Calling a command line utility from Python

Question

I am currently trying to utilize strace to automatically trace a programm 's system calls. To then parse and process the data obtained, I want to use a Python script.

I now wonder, how would I go about calling strace from Python? Strace is usually called via command line and I don't know of any C library compiled from strace which I could utilize.

What is the general way to simulate an access via command line via Python? alternatively: are there any tools similar to strace written natively in Python?

I'm thankful for any kind of help.

Nothing, as I'm clueless

user_1827457 · Answer

You need to use the subprocess module.

It has check_output to read the output and put it in a variable, and check_call to just check the exit code.

If you want to run a shell script you can write it all in a string and set shell=True, otherwise just put the parameters as strings in a list.

import subprocess
# Single process
subprocess.check_output(['fortune', '-m', 'ciao'])
# Run it in a shell
subprocess.check_output('fortune | grep a', shell=True)

Remember that if you run stuff in a shell, if you don't escape properly and allow user data to go in your string, it's easy to make security holes. It is better to not use shell=True.

user_1251220 · Answer

You can use commands as the following:

import commands
cmd = "strace command"
result = commands.getstatusoutput(cmd)
if result[0] == 0:
   print result[1]
else:
   print "Something went wrong executing your command"

result[0] contains the return code, and result[1] contains the output.

user_1927902 · Answer

Python 2 and Python 3 (prior 3.5)

Simply execute:

subprocess.call(["strace", "command"])

Execute and return the output for processing:

output = subprocess.check_output(["strace", "command"])

Reference: https://docs.python.org/2/library/subprocess.html

Python 3.5+

output = subprocess.run(["strace", "command"], caputure_output=True)

Reference: https://docs.python.org/3.7/library/subprocess.html#subprocess.run

Calling a command line utility from Python

3 Answers