How to execute a program or call a system command from Python

Created 18.09.2008 01:35
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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?

0
Answers 50
13
5099

Use the subprocess module in the standard library:

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

The advantage of subprocess.run over os.system is that it is more flexible (you can get the stdout, stderr, the "real" status code, better error handling, etc...).

Even the documentation for os.system recommends using subprocess instead:

The subprocess module provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using this function. See the Replacing Older Functions with the subprocess Module section in the subprocess documentation for some helpful recipes.

On Python 3.4 and earlier, use subprocess.call instead of .run:

subprocess.call(["ls", "-l"])
18.09.2008 01:39
Comments
Is there a way to use variable substitution? IE I tried to do echo $PATH by using call(["echo", "$PATH"]), but it just echoed the literal string $PATH instead of doing any substitution. I know I could get the PATH environment variable, but I'm wondering if there is an easy way to have the command behave exactly as if I had executed it in bash. by Kevin Wheeler, 01.09.2015 23:17
@KevinWheeler You'll have to use shell=True for that to work. by SethMMorton, 02.09.2015 20:38
@KevinWheeler You should NOT use shell=True, for this purpose Python comes with os.path.expandvars. In your case you can write: os.path.expandvars("$PATH"). @SethMMorton please reconsider your comment -> Why not to use shell=True by Murmel, 11.11.2015 20:24
does call block? i.e. if I want to run multiple commands in a for loop how do I do it without it blocking my python script? I don't care about the output of the command I just want to run lots of them. by Charlie Parker, 24.10.2017 19:07
If you want to create a list out of a command with parameters, a list which can be used with subprocess when shell=False, then use shlex.split for an easy way to do this docs.python.org/2/library/shlex.html#shlex.split by Daniel F, 20.09.2018 18:05
subprocess also allows you to directly pipe two commands together, there's an example in the docs. by Daniel F, 20.09.2018 18:15
@CharlieParker: call, check_call, check_output, and run blocks. If you want non-blocking, use subprocess.Popen. by Lie Ryan, 03.12.2018 13:44
You can also pass in as string instead of a list of strings: subprocess.run("ls -l") by tuket, 23.09.2019 14:33
If they want to name it run instead of call why not just make it an alias? Some types of backwards compatibility are difficult and allow error-prone patterns to continue, but making run == call is easy, simple, and almost no cost. Why call for versions of Python before 3.5 instead of both new and old versions? by Samuel Muldoon, 07.11.2019 04:28
@SamuelMuldoon The issue is that subprocess.call has completely different semantics from subprocess.run. subprocess.call still exists in Python 3.5–3.8 (for backwards compatibility), it's just that if you have it available, subprocess.run is better by far. by FeRD, 08.11.2019 12:54
I'm trying to use that command to run excalibur commands with python to connect with the localhost, but it doesn't work I get FileNotFoundError. import subprocess subprocess.run(["excalibur initdb", "excalibur webserver"]) by Chacho Fuva, 01.05.2020 20:26
@ChachoFuva the list you pass to subprocess.run is a list of words that represent 1 command, not a list of commands. It should be subprocess.run(["excalibur", "initdb"]) then subprocess.run(["excalibur", "webserver"]) by Boris, 17.11.2020 18:35
Any value in spinning a new thread with the subprocess.run() and having the original thread kill it if it exceeds some timeout? by grambo, 23.12.2020 20:12
Show remaining 8 comments
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3111

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

  1. os.system("some_command with args") passes the command and arguments to your system's shell. This is nice because you can actually run multiple commands at once in this manner and set up pipes and input/output redirection. For example:

    os.system("some_command < input_file | another_command > output_file")  
    

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.

  1. stream = os.popen("some_command with args") will do the same thing as os.system except that it gives you a file-like object that you can use to access standard input/output for that process. There are 3 other variants of popen that all handle the i/o slightly differently. If you pass everything as a string, then your command is passed to the shell; if you pass them as a list then you don't need to worry about escaping anything. See the documentation.

