How do I use a function return as a condition of an if else then shortcut?

Question

There are instances where a function needs to execute and return to the caller and this works for me.

if tst; then
    echo "success"
else
    echo "failure"
fi

function tst () {
    return 0;
}

However, I can't seem to be able to do it using shortcut syntax. I've tried many combinations of the following statement including testing if [[ tst = true ]] or if it = "0" but I haven't been able to figure it out.

[[ tst ]] && echo "success" || echo "failure"

What is the proper way to test the function in the if condition using bash shortcut syntax?

Answer 1

Assuming you want to use A && B || C, then simply call the function directly:

tst && echo "success" || echo "failure"

If you want to use [[, you'd have to use the exit value:

tst
if [[ $? -eq 0 ]]
then 
    ...

Answer 2

(this is not really an answer, more of a comment)

You have to be a bit careful about the a && b || c shortcuts:

• if a returns success, then b is executed
• if b subsequently returns an exit status, then c will be executed too.

$ [[ -f /etc/passwd ]] && { echo "file exists"; false; } || echo "file does not exist"
file exists
file does not exist

if a; then b; else c; fi is safer in this regard, in that c does not depend on b, only a

$ if [[ -f /etc/passwd ]];then { echo "file exists"; false; }; else echo "file does not exist"; fi
file exists
$ echo $?
1