set -e and trap to the rescue

gistfile1.txt

#! /bin/bash

set -e

LOCKFILE="/tmp/things.lock"

trap "rm -f $LOCKFILE" EXIT

if [ -e "$LOCKFILE" ]; then
    echo >&2 "Warning: $LOCKFILE present, not doing things."
    exit 1
else
    touch "$LOCKFILE"
fi

/usr/local/bin/things