Correct way to escape a subprocess call in python

Question

I am having problems correctly escaping a subprocess call

I want to call sed -n "$=" /path/to/file to count the number of lines in a file but fail to do so from python. My code is as follows:

import subprocess

filename = "/path/to/file"

cmd = subprocess.Popen(["sed", "-n '$='", filename], stdout=subprocess.PIPE)
cmd_out, cmd_err = cmd.communicate()
num_lines = int(cmd_out.strip())

print num_lines

I have tried different escaping combinations for "-n '$='" but nothing seems to work.

I think this mental model is helpful to understand: https://stackoverflow.com/a/70668916/247696 — Flimm, Jan 11 '22 at 15:00

score 4 · Answer 1 · answered Jul 24 '13 at 14:33

4

-n and $= are two separate arguments.

["sed", "-n", "$=", filename]

answered Jul 24 '13 at 14:33

Ignacio Vazquez-Abrams

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Thank you, I thought the arguments are passed in a single string not as a list. – markz Jul 24 '13 at 14:39

score 1 · Answer 2 · answered Jul 24 '13 at 14:41

By default, subprocess.Popen has shell=False.

Without invoking the shell, each argument is passed uninterpreted to sed So sed reads arguments as -n '$='

When you run the command sed -n '$=' , shell removes the '' before sending to the command sed.

So your $= should be without quotes

And as specified by Ignacio, -n $= arguments should be separate

cmd = subprocess.Popen(["sed", "-n", "$=", filename], stdout=subprocess.PIPE)

Correct way to escape a subprocess call in python

2 Answers2