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I have tried to get process count by using ps, when I execute command on my linux terminal it returns correct count. But when I am executing same command in python shell using os.popen(), then the returned count is always incremented by one 

root@dev:/home/admin# ps -ef | grep some_process | wc -l
1
root@dev:/home/admin# python
Python 2.7.6 (default, Nov 23 2017, 15:49:48) 
[GCC 4.8.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.popen('ps -ef | grep some_process | wc -l').read()
'2\n'
>>>
Sanket
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    try using: `ps -ef | grep [s]ome_process | wc -l` -first letter of process name in [] – Drako Jun 21 '18 at 07:17
  • `ps -ef | grep [s]ome_process | wc -l` now this returns 0, its not counting grep process now @Drako, count should be one, as in when we are doing grep it creates new process – Sanket Jun 21 '18 at 07:22

2 Answers2

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os.popen would have launched the process /bin/sh -c 'ps -ef | grep some_precess | wc -l' which will also be counted as a process matching your condition.

Instead try the cmd ps -ef | grep some_process | grep -v grep | wc -l both from shell and python, so that you be accidentally counting this launched process

Sunitha
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  • Thanks! @Sunitha that works for me, Can you able to explain in detail with modifying your answer , what happens in your command – Sanket Jun 21 '18 at 07:27
  • You should be able to understand better if you get rid of the `wc -l` at the end of your commands. Try both your version and mine, on shell and python without `wc -l`. – Sunitha Jun 21 '18 at 07:30
  • Yeah I got to know that, it not counting grep process in your case, I just want to you add information about `grep -v grep` in answer , so it is helpful for others as well – Sanket Jun 21 '18 at 07:33
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First, let's examine what os.popen('ps -ef | grep some_process | wc -l').read() does. It spawns a shell, with the given command line in its argument list. That then spawns a pipeline of three processes, and among those ps collects the list of processes. At this point, at least the first shell and the grep have some_process in their argument list; possibly the third pipeline process too, if it hasn't yet executed wc. grep filters the list, and wc counts the results. Note that the only reason the arguments were even in the listing for grep to find is the use of -f, which might have been redundant since wc doesn't care.

This should make it clear why someone suggested [s]ome_process; this pattern doesn't match itself, and would exclude all of those 2-3 processes, assuming the glob didn't work. It needs quotes to work should there happen to exist a file named some_process.

There may be far more reliable methods, however. We're already running a Python process, so we can easily count things, and ps has switches to select specific processes, for instance ps -C some_process to select on command name. Thus a more discriminate form of the task might be:

subprocess.check_output(["ps", "--no-heading", "-C", "python"]).count(b'\n')

Check for other relevant switches to command such as ps using man.

Yann Vernier
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