Run a Shell Command and Capture Output in Python

A script has to run another program and use what it prints.

import subprocess

r = subprocess.run(["ls", "/nonexistent"], capture_output=True, text=True)
print(r.returncode, repr(r.stdout), r.stderr.strip())

r = subprocess.run(["echo", "héllo"], capture_output=True, text=True, check=True)
print(repr(r.stdout))

try:
    subprocess.run(["false"], check=True)
except subprocess.CalledProcessError as e:
    print("raised:", e)

r = subprocess.run("echo hi | tr a-z A-Z", shell=True, capture_output=True, text=True)
print(r.stdout.strip())

2 '' ls: cannot access '/nonexistent': No such file or directory
'héllo\n'
raised: Command '['false']' returned non-zero exit status 1.
HI

subprocess.run waits for the command and returns a CompletedProcess. With capture_output=True the output and error streams are collected in stdout and stderr, and text=True decodes them to strings, so the accented word came back as a string with its trailing newline. A failed ls has return code 2 with its message in stderr. Adding check=True turns a non-zero exit into CalledProcessError, as the false example shows. Both keywords need Python 3.7 or later. See subprocess.run.

Pass the command as a list, which avoids the shell and its quoting. shell=True is needed for a pipe, as in the last call, but with input from outside it opens the door to command injection. The output is held in memory, so use Popen for very large or endless output.