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How to get stdout and stderr using Python's subprocess module

Date: 2008-09-23  |   Tags: processes, python   |   2 Comments

I wrote previously about how to get stdout and stderr using os.popen4. However, per the Python documentation, using the subprocess module is preferred:

The subprocess module allows you to spawn new processes, connect to their input/output/error pipes, and obtain their return codes. This module intends to replace several other, older modules and functions, such as:

os.system
os.spawn*
os.popen*
popen2.*
commands.*

See the subprocess module documentation for more information.

Here is how to get stdout and stderr from a program using the subprocess module:

from subprocess import Popen, PIPE, STDOUT

cmd = 'ls /etc/fstab /etc/non-existent-file'
p = Popen(cmd, shell=True, stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)
output = p.stdout.read()
print output

Results:

ls: cannot access /etc/non-existent-file: No such file or directory
/etc/fstab

2 Comments — feed icon Comments feed for this post


#1 Ian McCracken commented on 2008-09-23:

Here's a cleaner way:

from subprocess import Popen, PIPE
cmd = 'blah'
p = Popen(cmd, stdout=PIPE, stderr=PIPE)
stdout, stderr = p.communicate()

That way you wait for the output until the process has actually completed.

Also, take a look at the Process objects in the cliutils package. (Disclaimer: I wrote it.) They let you do some cool things.


#2 Eliot commented on 2008-09-23:

Ian,
This looks like an interesting alternative. I just took my example from the Replacing os.popen* section in the docs.

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