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Lockable script
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#!/bin/bash | |
# SPDX-License-Identifier: MIT | |
## Copyright (C) 2009 Przemyslaw Pawelczyk <przemoc@gmail.com> | |
## | |
## This script is licensed under the terms of the MIT license. | |
## https://opensource.org/licenses/MIT | |
# | |
# Lockable script boilerplate | |
### HEADER ### | |
LOCKFILE="/var/lock/`basename $0`" | |
LOCKFD=99 | |
# PRIVATE | |
_lock() { flock -$1 $LOCKFD; } | |
_no_more_locking() { _lock u; _lock xn && rm -f $LOCKFILE; } | |
_prepare_locking() { eval "exec $LOCKFD>\"$LOCKFILE\""; trap _no_more_locking EXIT; } | |
# ON START | |
_prepare_locking | |
# PUBLIC | |
exlock_now() { _lock xn; } # obtain an exclusive lock immediately or fail | |
exlock() { _lock x; } # obtain an exclusive lock | |
shlock() { _lock s; } # obtain a shared lock | |
unlock() { _lock u; } # drop a lock | |
### BEGIN OF SCRIPT ### | |
# Simplest example is avoiding running multiple instances of script. | |
exlock_now || exit 1 | |
# Remember! Lock file is removed when one of the scripts exits and it is | |
# the only script holding the lock or lock is not acquired at all. |
An ugly test - if I
kill -9 pid-of-my-script
, then it doesn't have a chance to remove the lock.
This will always be true. kill -9 is "non-catchable, non-ignorable kill", the OS won't let you do anything when you get one, it's the equivalent of pulling taking an axe to the power supply. It's why you should escalate to kill -9
when trying to fix something, not start there.
This is beautiful. Thank you.
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An ugly test - if I
kill -9 pid-of-my-script
, then it doesn't have a chance to remove the lock.