Python subprocess kill with timeout

Question

I am running some shell scripts with the subprocess module in python. If the shell scripts is running to long, I like to kill the subprocess. I thought it will be enough if I am passing the timeout=30 to my run(..) statement.

Here is the code:

try:
    result=run(['utilities/shell_scripts/{0} {1} {2}'.format(
                        self.language_conf[key][1], self.proc_dir, config.main_file)],
                shell=True,
                check=True,
                stdout=PIPE,
                stderr=PIPE, 
                universal_newlines=True, 
                timeout=30,
                bufsize=100)
except TimeoutExpired as timeout:

I have tested this call with some shell scripts that runs 120s. I expected the subprocess to be killed after 30s, but in fact the process is finishing the 120s script and than raises the Timeout Exception. Now the Question how can I kill the subprocess by timeout?

Answer 1

The documentation explicitly states that the process should be killed:

from the docs for subprocess.run:

"The timeout argument is passed to Popen.communicate(). If the timeout expires, the child process will be killed and waited for. The TimeoutExpired exception will be re-raised after the child process has terminated."

But in your case you're using shell=True, and I've seen issues like that before, because the blocking process is a child of the shell process.

I don't think you need shell=True if you decompose your arguments properly and your scripts have the proper shebang. You could try this:

result=run(
  [os.path.join('utilities/shell_scripts',self.language_conf[key][1]), self.proc_dir, config.main_file],  # don't compose argument line yourself
            shell=False,  # no shell wrapper
            check=True,
            stdout=PIPE,
            stderr=PIPE, 
            universal_newlines=True, 
            timeout=30,
            bufsize=100)

note that I can reproduce this issue very easily on Windows (using Popen, but it's the same thing):

import subprocess,time

p=subprocess.Popen("notepad",shell=True)
time.sleep(1)
p.kill()

=> notepad stays open, probably because it manages to detach from the parent shell process.

import subprocess,time

p=subprocess.Popen("notepad",shell=False)
time.sleep(1)
p.kill()

=> notepad closes after 1 second

Funnily enough, if you remove time.sleep(), kill() works even with shell=True probably because it successfully kills the shell which is launching notepad.

I'm not saying you have exactly the same issue, I'm just demonstrating that shell=True is evil for many reasons, and not being able to kill/timeout the process is one more reason.

However, if you need shell=True for a reason, you can use psutil to kill all the children in the end. In that case, it's better to use Popen so you get the process id directly:

import subprocess,time,psutil

parent=subprocess.Popen("notepad",shell=True)
for _ in range(30): # 30 seconds
    if parent.poll() is not None:  # process just ended
      break
    time.sleep(1)
else:
   # the for loop ended without break: timeout
   parent = psutil.Process(parent.pid)
   for child in parent.children(recursive=True):  # or parent.children() for recursive=False
       child.kill()
   parent.kill()

(source: how to kill process and child processes from python?)

that example kills the notepad instance as well.