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pythonlinuxbashsubprocessgnu-screen

Converting `os.system` to `subprocess` in Python

I want to grep a Linux Screen ID in Python to check if this Screen exists. I would like to convert my os.system command to subprocess command.

From this:

os.system('screen -ls | grep -i ' + INSTANCE_NAME + ' >/dev/null')

to this:

subprocess.check_call(['screen', '-ls', '|', 'grep', '-i', INSTANCE_NAME])

The os.system command works fine, but not the subprocess command. And what are the differences between subprocess.run, subprocess.call and subprocess.check_call? Can someone help me out?

I'm using Python 3.10 on Linux Ubuntu Server 20.04 LTS

Solution

  • The pipe | is a shell command that connects the output of a process to the input of another. If you want to use it in subprocess, you have to tell the subprocess function that you want to run the command in a shell like this:

    subprocess.check_call('screen -ls | grep -i ' + INSTANCE_NAME, shell=True)

    If you use this, you should take into account the security considerations of the shell invocation. If you want to achieve the same thing without a shell, you must use two subprocesses. Here is an example:

    p1 = subprocess.Popen(["screen", "-ls"], stdout=subprocess.PIPE)
p2 = subprocess.Popen(
    ["grep", "-i", INSTANCE_NAME], stdin=p1.stdout, stdout=subprocess.PIPE
)
p1.stdout.close()  # Allow p1 to receive a SIGPIPE if p2 exits.
output = p2.communicate()[0].decode()

    (See https://docs.python.org/3/library/subprocess.html#replacing-shell-pipeline)

    And what are the differences between subprocess.run, subprocess.call and subprocess.check_call? Can someone help me out?

    The documentation provides extensive information about the difference between these functions. The TL;DR is:

    1. call and check_call are part of the old high-level API while run is the new recommended approach to invoke subprocesses.
    2. call: runs a command and returns its return code
    3. check_call: like call but raises an exception if the return code is not 0
    4. run: well, you have access to the return code, the output, you can raise for non-zero codes, etc, etc. The documentation has all the information.