Watch dog which knows only one trick

watch_bro.py

"""
Watchdog which can do one trick only - to sniff for a pattern in a log file and
"bark" by running some command. The program won't stop after "barking".

Usage:
    python watch_bro.py <filename> <pattern> <command>

    filename: A file to watch for changes
    pattern: A string to look for in the file
    command: What to execute when the pattern occurs

Example:
    python watch_bro.py nginx.log 111.222.33.44 reboot
"""

import re
import subprocess
import sys
import time


def iter_changes(filename):
    with open(filename) as input:
        input.seek(0, 2)
        while True:
            line = input.readline()
            if line == '':
                time.sleep(1)
            else:
                yield line


def run(cmd):
    try:
        return subprocess.check_call(cmd.split(' '), shell=True)
    except subprocess.CalledProcessError:
        print('Command %r return code is not zero' % cmd)
        exit(1)


def main(filename, pattern, cmd):
    re_pattern = re.compile(pattern, re.U)
    for line in iter_changes(filename):
        search = re_pattern.search(line)
        if search:
            run(cmd)


if __name__ == '__main__':
    if len(sys.argv) != 4:
        print(__doc__)
        exit(1)

    file_to_watch, pattern_to_look, cmd_to_run = sys.argv[1:]
    exit(main(
        file_to_watch,
        pattern_to_look,
        cmd_to_run,
    ))