use flock(1) to serialize concurrent runs of the same script and prevent overlapping cron/systemd-timer executions

Steps

1. Wrap a script invocation in a lock using a dedicated lock file: `flock -n /var/lock/myjob.lock -c '/path/to/script.sh'`, where -n/--nonblock exits immediately if another instance already holds the lock.
2. To wait up to a bounded time instead of failing immediately, use -w/--timeout <seconds> (fractional seconds allowed) so a second invocation waits briefly for the first to finish.
3. Inside a shell script, acquire a lock on a file descriptor with `( flock -n 9 || exit 1; ...commands under lock... ) 9>/var/lock/myjob.lock` to keep locking self-contained.
4. Make a script self-locking on its own path with the boilerplate `[ "${FLOCKER}" != "$0" ] && exec env FLOCKER="$0" flock -en "$0" "$0" "$@" || :` at the top, so it always re-execs itself under its own lock.
5. Use -s/--shared for read-style locks that multiple concurrent readers can hold simultaneously, reserving the default exclusive lock for writer-style critical sections.