flock(2)

Synopsis

#include <sys/file.h>

int flock(int fd, int op);

Rod's take

The pidfile you were about to write is broken three ways: test-and-set races, a SIGKILL leaves it stale forever, and PIDs get recycled. flock(2) is one kernel operation on an open file description, so when the holder dies, however it dies, the lock goes with it. Wrap the cron job in flock -n and an overlapping run just exits. Lock the contested resource, not the job, and share the lock file across every script that touches it. It's advisory: anything that doesn't ask walks straight through. Across hosts or NFS, use something built for distributed locking, not a file and a prayer.

Flags and arguments that matter

Errors you will actually hit

Example

# crontab: skip this run if the last one is still going
# */5 * * * * /usr/bin/flock -n /var/lock/report.lock /usr/local/bin/report.sh

#!/bin/bash
# self-locking script: fd 9 holds the lock until the script exits
exec 9>/var/lock/job.lock
flock -n 9 || { echo "already running"; exit 0; }
# ... work ...

# wait up to 60s instead of skipping
flock -w 60 /var/lock/job.lock /usr/local/bin/job.sh

Trimmed from the full walkthrough in Your cron job is running twice and that's why the numbers are wrong.

BSD equivalent

FreeBSD: flock(2) (appeared in 4.2BSD). OpenBSD: flock(2) (first appeared in 4.1cBSD per its man page). On the modern BSDs flock and fcntl locks interact; on Linux they don't (except over NFS/SMB).