  2. The Popen class of the subprocess module. This is intended as a replacement for os.popen but has the downside of being slightly more complicated by virtue of being so comprehensive. For example, you'd say:

    print subprocess.Popen("echo Hello World", shell=True, stdout=subprocess.PIPE).stdout.read()
    

    instead of:

    print os.popen("echo Hello World").read()
    

    but it is nice to have all of the options there in one unified class instead of 4 different popen functions. See the documentation.

  3. The call function from the subprocess module. This is basically just like the Popen class and takes all of the same arguments, but it simply waits until the command completes and gives you the return code. For example:

    return_code = subprocess.call("echo Hello World", shell=True)  
    

    See the documentation.

  4. If you're on Python 3.5 or later, you can use the new subprocess.run function, which is a lot like the above but even more flexible and returns a CompletedProcess object when the command finishes executing.

  5. The os module also has all of the fork/exec/spawn functions that you'd have in a C program, but I don't recommend using them directly.

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).stdout.read()

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

18.09.2008 13:11
Comments
Nice answer/explanation. How is this answer justifying Python's motto as described in this article ? fastcompany.com/3026446/… "Stylistically, Perl and Python have different philosophies. Perl’s best known mottos is " There’s More Than One Way to Do It". Python is designed to have one obvious way to do it" Seem like it should be the other way! In Perl I know only two ways to execute a command - using back-tick or open. by Jean, 26.05.2015 21:16
If using Python 3.5+, use subprocess.run(). docs.python.org/3.5/library/subprocess.html#subprocess.run by phoenix, 07.10.2015 16:37
What one typically needs to know is what is done with the child process's STDOUT and STDERR, because if they are ignored, under some (quite common) conditions, eventually the child process will issue a system call to write to STDOUT (STDERR too?) that would exceed the output buffer provided for the process by the OS, and the OS will cause it to block until some process reads from that buffer. So, with the currently recommended ways, subprocess.run(..), what exactly does "This does not capture stdout or stderr by default." imply? What about subprocess.check_output(..) and STDERR? by Evgeni Sergeev, 01.06.2016 10:44
which of the commands you recommended block my script? i.e. if I want to run multiple commands in a for loop how do I do it without it blocking my python script? I don't care about the output of the command I just want to run lots of them. by Charlie Parker, 24.10.2017 19:08
@phoenix I disagree. There is nothing preventing you from using os.system in python3 docs.python.org/3/library/os.html#os.system by Qback, 08.12.2017 09:27
This is arguably the wrong way around. Most people only need subprocess.run() or its older siblings subprocess.check_call() et al. For cases where these do not suffice, see subprocess.Popen(). os.popen() should perhaps not be mentioned at all, or come even after "hack your own fork/exec/spawn code". by tripleee, 03.12.2018 06:00
Show remaining 1 comments
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389

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().

18.09.2008 18:20
Comments
.readlines() reads all lines at once i.e., it blocks until the subprocess exits (closes its end of the pipe). To read in real time (if there is no buffering issues) you could: for line in iter(p.stdout.readline, ''): print line, by jfs, 16.11.2012 14:12
Could you elaborate on what you mean by "if there is no buffering issues"? If the process blocks definitely, the subprocess call also blocks. The same could happen with my original example as well. What else could happen with respect to buffering? by EmmEff, 17.11.2012 13:25
the child process may use block-buffering in non-interactive mode instead of line-buffering so p.stdout.readline() (note: no s at the end) won't see any data until the child fills its buffer. If the child doesn't produce much data then the output won't be in real time. See the second reason in Q: Why not just use a pipe (popen())?. Some workarounds are provided in this answer (pexpect, pty, stdbuf) by jfs, 17.11.2012 13:51
the buffering issue only matters if you want output in real time and doesn't apply to your code that doesn't print anything until all data is received by jfs, 17.11.2012 13:53
This answer was fine for its time, but we should no longer recommend Popen for simple tasks. This also needlessly specifies shell=True. Try one of the subprocess.run() answers. by tripleee, 03.12.2018 05:39
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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, "longtask.py"]) # Call subprocess

# Some more code here

The idea here is that you do not want to wait in the line 'call subprocess' until the longtask.py 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 longtask.py 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:

DETACHED_PROCESS = 0x00000008

pid = subprocess.Popen([sys.executable, "longtask.py"],
                       creationflags=DETACHED_PROCESS).pid

/* 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, "longtask.py"], 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.

