flock
ShellPackage: util-linux
Takes a lock on a file for the duration of a command, so a second instance waits or gives up. It is what keeps a timer-launched backup from overlapping the previous one.
What its options do in the lessons
From the same glossary the lessons render under their commands, so the two cannot disagree.
flock -n- Gives up at once if the lock is already held, instead of waiting. Exits 1 and prints nothing: the option for a scheduled job, which will be started again anyway.
flock -w- Waits for the lock for at most that many seconds, fractions allowed, then fails the way
-ndoes.-w 0means the same as-n. Keep the timeout shorter than the scheduling interval, or waiting runs pile up. flock -o- Closes the descriptor holding the lock before running the command, so a background process does not inherit it and keep the lock alive after the parent has finished.
flock -u- Drops the lock on an already-open descriptor (
flock -u 9). Rarely needed — a lock goes when the descriptor closes — except when a background child might keep holding it. flock --verbose- Says on standard error why the lock could not be taken (
failed to get lock,timeout while waiting to get lock), or how long it took to get it. Without it a skipped run leaves no trace at all.