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

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