12.02.2010 10:15
Comments
i noticed a possible "quirk" with developing py2exe apps in pydev+eclipse. i was able to tell that the main script was not detached because eclipse's output window was not terminating; even if the script executes to completion it is still waiting for returns. but, when i tried compiling to a py2exe executable, the expected behavior occurs (runs the processes as detached, then quits). i am not sure, but the executable name is not in the process list anymore. this works for all approaches (os.system("start *"), os.spawnl with os.P_DETACH, subprocs, etc.) by maranas, 09.04.2010 08:09
Windows gotcha: even though I spawned process with DETACHED_PROCESS, when I killed my Python daemon all ports opened by it wouldn't free until all spawned processes terminate. WScript.Shell solved all my problems. Example here: pastebin.com/xGmuvwSx by Alexey Lebedev, 16.04.2012 10:04
you might also need CREATE_NEW_PROCESS_GROUP flag. See Popen waiting for child process even when the immediate child has terminated by jfs, 16.11.2012 14:16
I'm seeing import subprocess as sp;sp.Popen('calc') not waiting for the subprocess to complete. It seems the creationflags aren't necessary. What am I missing? by ubershmekel, 27.10.2014 21:01
@ubershmekel, I am not sure what you mean and don't have a windows installation. If I recall correctly, without the flags you can not close the cmd instance from which you started the calc. by newtover, 28.10.2014 12:25
I'm on Windows 8.1 and calc seems to survive the closing of python. by ubershmekel, 30.10.2014 05:45
Is there any significance to using '0x00000008'? Is that a specific value that has to be used or one of multiple options? by SuperBiasedMan, 05.05.2015 13:13
The following is incorrect: "[o]n windows (win xp), the parent process will not finish until the longtask.py has finished its work". The parent will exit normally, but the console window (conhost.exe instance) only closes when the last attached process exits, and the child may have inherited the parent's console. Setting DETACHED_PROCESS in creationflags avoids this by preventing the child from inheriting or creating a console. If you instead want a new console, use CREATE_NEW_CONSOLE (0x00000010). by Eryk Sun, 27.10.2015 00:27
I didn't mean that executing as a detached process is incorrect. That said, you may need to set the standard handles to files, pipes, or os.devnull because some console programs exit with an error otherwise. Create a new console when you want the child process to interact with the user concurrently with the parent process. It would be confusing to try to do both in a single window. by Eryk Sun, 27.10.2015 17:37
stdout=subprocess.PIPE will make your code hang up if you have long output from a child. For more details see thraxil.org/users/anders/posts/2008/03/13/… by Dr_Zaszuś, 08.03.2018 08:56
is there not an OS-agnostic way to have the process run in the background? by Charlie Parker, 24.02.2019 19:05
your answer seems strange to me. I just opened a subprocess.Popen and nothing bad happened (not had to wait). Why exactly do we need to worry about the scenario you are pointing out? I'm skeptical. by Charlie Parker, 24.02.2019 19:38
Show remaining 7 comments
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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.

18.09.2008 01:37
Comments
No idea what I meant nearly a decade ago (check the date!), but if I had to guess, it would be that there's no validation done. by nimish, 06.06.2018 16:01
This should now point to subprocess as a slightly more versatile and portable solution. Running external commands is of course inherently unportable (you have to make sure the command is available on every architecture you need to support) and passing user input as an external command is inherently unsafe. by tripleee, 03.12.2018 05:11
Note the timestamp on this guy: the "correct" answer has 40x the votes and is answer #1. by nimish, 03.12.2018 18:41
The one solution that worked for me for running NodeJS stuff. by Nikolay Shindarov, 29.10.2019 20:49
2
163

