Python: start a process if it's not running, or if the process's timestamp is newer than the start time

start_if_not_running.py

'''This script will start a process if it's not already running, or if the
   timestamp of the process file is newer than the start time.
'''

import sys
import os
import pwd
import subprocess
import re
import time

if len(sys.argv) != 2:
   print "Usage: %s name" % sys.argv[0]
   sys.exit(1)
name = sys.argv[1]
if not os.path.isfile(name):
   print "ERROR: '%s' does not exist!" % name
   sys.exit(1)
whoami = pwd.getpwuid(os.getuid())[0]

##
## Get the pid of the process.  Returns empty string if it's not running.
##
def getPID():
   ps_cmd = "/bin/ps -fwwu %s | /bin/grep %s | /bin/grep -v grep | " \
            "/bin/awk '{print $2}' | /bin/grep -v %d" % \
            (whoami, name, os.getpid())
   pipe = subprocess.Popen(ps_cmd, stdout=subprocess.PIPE, shell=True)
   PID = [p.strip('\n') for p in pipe.stdout.readlines()]
   rc = pipe.wait()
   return PID

##
## Get the starttime of a process in number of seconds since the epoch.
##
def proc_starttime(pid):
   p = re.compile(r"^btime (\d+)$", re.MULTILINE)
   m = p.search(open("/proc/stat").read())
   btime = int(m.groups()[0])
   clk_tck = os.sysconf(os.sysconf_names["SC_CLK_TCK"])
   stime = int(open("/proc/%d/stat" % pid).read().split()[21]) / clk_tck
   return btime + stime

##
## Start the process and return the PID.
##
def start():
   log = name + ".log"
   try:
      os.unlink(log)
   except:
      pass
   cmd = "/usr/bin/nohup %s >%s 2>&1 &" % (name, log)
   subprocess.Popen(cmd, shell=True)
   return(getPID())
   
PID = getPID()
if len(PID) == 0:
   ##
   ## The process is not running; start it now.
   ##
   PID = start()
   print "Started %s, pid %s" % (name, PID[0])
elif len(PID) == 1:
   ##
   ## The process is already running.  If the file's timestamp is newer than
   ## the start time, then restart the process.
   ##
   start_time = proc_starttime(int(PID[0]))
   file_time = os.path.getmtime(name)
   if start_time < file_time:
      os.kill(int(PID[0]),15)
      PID = start()
      print "Restarted %s, pid %s" % (name, PID[0])
   else:
      print "%s already running, pid %s" % (name, PID[0])
else:
   print "WARNING: found %d instances of %s" % (len(PID), name)