Complex lock management with one command.

lock

#!/bin/bash

########################################################
# Locking library
########################################################

__LOCK_DIR="${_LOCK_DIR:-/var/lock/backup}"

__clear_locks() {
    local _lock_dir="${1:-${__LOCK_DIR}}"
    find "${_lock_dir}" -name '*.lock' | \
    while read _lf; do
        cat "${_lf}" | read _pid _resource
        kill -0 "${_pid}" || rm -f "${_lf}"
    done
}

lock() {
    local _action="$1"
    local _resource="${2:-main}"
    local _lock_dir="${3:-${__LOCK_DIR}}"

    [[ -z "${_action}" ]] && return 255
    [[ -z "${_resource}" ]] && return 255
    [[ -d "${_lock_dir}" ]] || mkdir -p "${_lock_dir}"

    # Clear unneeded locks
    clear_locks "${_lock_dir}"

    case "${_action}" in
        acquire|lock)
            local _lf="`mktemp --tmpdir="${_lock_dir}" 'XXXXXXXX.lock'`"
            echo "$$ ${_resource}" > "${_lf}" || return 1
            return 0
            ;;
        release|free)
            # find all locks for current PID and release (remove) first one
            local _lf="`grep "$$ ${_resource}" "${_lock_dir}/*.lock" | cut -d ':' -f 1 | uniq | head -n 1`"
            [[ -f "${_lf}" ]] && rm -f "${_lf}"
            return $?
            ;;
        status|list)
            # list all locker PIDs
            grep "[0-9]\+ ${_resource}" "${_lock_dir}/*.lock" | cut -d ':' -f 2 | cut -d ' ' -f 1 | uniq
            return 0
            ;;
        check)
            # return 0 if resource is not locked by any PID
            local _lf="`grep "[0-9]\+ ${_resource}" "${_lock_dir}/*.lock" | cut -d ':' -f 1 | uniq | head -n 1`"
            [[ -f "${_lf}" ]] && return 1
            return 0
            ;;
        *)
            return 255
            ;;
    esac
}