I'd recommend using the subprocess module instead of os.system because it does shell escaping for you and is therefore much safer.

subprocess.call(['ping', 'localhost'])
18.09.2008 01:42
Comments
If you want to create a list out of a command with parameters, a list which can be used with subprocess when shell=False, then use shlex.split for an easy way to do this docs.python.org/2/library/shlex.html#shlex.split (it's the recommended way according to the docs docs.python.org/2/library/subprocess.html#popen-constructor) by Daniel F, 20.09.2018 18:07
This is incorrect: "it does shell escaping for you and is therefore much safer". subprocess doesn't do shell escaping, subprocess doesn't pass your command through the shell, so there's no need to shell escape. by Lie Ryan, 04.12.2018 08:36
2
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import os
cmd = 'ls -al'
os.system(cmd)

If you want to return the results of the command, you can use os.popen. However, this is deprecated since version 2.6 in favor of the subprocess module, which other answers have covered well.

18.09.2008 01:37
Comments
popen is deprecated in favor of subprocess. by Tris - archived, 08.08.2014 00:22
You can also save your result with the os.system call, since it works like the UNIX shell itself, like for example os.system('ls -l > test2.txt') by Stefan Gruenwald, 07.11.2017 23:19
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There are lots of different libraries which allow you to call external commands with Python. For each library I've given a description and shown an example of calling an external command. The command I used as the example is ls -l (list all files). If you want to find out more about any of the libraries I've listed and linked the documentation for each of them.

Sources

These are all the libraries

Hopefully this will help you make a decision on which library to use :)

subprocess

Subprocess allows you to call external commands and connect them to their input/output/error pipes (stdin, stdout, and stderr). Subprocess is the default choice for running commands, but sometimes other modules are better.

subprocess.run(["ls", "-l"]) # Run command
subprocess.run(["ls", "-l"], stdout=subprocess.PIPE) # This will run the command and return any output
subprocess.run(shlex.split("ls -l")) # You can also use the shlex library to split the command

os

os is used for "operating system dependent functionality". It can also be used to call external commands with os.system and os.popen (Note: There is also a subprocess.popen). os will always run the shell and is a simple alternative for people who don't need to, or don't know how to use subprocess.run.

os.system("ls -l") # Run command
os.popen("ls -l").read() # This will run the command and return any output

sh

sh is a subprocess interface which lets you call programs as if they were functions. This is useful if you want to run a command multiple times.

sh.ls("-l") # Run command normally
ls_cmd = sh.Command("ls") # Save command as a variable
ls_cmd() # Run command as if it were a function

plumbum

plumbum is a library for "script-like" Python programs. You can call programs like functions as in sh. Plumbum is useful if you want to run a pipeline without the shell.

ls_cmd = plumbum.local("ls -l") # Get command
ls_cmd() # Run command

pexpect

pexpect lets you spawn child applications, control them and find patterns in their output. This is a better alternative to subprocess for commands that expect a tty on Unix.

pexpect.run("ls -l") # Run command as normal
child = pexpect.spawn('scp foo user@example.com:.') # Spawns child application
child.expect('Password:') # When this is the output
child.sendline('mypassword')

fabric

fabric is a Python 2.5 and 2.7 library. It allows you to execute local and remote shell commands. Fabric is simple alternative for running commands in a secure shell (SSH)

fabric.operations.local('ls -l') # Run command as normal
fabric.operations.local('ls -l', capture = True) # Run command and receive output

envoy

envoy is known as "subprocess for humans". It is used as a convenience wrapper around the subprocess module.

r = envoy.run("ls -l") # Run command
r.std_out # Get output

commands

commands contains wrapper functions for os.popen, but it has been removed from Python 3 since subprocess is a better alternative.

29.10.2016 14:02
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80

I always use fabric for this things like:

from fabric.operations import local
result = local('ls', capture=True)
print "Content:/n%s" % (result, )

But this seem to be a good tool: sh (Python subprocess interface).

