using Python's subprocess module

2011-01-26

There are at least 2 ways to run a simple command like echo test in Python: via the shell, or directly (via the kernel).

This is via the shell:

import subprocess
subprocess.Popen("echo test", shell=True)

Output:

test

This is via the kernel:

import subprocess
p = subprocess.Popen(["echo", "test"], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
output = p.communicate()
print(output)

Output:

(b'testn', b'')

Not exactly what we want. The output is actually a tuple (stdout, stderr). Note that by default, shell=False, and that's why it's not stated here.

To get the equivalent of what we get from the first example, we have to do a bit more work:

import subprocess
p = subprocess.Popen(["echo", "test"], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
output = p.communicate()
print(output[0].decode(), end="")

Output:

test

The b thing indicates that the string is in bytes format. To get a normal string from that, just run the string's decode method. Also, that I had to use the end argument on the print function, was to avoid an empty line on the terminal output.

As can be seen, this means we do much of the work ourself, but the benefit is that we now have finer-grained control (e.g. we get to control when to display the output). Another benefit is that it is more secure to do it this way (I don't know the details, so see shell injection for the argument).

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