# Created on savesnippets.com · https://savesnippets.com/Bv7SeQ9Ku9ZWyV
#!/usr/bin/env bash
LOCKFILE="/var/lock/myjob.lock"

(
    # -n = non-blocking, -x = exclusive
    flock -n -x 9 || { echo "Another instance is running, exiting." >&2; exit 1; }
    # ─── critical section ──────────────────────────────
    echo "[$(date)] starting work…"
    sleep 30
    echo "[$(date)] done."
) 9>"$LOCKFILE"

# One-liner in crontab:
# */5 * * * * flock -n /var/lock/myjob.lock -c '/path/to/myjob.sh'