Posted in Information Technology, Software Engineering

Python subprocess Examples

Syntax

Here is the syntax of important subprocess functions

<status> = subprocess.call         (<command array>[, <options>])
<status> = subprocess.check_call   (<command array>[, <options>])
<string> = subprocess.check_output (<command array>[, <options>])
<Popen>  = subprocess.Popen        (<command array>[, <options>])

Basic Example

import subprocess

subprocess.call(['find', '/etc', '-name', '*.conf'])

In the example a program named ‘find’ is executed without a shell. To launch the command in a shell pass ‘shell=True’ in the list of options

subprocess.call(['find', '/'], shell=True)

Catching STDOUT

To collect all output of a command executed use check_output()

output = subprocess.check_output(["ls"])

Redirecting STDERR

When you only want to redirect STDERR to STDIN and catch this output too, simply add ‘stderr=subprocess.STDOUT’ as an option when calling check_output()

output = subprocess.check_output(["unknown_command"], stderr=subprocess.STDOUT)

Complex Pipe Use Cases

To also handle STDIN, STDOUT and STDERR you need use Popen() and Popen.communicate() to write and read from/to those pipes.

# Launch command with all pipes connected
p = subprocess.Popen(['rm', '-i', '*'], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)

# Pass exactly one 'y' and read output
(out, err) = p.communicate('y')

# Check return code
if p.returncode != 0:
    print("Error!")

Don’t forget to pass a ‘stdxx=subprocess.PIPE’ option for each pipe you want to use

Passing Environment Variables

Popen(["ls"], env={"PATH": "/usr/local/bin"})

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