CommandMonitoring.py

# <CommandMonitoring.py>
# Generic interface library for monitoring commands for new lines. Calls a 
# user-specified method whenever a new line is added to the file.

import subprocess

# Monitor the output of a command and called function(line) with each new line
# of output the command generates.
# Input:
#   args: An iteratable list of arguments (including the command name). For
#   example, passing ("ls", "-l") would be the equivalent of calling `ls -l`
#   from the command line.
def monitorCommand(args, function):
    p = subprocess.Popen(args, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
    for line in iter(p.stdout.readline,''):
        function(line)

# Monitors a specific file and calls function(line) for every new line that is
# written to the end. Currently implemented as a special case of monitorCommand
# using `tail -f` to watch the file.
# Input:
#   filepath: Path to the file.
#   function: Function to be called for each line. Takes one argument: the line
#       as a string.
#   backScroll: The number of previously written lines to iterate over when the
#       method is first called. Defaults to 0, meaning only lines add to the
#       file since the monitoring started will be seen.
def monitorFile(filepath, function, backScroll=0):
    print("Monitoring file at %s (%d lines of backScroll" % (filepath,backScroll))
    monitorCommand(["tail", "-fn", str(backScroll), filepath], function)