BASH: How to correctly return true and false from a function

Question

I have a function in a bash script that I want to return true if a process is running and false if not. The code for my function is:

    checkProcess() {
        if [ kill -s 0 $(getPid) > /dev/null 2>&1 ]; then
            return 0
        else
            return 1
        fi 
    }

When I use this function it does not seem to work. I use it like so:

    if [ ! checkProcess ]; then
        #do something, like rm file containing PID
    fi

However if I just use the below implementation it works fine:

    if [ ! kill -s 0 $(getPid) > /dev/null 2>&1 ]; then
        #do something, like rm file containing PID
    fi

Is there a reason for this? Am I not returning the correct values in my function? Or is it all just wrongly implemented?

Answer 1

[ is not syntax; it is a command. It is commonly used with if, which works based on its return code.

To directly check the return code of a different command, e.g. kill, you should remove the [ and ] (the last argument to [):

if kill -s 0 $(getPid) &>/dev/null; then
    # ...
fi

Note that &> is a Bash extension, equivalent to >/dev/null 2>&1.