bash syntax allows you to combine exit statuses logically, so that you can test more than one thing at a time.

The syntax statement1 && statement2 means, "execute statement1, and if its exit status is 0, execute statement2." The syntax statement1 || statement2 is the converse: it means, "execute statement1, and if its exit status is not 0, execute statement2." At first, these look like "if/then" and "if not/then" constructs, respectively. But they are really intended for use within conditions of if constructs—as C programmers will readily understand.

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It's much more useful to think of these constructs as "and" and "or," respectively. Consider this:

if statement1 && statement2
then
    ...
fi

In this case, statement1 is executed. If it returns a 0 status, then presumably it ran without error. Then statement2 runs. The then clause is executed if statement2 returns a 0 status. Conversely, if statement1 fails (returns a non-zero exit status), then statement2 doesn't even run; the last statement that actually ran was statement1, which failed—so the then clause doesn't run, either. Taken all together, it's fair to conclude that the then clause runs if statement1 and statement2 both succeeded.

Similarly, consider this:

if statement1 || statement2
then
    ...
fi

If statement1 succeeds, then statement2 does not run. This makes statement1 the last statement, which means that the then clause runs. On the other hand, if statement1 fails, then statement2 runs, and whether the then clause runs or not depends on the success of statement2. The upshot is that the then clause runs if statement1 or statement2 succeeds.

bash also allows you to reverse the return status of a statement with the use of !, the logical "not". Preceding a statement with ! will cause it to return 0 if it fails and 1 if it succeeds. We'll see an example of this at the end of this chapter.

As a simple example of testing exit statuses, assume that we need to write a script that checks a file for the presence of two words and just prints a message saying whether either word is in the file or not. We can use grep for this: it returns exit status 0 if it found the given string in its input, non-zero if not:

filename=$1
word1=$2
word2=$3
    
if grep $word1 $filename || grep $word2 $filename
then
    echo "$word1 or $word2 is in $filename."
fi

The then clause of this code runs if either grep statement succeeds. Now assume that we want the script to say whether the input file contains both words. Here's how to do it:

filename=$1
word1=$2
word2=$3
    
if grep $word1 $filename && grep $word2 $filename
then
    echo "$word1 and $word2 are both in $filename."
fi

We'll see more examples of these logical operators later in this chapter.