Question

What’s a quick-and-dirty way to make sure that only one instance of a shell script is running at a given time?

Answer 1

Here’s an implementation that uses a lockfile and echoes a PID into it. This serves as a protection if the process is killed before removing the pidfile:

LOCKFILE=/tmp/lock.txt if [ -e ${LOCKFILE} ] && kill -0 `cat ${LOCKFILE}`; then     echo 'already running'     exit fi  # make sure the lockfile is removed when we exit and then claim it trap 'rm -f ${LOCKFILE}; exit' INT TERM EXIT echo $$ > ${LOCKFILE}  # do stuff sleep 1000  rm -f ${LOCKFILE} 

The trick here is the kill -0 which doesn’t deliver any signal but just checks if a process with the given PID exists. Also the call to trap will ensure that the lockfile is removed even when your process is killed (except kill -9).