How can i read the output from an executed shell command?

Question

I've read numerous questions about this subject, and even 2 which have accepted answers, which then have in the comment the same issue as i'm experiencing.

So what I want to do is catch the output of this command (which works in the command line)

 sudo /usr/bin/atq

in my Python program.

This is my code (which is an accepted answer in another question)

from subprocess import Popen, PIPE

output = Popen(['sudo /usr/bin/atq', ''], stdout=PIPE)
print output.stdout.read()

and this is the result:

  File "try2.py", line 3, in <module>
    output = Popen(['sudo /usr/bin/atq', ''], stdout=PIPE)
  File "/usr/lib/python2.7/subprocess.py", line 679, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1259, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

Why oh why is this the result (in Python 2.7, on a debian Raspbarry Wheezy install)?

Answer 1

I believe all you need to do is change,

output = Popen(['sudo /usr/bin/atq'], stdout=PIPE)

to

output = Popen(['sudo', '/usr/bin/atq'], stdout=PIPE)

I get the same error when I include multiple arguments as a single string in the args list.

Answer 2

The arguments to Popen need to be a list, you could use shlex to handle this automatically for you

import shlex
args = shlex.split('sudo /usr/bin/atq')
print args

produces

['sudo', '/usr/bin/atq']

which you can then pass to Popen . Then you'll need to communicate with the process you've created. Have a go with .communicate() (note arguments to Popen here are a list!) ie

prc = Popen(['sudo', '/usr/bin/atq'], stdout=PIPE, stderr=PIPE)
output, stderr = prc.communicate()
print output

Popen returns the subprocess handle, which you need to communicate with to get the output. Note - adding stderr=PIPE will give you access to STDERR as well as STDOUT .

Answer 3

You can use subprocess.check_output() :

subprocess.check_output(['sudo', '/usr/bin/atq'])

example:

In [11]: print subprocess.check_output(["ps"])
  PID TTY          TIME CMD
 4547 pts/0    00:00:00 bash
 4599 pts/0    00:00:00 python
 4607 pts/0    00:00:00 python
 4697 pts/0    00:00:00 ps

help() :

In [12]: subprocess.check_output?
Type:       function
String Form:<function check_output at 0xa0e9a74>
File:       /usr/lib/python2.7/subprocess.py
Definition: subprocess.check_output(*popenargs, **kwargs)
Docstring:
Run command with arguments and return its output as a byte string.

If the exit code was non-zero it raises a CalledProcessError.  The
CalledProcessError object will have the return code in the returncode
attribute and output in the output attribute.

The arguments are the same as for the Popen constructor.