Lockable script

lockable_script_boilerplate.sh

#!/bin/bash
# SPDX-License-Identifier: MIT
 
## Copyright (C) 2009 Przemyslaw Pawelczyk <[email protected]>
##
## 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.

Thanks for the hardwork!

Why are we evaling the exec statement?