Python System Command – os.system(), subprocess.call()

Filed Under: Python

In this tutorial we will learn about Python System Command. Previously we learned about Python Random Number.

Python System Command

While making a program in python, you may need to exeucte some shell commands for your program. For example, if you use Pycharm IDE, you may notice that there is option to share your project on github. And you probably know that file transferring is done by git, which is operated using command line. So, Pycharm executes some shell commands in background to do it.

However, In this tutorial we will learn some basics about executing shell commands from your python code.

Python os.system() function

We can execute system command by using os.system() function. According to the official document, it has been said that

This is implemented by calling the Standard C function system(), and has the same limitations.

However, if command generates any output, it is sent to the interpreter standard output stream. Using this command is not recommended. In the following code we will try to know the version of git using the system command git --version.


import os

cmd = "git --version"

returned_value = os.system(cmd)  # returns the exit code in unix
print('returned value:', returned_value)

The following output found in ubuntu 16.04 where git is installed already.


git version 2.14.2
returned value: 0

Notice that we are not printing the git version command output to console, it’s being printed because console is the standard output stream here.

Python subprocess.call() Function

In the previous section, we saw that os.system() function works fine. But it’s not recommended way to execute shell commands. We will use Python subprocess module to execute system commands.

We can run shell commands by using subprocess.call() function. See the following code which is equivalent to the previous code.


import subprocess

cmd = "git --version"

returned_value = subprocess.call(cmd, shell=True)  # returns the exit code in unix
print('returned value:', returned_value)

And the output will be same also.
Python System Command

Python subprocess.check_output() function

So far, we executed the system commands with the help of python. But we could not manipulate the output produced by those commands. Using subprocess.check_output() function we can store the output in a variable.


import subprocess

cmd = "date"

# returns output as byte string
returned_output = subprocess.check_output(cmd)

# using decode() function to convert byte string to string
print('Current date is:', returned_output.decode("utf-8"))

It will produce output like the following


Current date is: Thu Oct  5 16:31:41 IST 2017

So, in the above sections we have discussed about basic ideas about executing python system command. But there is no limit in learning. If you wish, you can learn more about Python System command using subprocess module from official documentation.

Comments

  1. Ubaid says:

    Hey Guys,

    I am using python3.6.9, need some help here – I am coming from a perl background, trying to find a way to run a command (say for example ‘df’ command and store the output of columns 2 and 6 (size and name) in a key,value pair in a hash(dict in python world) and/or push these into a list which can be called from anywhere in the script- I have not seen very many examples of this kind of logic – any help is appreciated.

    Thanks

  2. uttam kumar says:

    Q.
    import subprocess
    cmd = “ps -ef | wc -l”
    returned_output = subprocess.check_output(cmd)
    print(returned_output.decode(“utf-8”))

    not working in Case command using with pipe

    1. Luis Raúl Carmona RIOS says:

      How to solution this problem?

    2. Pravi says:

      Have you find the solution for this ?

  3. abdallah says:

    with python 2 no need to decode ,

  4. shruhthilaya says:

    I want include a variable which stores the result of previous command in subprocess.Popen({“command here”],subprocess.PIPE)

    example:
    state = “here”
    cmd = [“”” grep -i $state /log/messages”””]

  5. Paulius says:

    Hello, what if after executing command, cmd prompts something like “Press any key to continue or ctrl-c to cancel” and I basically need to somehow send any key press. Thanks for post.

  6. M says:

    someone can share me the python os system need to try it this my mail.marckeef@hotmail.com

  7. adk says:

    If you just wanted to accomplish that then you could just do this:

    >>> text=’abc\ncde’
    >>> print(text,file=open(‘file1′,’w’))

    The file ‘file1’ will be saved to the current working directory. You can view this by:

    >>> import os
    >>> os.getcwd()

  8. divya says:

    I’m pretty new to python scripting, I’m trying to achieve the python equivalent of shell cmd = “echo -e “abc\ncde” >file1″

    The contents of file1 then looks like this:

    abc
    cde

    My python script has:

    cmd = “echo -e \”abc\ncde\” >file”
    os.system(cmd)

    However, when executing this my file looks like this:

    -e abc
    cde

    -e is an option for echo to recognise \n as new line character and should not be written to the file. Is there a way around this?

    1. Andy says:

      I’m using python 3.6.8.

      Here’s the fix:
      Just wrap it in triple quotes, remove the escapes on the quotes you had and specify the path to echo, such as /bin/echo.

      cmd = “””/bin/echo -e ”abc\ncde” >file”””
      os.system(cmd)

  9. Priyasha says:

    I am trying to use the check_output command to call “git branch -r –merged” as follows:

    import subprocess
    import os
    import sys
    import csv

    cmd = “git branch -r –merged”
    listed_merged_branches = subprocess.check_output(cmd)
    print (listed_merged_branches.decode(“utf-8”))

    i am getting callprocesserror

    Could you help please ?

    1. Thiago Bezerra says:

      Please try:

      listed_merged_branches = subprocess.check_output(([‘git’,’branch’,’-r’,’-merged’]))
      print (listed_merged_branches.decode(“utf-8”))

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