Question

I can’t wrap my head around this. Why would /dev/null be used as input to an if statement? What is the use of < /dev/null in the following?

if ( $PROG --version ) < /dev/null > /dev/null 2>&1; then
        $PROG
else
        echo "failed"
        exit 1
fi

I (think) I understand that > /dev/null 2>&1 is just used to suppress any output from both stdout and stderr.

Answer 1

If $PROG is a program that expects input from stdin, your script might stall forever waiting for you to type something in. The use of < /dev/null is to provide an empty input to such a program. You’re right about the > and 2>&1.

This snippet of script is checking to see if $PROG --version exits with a 0 status (success), and then runs that program without any flags. If $PROG --version fails, the script echoes “failed” and exits.