Calling an external command or script in Python

run_cmd.py

#!/usr/bin/env python

import subprocess
import os

# 1) Calling an external command in Python
# Simple way to call external command is using os.system(...). And this funciton returns the exit value of the command. 
# But the drawback is we wont get stdout and stderr.
ret = os.system('some_cmd.sh')
if ret != 0 :
    print 'some_cmd.sh execution returned failure'
    
# 2) Calling an external command in Python in background
# subprocess.Popen provides more flexibility for running an external command rather than using os.system. 
# We can start a command in background and wait for its finish. And after that we can get the stdout and stderr
proc = subprocess.Popen(["./some_cmd.sh"], stdout=subprocess.PIPE)
print 'waiting for ' + str(proc.pid)
proc.wait()
print 'some_cmd.sh execution finished'
(out, err) = proc.communicate()
print 'some_cmd.sh output : ' + out

# 3) Calling a long run external command in Python in background and stop after sometime
# Even we can start a long run process in backgroup using subprocess. Popen and kill it after sometime 
# once its taks is done.
proc = subprocess.Popen(["./some_long_run_cmd.sh"], stdout=subprocess.PIPE)
# Do something else
# Now some_long_run_cmd.sh exeuction is no longer needed，so kill it
os.system('kill -15 ' + str(proc.pid))
print 'Output : ' proc.communicate()[0]