Wait until a certain process (knowing the "pid") end

Question

I have this:

def get_process():
    pids = []
    process = None
    for i in os.listdir('/proc'):
        if i.isdigit():
            pids.append(i)

    for pid in pids:
        proc = open(os.path.join('/proc', pid, 'cmdline'), 'r').readline()
        if proc == "Something":
            process = pid

    return process          

def is_running(pid):
    return os.path.exists("/proc/%s" % str(pid))

Then i do this:

process = get_process()
if process == None:
    #do something
else:
    #Wait until the process end
    while is_running(process):
        pass

I think this is not the best way to wait for the process to terminate, there must be some function wait or something, but i can't find it.

Disclaimer: The process is not a child process

Answer 1

I'm not really a Python programmer, but apparently Python does have os.waitpid(). That should consume less CPU time and provide a much faster response than, say, trying to kill the process at quarter-second intervals.

---

Addendum: As Niko points out, os.waitpid() may not work if the process is not a child of the current process. In that case, using os.kill(pid, 0) may indeed be the best solution. Note that, in general, there are three likely outcomes of calling os.kill() on a process:

1. If the process exists and belongs to you, the call succeeds.
2. If the process exists but belong to another user, it throws an OSError with the errno attribute set to errno.EPERM.
3. If the process does not exist, it throws an OSError with the errno attribute set to errno.ESRCH.

Thus, to reliably check whether a process exists, you should do something like

def is_running(pid):        
    try:
        os.kill(pid, 0)
    except OSError as err:
        if err.errno == errno.ESRCH:
            return False
    return True

Answer 2

Since that method would only work on linux, for linux/osx support, you could do:

import time
import os

def is_running(pid):
    stat = os.system("ps -p %s &> /dev/null" % pid)
    return stat == 0

pid = 64463

while is_running(pid):
    time.sleep(.25)

Edit - Per tMc's comment about excessive processes

Referencing: How to check if there exists a process with a given pid in Python?

Wouldn't this use less resources (I havent tested), than listing on the filesystem and opening FDs to all the results?

import time
import os

def is_running(pid):        
    try:
        os.kill(pid, 0)
    except OSError:
        return False

    return True

pid = 64463

while is_running(pid):
    time.sleep(.25)

Answer 3

import time, then use time.sleep(#):

import time
process = get_process()
if process == None:
    #do something
else:
    #Wait until the process end
    while is_running(process):
        time.sleep(0.25)

I also have that exact same function in several of my scrips to read through /proc/#/cmdline to check for a PID.