Call an external command in Python

Python Programming

Question or problem about Python programming:

How do you call an external command (as if I’d typed it at the Unix shell or Windows command prompt) from within a Python script?

How to solve the problem:

Solution 1:

Look at the subprocess module in the standard library:

import subprocess["ls", "-l"])

The advantage of subprocess vs. system is that it is more flexible (you can get the stdout, stderr, the “real” status code, better error handling, etc…).

The official documentation recommends the subprocess module over the alternative os.system():

The Replacing Older Functions with the subprocess Module section in the subprocess documentation may have some helpful recipes.

For versions of Python before 3.5, use call:

import subprocess["ls", "-l"])

Solution 2:

Here’s a summary of the ways to call external programs and the advantages and disadvantages of each:

However, while this is convenient, you have to manually handle the escaping of shell characters such as spaces, etc. On the other hand, this also lets you run commands which are simply shell commands and not actually external programs. See the documentation.

The subprocess module should probably be what you use.

Finally please be aware that for all methods where you pass the final command to be executed by the shell as a string and you are responsible for escaping it. There are serious security implications if any part of the string that you pass can not be fully trusted. For example, if a user is entering some/any part of the string. If you are unsure, only use these methods with constants. To give you a hint of the implications consider this code:

print subprocess.Popen("echo %s " % user_input, stdout=PIPE)

and imagine that the user enters something “my mama didnt love me && rm -rf /” which could erase the whole filesystem.

Solution 3:

Typical implementation:

import subprocess

p = subprocess.Popen('ls', shell=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
for line in p.stdout.readlines():
    print line,
retval = p.wait()

You are free to do what you want with the stdout data in the pipe. In fact, you can simply omit those parameters (stdout= and stderr=) and it’ll behave like os.system().

Solution 4:

Some hints on detaching the child process from the calling one (starting the child process in background).

Suppose you want to start a long task from a CGI script. That is, the child process should live longer than the CGI script execution process.

The classical example from the subprocess module documentation is:

import subprocess
import sys

# Some code here

pid = subprocess.Popen([sys.executable, ""]) # Call subprocess

# Some more code here

The idea here is that you do not want to wait in the line ‘call subprocess’ until the is finished. But it is not clear what happens after the line ‘some more code here’ from the example.

My target platform was FreeBSD, but the development was on Windows, so I faced the problem on Windows first.

On Windows (Windows XP), the parent process will not finish until the has finished its work. It is not what you want in a CGI script. The problem is not specific to Python; in the PHP community the problems are the same.

The solution is to pass DETACHED_PROCESS Process Creation Flag to the underlying CreateProcess function in Windows API.
If you happen to have installed pywin32, you can import the flag from the win32process module, otherwise you should define it yourself:


pid = subprocess.Popen([sys.executable, ""],

/* UPD 2015.10.27 @eryksun in a comment below notes, that the semantically correct flag is CREATE_NEW_CONSOLE (0x00000010) */

On FreeBSD we have another problem: when the parent process is finished, it finishes the child processes as well. And that is not what you want in a CGI script either. Some experiments showed that the problem seemed to be in sharing sys.stdout. And the working solution was the following:

pid = subprocess.Popen([sys.executable, ""], stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=subprocess.PIPE)

I have not checked the code on other platforms and do not know the reasons of the behaviour on FreeBSD. If anyone knows, please share your ideas. Googling on starting background processes in Python does not shed any light yet.

Solution 5:

import os
os.system("your command")

Note that this is dangerous, since the command isn’t cleaned. I leave it up to you to google for the relevant documentation on the ‘os’ and ‘sys’ modules. There are a bunch of functions (exec* and spawn*) that will do similar things.

Hope this helps!