Shell Variable Checks

Check if Set

To check if a value IS set, use -n:

SOME_ARG="$2"

if [ -n "$SOME_ARG" ]; then
	echo "Now I can do some thing! Wow!"
fi

To check if a value IS NOT set, use -z

kill_port() {
	pid="$(lsof -i :${1} | awk 'NR > 1 {print $2}')"

	# If $pid is unset, this branch happens.
	if [ -z "${pid}" ]; then
		echo "No process using port ${port}"
		return 0
	fi

	kill -${signal:-15} $pid
}

Check for Equality

To check for equality, or non-equality it's -eq or -ne respectively:

kill_pid() {
	pid="${1}"
	kill -15 ${pid}

	# If this worked, $? will be 0 to indicate success
	if [ $? -eq 0 ]; then
		echo "Killed ${pid}"
		return 0
	fi

	echo "Could not kill: ${pid}"
	return 1
}

Check for Pattern Match

To check for matches against a regular expression (regex pattern), use the =~ matching operator:

NUMBER_IN_QUOTES='"3"'

# Note the double braces! The operator only works in double braces.
if [[ "$NUMBER_IN_QUOTES" =~ ^\"[0-9]+\"$ ]]; then
	echo "It is what it is"
fi