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:
call and check_call are part of the old high-level API while run is the new recommended approach to invoke subprocesses.call: runs a command and returns its return codecheck_call: like call but raises an exception if the return code is not 0run: 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.
