python subprocess

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The subprocess module enables you to start new applications from your Python program. How cool is that?

Start a process in Python:

You can start a process in Python using the Popen function call. The program below starts the unix program ‘cat’ and the second parameter is the argument. This is equivalent to ‘cat test.py’. You can start any program with any parameter.

#!/usr/bin/env python

from subprocess import Popen, PIPE

process = Popen(['cat', 'test.py'], stdout=PIPE, stderr=PIPE)
stdout, stderr = process.communicate()
print stdout

The process.communicate() call reads input and output from the process. stdout is the process output. stderr will be written only if an error occurs. If you want to wait for the program to finish you can call Popen.wait().

Subprocess call():

Subprocess has a method call() which can be used to start a program. The parameter is a list of which the first argument must be the program name. The full definition is:

subprocess.call(args, *, stdin=None, stdout=None, stderr=None, shell=False)
# Run the command described by args.
# Wait for command to complete, then return the returncode attribute.

In the example below the full command would be “ls -l”

#!/usr/bin/env python

import subprocess
subprocess.call(["ls", "-l"])

Save process output (stdout)

We can get the output of a program and store it in a string directly using check_output. The method is defined as:

 subprocess.check_output(args, *, stdin=None, stderr=None, shell=False, universal_newlines=False)
# Run command with arguments and return its output as a byte string.

Example usage:

#!/usr/bin/env python
import subprocess

s = subprocess.check_output(["echo", "Hello World!"])
print("s = " + s)

If you are new to Python programming, I highly recommend this book.

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Posted in Beginner

2015-08-29

Leave a Reply:

Kheenan • Thu, 16 Jul 2015

I just want to say These are the best tutorials of any anywhere. I use tutorials all the time and these are just so concise and effective. Thanks a lot and keep at it!

Frank • Thu, 16 Jul 2015

Thanks Kheenan!

Benben • Tue, 01 Sep 2015

Hi,Frank

Could tell me why?

I've copy it and run it . it show me:

from subprocess import Popen, PIPE
 
process = Popen(['cat', 'test.py'], stdout=PIPE, stderr=PIPE)
stdout, stderr = process.communicate()
print stdout

Traceback (most recent call last):
  File "C:\Python27\learn\subprocess.py", line 5, in 
    process = Popen(['cat', 'test.py'], stdout=PIPE, stderr=PIPE)
  File "C:\Python27\lib\subprocess.py", line 710, in __init__
    errread, errwrite)
  File "C:\Python27\lib\subprocess.py", line 958, in _execute_child


   startupinfo)
WindowsError: [Error 2]

Frank • Tue, 01 Sep 2015

Hi,
The first parameter of Popen() is 'cat', this is a unix program.
I'm not sure if this program is available on windows, try changing it to notepad.exe

Hina • Mon, 11 Jan 2016

Hi Frank,
i have found your website very useful as i am python learner. please if you could guide me the way to read pdf file in python. Secondly, i want tutorials about natural language processing in python. can you help in this regard.
Many Thanks

Frank • Thu, 28 Jan 2016

Hi Hina,
Python has no standard method to read pdf. To read pdf you need to use a module. You can find related projects here: https://github.com/search?l...
Alternatively you can convert the pdf document to a txt method, using the Pdftotext tool: https://en.wikipedia.org/wi...

Francisco Javier Martin • Tue, 30 May 2017

If you start notepad.exe as a windowed app then python will not get the output.
The MSDOS command similar to "cat" is "type".

process = Popen(['type', 'test.py'], stdout=PIPE, stderr=PIPE)

Frank • Wed, 31 May 2017

Thanks Francisco!

M.d Rehman • Mon, 10 Jul 2017

Hi frank. I wanted to know if i could store many values using (check_output). I want to use ping operation in cmd as subprocess and store the ping statistics in the variable to use them.

Frank • Tue, 11 Jul 2017

Hi Rehman, you can store the entire output like this:


#!/usr/bin/env python
import subprocess
s = subprocess.check_output(["ping", "-c 1", "google.com"])
print("s = " + str(s))

Python 3 returns this as bytes, so we convert it to string with the function decode.
Then we split on the newline character:


#!/usr/bin/env python
import subprocess
s = subprocess.check_output(["ping", "-c 4", "google.com"])
output = s.decode("utf-8")
lines = output.split('\n')
for line in lines:
    print(line)

Now we have a few strings. We can parse the line or lines we want. We'll split that string into parts.
I'll take the statistics summary.


#!/usr/bin/env python
import subprocess
s = subprocess.check_output(["ping", "-c 4", "google.com"])
output = s.decode("utf-8")
lines = output.split('\n')
statistics = (lines[7])
values = statistics.split(", ")
for v in values:
    print(v)

All variables are stored in the list values. You can then store them in variables if you want. v[0] contains the first variable of the line.