What is the order of evaluation of the sentences of a script in bash?

manpreet Best Answer 3 years ago

from left to right? or vice versa?

example:

If [! -z "$ foo"] && [$ foo2 -eq 0]; then
    echo "something"
fi

What is the evaluation done first?

[! -z "$ foo"]

or

[$ foo2 -eq 0]

thanks!

manpreet 3 years ago

In a shell script saying

if [ ! -z "$foo" ] && [ "$foo2" -eq 0 ]; then
    echo 'something'
fi

the first test, [ ! -z "$foo" ] (better written as [ -n "$foo" ]), will run first. The second test will only run if the first test succeeded. If both tests succeed, meaning $foo is a value that is not empty and $foo2 has an arithmetic value equal to zero, then the echo will be executed.

The && operator works like similar operators in other languages. The shell implements short-circuit evaluation and will only evaluate the right hand side of && if the left hand side is true.

This particular if statement could also be written without using the if keyword, as

[ ! -z "$foo" ] && [ "$foo2" -eq 0 ] && echo 'something'

Although using if ...; then ...; fi is IMHO clearer.

Again, the echo would only execute if both preceding tests succeeded.