Running a linux command from python

Question

I need to run this linux command from python and assign the output to a variable.

ps -ef | grep rtptransmit | grep -v grep

I've tried using pythons commands library to do this.

import commands
a = commands.getoutput('ps -ef | grep rtptransmit | grep -v grep')

But a gets the end of cut off. The output I get is:

'nvr 20714 20711 0 10:39 ? 00:00:00 /opt/americandynamics/venvr/bin/rtptransmit setup_req db=media camera=6 stream=video substream=1 client_a'

but the expected output is:

nvr 20714 20711 0 10:39 ? 00:00:00 /opt/americandynamics/venvr/bin/rtptransmit setup_req db=media camera=6 stream=video substream=1 client_address=192.168.200.179 client_rtp_port=6970 override_lockout=1 clienttype=1

Does anyone know how to stop the output from getting cut off or can anyone suggest another method?

Answer 1

ps apparently limits its output to fit into the presumed width of the terminal. You can override this width with the $COLUMNS environment variable or with the --columns option to ps.

The commands module is deprecated. Use subprocess to get the output of ps -ef and filter the output in Python. Do not use shell=True as suggested by other answers, it is simply superfluous in this case:

ps = subprocess.Popen(['ps', '-ef', '--columns', '1000'], stdout=subprocess.PIPE)
output = ps.communicate()[0]
for line in output.splitlines():
 if 'rtptransmit' in line:
 print(line)

You may also want to take a look the pgrep command by which you can directly search for specific processes.

Answer 2

I usually use subprocess for running an external command. For your case, you can do something like the following

from subprocess import Popen, PIPE
p = Popen('ps -ef | grep rtptransmit | grep -v grep', shell=True,
 stdout=PIPE, stderr=PIPE)
out, err = p.communicate()

The output will be in out variable.

• fajran: The shell is needed if you want to use pipe in the command. About the grep, I actually just copied and pasted the command from the question. That second grep I suspect is there because sometimes the grep command that we execute also appears to be grep-ed and therefore need to be removed. This actually can be avoided using grep [r]tptransmit
• user355252: I know that shell=True is required to use a pipe in the command, and that the second grep is to remove the first grep command from the filtered process list. But both, the pipe and grep are superfluous in this case.
• jfs: @lunaryorn it is worse. The Python code produces a truncated output in my environment: it seems if $COLUMNS is set implicitly then ps ignores it e.g., the output (in the shell) is longer than shutil.get_terminal_size() and os.environ['COLUMNS'] raises KeyError but echo $COLUMNS corresponds to shutil.get_terminal_size().

Answer 3

commands is deprecated, you should not use it. Use subprocess instead

import subprocess
a = subprocess.check_output('ps -ef | grep rtptransmit | grep -v grep', shell=True)

• vartec: @lunaryorn: question is how to run that command in Python, not how to grep that in Python.
• user355252: Actually the question is how to avoid truncation of the ps output. But that doesn't contradict my point...
• vartec: @lunaryorn: also, without shell=True shell pipes won't work, so you wrong on that one too.
• user355252: So what? The point is that you don't need shell pipes in this case...
• fajran: There are many ways to get the output of ps, from a bad one to a good one. However the output will get truncated if the pscolumn width is not set and this is the point of the question and answer. So, arguing that using shell and pipes is not necessary is just silly because it's not the point.

Answer 4

#!/usr/bin/python
import os
a = os.system("cat /var/log/syslog")
print a
from subprocess import call
b = call("ls -l", shell=True)
print b
import subprocess
cmd = subprocess.check_output('ps -ef | grep kernel', shell=True)
print cmd

Any of the above script will work for you :-)

Answer 5

nano test.py

import os
a = os.system('ps -ef | grep rtptransmit | grep -v grep')
print(a)

python test.py

python3 test.py

Run python file using both python and python3

Run python script using python and python3

• Celius Stingher: Could you please add more information on how this solution solves the issue?
• Trenton McKinney: Here are some guidelines for How do I write a good answer?. This provided answer may be correct, but it could benefit from an explanation.