python subprocess.check_output giving wrong output

Question

I have problem execute this code here

subprocess.check_output(['ps -ef | grep ftp | wc -l'],env=environ,shell=True)

When I execute from terminal

ps -ef | grep ftp | wc -l

I get "1" as output which is fine.

Now, I execute same code from my python files as subprocess.check_output and it gives me 2. That is strange. Any Ideas why is it happening. Here is the complete code:

 def countFunction(): environ = dict(os.environ) return subprocess.check_output(['ps -ef | grep ftp | wc -l'],env=environ,shell=True) count = countFunction() print count

EDIT: Just to update , I do not have any ftp connections on.So command line is printing 1 on command which is fine. Thanks Arvind

Does your python process have the name ftp in it by any chance? — Martin Konecny
– Martin Konecny, Commented May 14, 2015 at 16:23
No, just the above code. No ftp text in the process executed — 89neuron
– 89neuron, Commented May 14, 2015 at 16:26
You can fix this easily by simply running ps -ef | grep ftp and finding where the extra process is coming from. — Martin Konecny
– Martin Konecny, Commented May 14, 2015 at 16:39
Hint: ps -ef | grep ftp might, depending on timing, find not only ftp, but also grep ftp... — twalberg
– twalberg, Commented May 14, 2015 at 17:07
@twalberg: Both processes are created before they run. Otherwise there would be no way to connect their pipes. — wallyk
– wallyk, Commented May 14, 2015 at 17:11

wallyk · Accepted Answer · 2015-05-14 16:38:59Z

2

The grep command will find itself:

$ ps -ef | grep ftp wallyk 12546 12326 0 16:25 pts/3 00:00:00 grep ftp

If you don't want that, exclude the grep command:

$ ps -ef | grep ftp | grep -v ftp $

It would be better to drop the -f switch to ps so that the command line arguments are not searched. That way, it won't find the grep ftp running:

$ ps -e | grep ftp | wc -l

edited May 14, 2015 at 16:38

answered May 14, 2015 at 16:27

wallyk

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7 Comments

Martin Konecny Over a year ago

He doesn't have any FTP processes running, so he wants it to return only 1.

89neuron Over a year ago

Thanks for your reply wallyk . Yes but with python code , your above command does not print anything, even if ftp is on.

wallyk Over a year ago

@MartinKonecny: Exactly! By excluding the grep command from the results, only other processes with ftp in their arguments will count.

89neuron Over a year ago

Thank you wallyk, -f flag was causing problem. Cheers!

89neuron Over a year ago

@wallyk any ideas why output of ps -e | grep ftp | wc -l shows 1 in command line output while 2 form python code!

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