Python Cookbook

Running a Command Repeatedly

Credit: Philip Nunez

Problem

You need to run a command repeatedly, with arbitrary periodicity.

Solution

The time.sleep function offers a simple approach to this task:

import time, os, sys, string

def main(cmd, inc=60):
    while 1:
        os.system(cmd)
        time.sleep(inc)

if _ _name_ _ == '_ _main_ _' :
    if len(sys.argv) < 2 or len(sys.argv) > 3:
        print "usage: " + sys.argv[0] + " command [seconds_delay]"
        sys.exit(1)

    cmd = sys.argv[1]
    if len(sys.argv) < 3:
        main(cmd)
    else:
        inc = string.atoi(sys.argv[2])
        main(cmd, inc)

Discussion

You can use this recipe with a command that periodically checks for something (e.g., polling) or performs an endlessly-repeating action, such as telling a browser to reload a URL whose contents change often, so you always have a recent version of that URL up for viewing. The recipe is structured into a function called main and a body that is preceded by the usual if _ _name_ _=='_ _main_ _': idiom, which ensures that the body executes only if the script runs as a main script. The body examines the command-line arguments you used with the script and calls main appropriately (or gives a usage message if there are too many or too few arguments). This is always the best way to structure a script, so its key functionality is also available to other scripts that may import it as a module.

The main function accepts a cmd string, which is a command you should pass periodically to the operating system’s shell, and, optionally, a period of time in seconds, ...