Simple bash locking based on file existence

locklib.sh

#!/bin/bash

# Simple locking mechanism based on file existence

LOCK_ERR_ARGS=64
LOCK_EXISTS=1
LOCK_MISSING=2
LOCK_ROOT=/var/run/amos

#
# Helper function to report an error
# param: message to display
#
function lock_error() {
    echo "$@" > /dev/stderr
}

#
# Acquire a lock or die
# param: lock name
#
function lock_get() {

    if [[ $# -lt 1 ]]; then
        lock_error "Missing lock name"
        exit $LOCK_ERR_ARGS
    fi

    if [[ -f $LOCK_ROOT/$1.lock ]]; then
        lock_error "Lock $1 exists - unable to acquire"
        exit $LOCK_EXISTS
    fi

    date > $LOCK_ROOT/$1.lock
}

#
# Release an existing lock or die
# param: lock name
#
function lock_release() {

    if [[ $# -lt 1 ]]; then
        lock_error "Missing lock name"
        exit $LOCK_ERR_ARGS
    fi

    if [[ ! -f $LOCK_ROOT/$1.lock ]]; then
        lock_error "Non-existing lock $1 - unable to release"
        exit $LOCK_MISSING
    fi

    unlink $LOCK_ROOT/$1.lock
}

#
# Confirm an existing lock or die
# param: lock name
#
function lock_confirm() {

    if [[ $# -lt 1 ]]; then
        lock_error "Missing lock name"
        exit $LOCK_ERR_ARGS
    fi

    if [[ ! -f $LOCK_ROOT/$1.lock ]]; then
        lock_error "Non-existing lock $1 - unable to confirm"
        exit $LOCK_MISSING
    fi

    date >> $LOCK_ROOT/$1.lock
}

usage.sh

#!/bin/bash

source /path/to/locklib.sh

lock_get jobname

echo "Executing the job!"

lock_release jobname

The problem with this is that you have a race condition -- if you have two processes trying to get a lock, and they both run the "if [[ -f $LOCK_ROOT/$1.lock ]];" check before the other creates the lock file, then both processes will get the lock.

Yes. Everybody here and there comments and repeats this race condition like a mantra. But honestly, this is expected just to guard cron jobs so that the same line from crontab is not executed until its previous incarnation finishes. If there are two threads at the machine trying to execute cronjobs at the same time, the server is in much bigger trouble anyway.

http://www.madmadmod.com/programming/31-lockfile-conditional-semaphore-file-creator.html