Lock a file using lockf(3)

lockf.py

#!/usr/bin/env python
#
# Lock through a file using fcntl(2), i.e. lockf(3), rather than flock(2)
# as used in flock(1)
#
# Example: lockf /var/lib/dpkg/lock apt-get update
#
# Copyright 2018 Diomidis Spinellis
#
#   Licensed under the Apache License, Version 2.0 (the "License");
#   you may not use this file except in compliance with the License.
#   You may obtain a copy of the License at
#
#       http://www.apache.org/licenses/LICENSE-2.0
#
#   Unless required by applicable law or agreed to in writing, software
#   distributed under the License is distributed on an "AS IS" BASIS,
#   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
#   See the License for the specific language governing permissions and
#   limitations under the License.
#


import fcntl
import errno
import sys
import os
import time

if len(sys.argv) < 2:
    sys.exit('Usage: lockf file [command [args ...]]')

try:
    f = open(sys.argv[1], 'w')
except IOError as e:
    sys.exit('lockf: Unable to open {} for writing: {}'.format(
        sys.argv[1], os.strerror(e.errno)))

while True:
    try:
        fcntl.lockf(f, fcntl.LOCK_EX | fcntl.LOCK_NB)
        if len(sys.argv) > 2:
            try:
                os.execvp(sys.argv[2], sys.argv[2:])
            except OSError as e:
                sys.exit('lockf: Unable to execute {}: {}'.format(
                    sys.argv[2], os.strerror(e.errno)))
        sys.exit(0)
    except IOError as e:
        if e.errno != errno.EAGAIN:
            sys.exit('lockf: Unable to lock {}: {}'.format(
                sys.argv[1], os.strerror(e.errno)))
    time.sleep(1)

See also with-lock-ex. The advantage of this lockf.py small script over the with-lock-ex program is that lockf.py doesn't require installation via a package manager.

Informational: It would be more elegant if you use argparse, in order to give a hint about the usage of commandline arguments!