Wednesday, April 24, 2013

Locking processes with flock

Got a cronjob that might overlap if it runs slower than usual and you need to avoid multiple instances running?
Here's how to lock them using flock in bash:
function delicate_process() {
  # the 'locked down' code
  return 0;

function main() {
    if ! flock -x --nonblock 200; then
      return 1;


  ) 200>/var/lock/.my.lock

main "${@}";
This is essentially opening the file /var/lock/.my.lock and assigning it the FD 200, then inside flock attempts a non blocking exclusive lock on the FD 200, returning `1` on failure.