determining if process has terminated successfully

Question

Hi,

I have some code to execute the unix shell command in background in python

 import subprocess
 process = subprocess.Popen('find / > tmp.txt &',shell=True)

I need to capture the scenario where i come to know that process has finished successful completion .

Please explain with sample code

Tazim

Answer 1

There is no need for the &: the command is launched in a separate process, and runs independently.

If you want to wait until the process terminates, use wait():

process = subprocess.Popen('find / > tmp.txt', shell = True)
exitcode = process.wait()
if exitcode == 0:
    # successful completion
else:
    # error happened

If your program can do something meaningful in the meantime, you can use poll() to determine if the process has finished.

Furthermore, instead of writing the output to a temporary file and then reading it from your Python program, you can read from the pipe directly. See the subprocess documentation for details.

Answer 2

Don't use shell=True. It is bad for your health.

proc = subprocess.Popen(['find', '/'], stdout=open('tmp.txt', 'w'))
if proc.wait() == 0:
  pass

If you really need a file, use import tempfile instead of a hard-coded temporary file name. If you don't need the file, use a pipe (see the subprocess docs as Thomas suggests).

Also, don't write shell scripts in Python. Use the os.walk function instead.