Look at an example:

from sh import vgdisplay
print vgdisplay()
print vgdisplay('-v')
print vgdisplay(v=True)
13.03.2012 00:12
0
80

With the standard library

Use the subprocess module (Python 3):

import subprocess
subprocess.run(['ls', '-l'])

It is the recommended standard way. However, more complicated tasks (pipes, output, input, etc.) can be tedious to construct and write.

Note on Python version: If you are still using Python 2, subprocess.call works in a similar way.

ProTip: shlex.split can help you to parse the command for run, call, and other subprocess functions in case you don't want (or you can't!) provide them in form of lists:

import shlex
import subprocess
subprocess.run(shlex.split('ls -l'))

With external dependencies

If you do not mind external dependencies, use plumbum:

from plumbum.cmd import ifconfig
print(ifconfig['wlan0']())

It is the best subprocess wrapper. It's cross-platform, i.e. it works on both Windows and Unix-like systems. Install by pip install plumbum.

Another popular library is sh:

from sh import ifconfig
print(ifconfig('wlan0'))

However, sh dropped Windows support, so it's not as awesome as it used to be. Install by pip install sh.

11.04.2013 17:17
0
78

Check the "pexpect" Python library, too.

It allows for interactive controlling of external programs/commands, even ssh, ftp, telnet, etc. You can just type something like:

child = pexpect.spawn('ftp 192.168.0.24')

child.expect('(?i)name .*: ')

child.sendline('anonymous')

child.expect('(?i)password')
07.10.2010 07:09
1
76

If you need the output from the command you are calling, then you can use subprocess.check_output (Python 2.7+).

>>> subprocess.check_output(["ls", "-l", "/dev/null"])
'crw-rw-rw- 1 root root 1, 3 Oct 18  2007 /dev/null\n'

Also note the shell parameter.

If shell is True, the specified command will be executed through the shell. This can be useful if you are using Python primarily for the enhanced control flow it offers over most system shells and still want convenient access to other shell features such as shell pipes, filename wildcards, environment variable expansion, and expansion of ~ to a user’s home directory. However, note that Python itself offers implementations of many shell-like features (in particular, glob, fnmatch, os.walk(), os.path.expandvars(), os.path.expanduser(), and shutil).

28.04.2011 20:29
Comments
Note that check_output requires a list rather than a string. If you don't rely on quoted spaces to make your call valid, the simplest, most readable way to do this is subprocess.check_output("ls -l /dev/null".split()). by Bruno Bronosky, 30.01.2018 18:18
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This is how I run my commands. This code has everything you need pretty much

from subprocess import Popen, PIPE
cmd = "ls -l ~/"
p = Popen(cmd , shell=True, stdout=PIPE, stderr=PIPE)
out, err = p.communicate()
print "Return code: ", p.returncode
print out.rstrip(), err.rstrip()
28.10.2012 05:14
Comments
I think it's acceptable for hard-coded commands, if it increases readability. by Adam Matan, 02.04.2014 13:07
An explanation would be in order. E.g., what is the idea/gist? by Peter Mortensen, 07.04.2021 17:18
0
60

Update:

subprocess.run is the recommended approach as of Python 3.5 if your code does not need to maintain compatibility with earlier Python versions. It's more consistent and offers similar ease-of-use as Envoy. (Piping isn't as straightforward though. See this question for how.)

Here's some examples from the documentation.

Run a process:

>>> subprocess.run(["ls", "-l"])  # Doesn't capture output
CompletedProcess(args=['ls', '-l'], returncode=0)

Raise on failed run:

>>> subprocess.run("exit 1", shell=True, check=True)
Traceback (most recent call last):
  ...
subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1

Capture output:

>>> subprocess.run(["ls", "-l", "/dev/null"], stdout=subprocess.PIPE)
CompletedProcess(args=['ls', '-l', '/dev/null'], returncode=0,
stdout=b'crw-rw-rw- 1 root root 1, 3 Jan 23 16:23 /dev/null\n')

Original answer:

I recommend trying Envoy. It's a wrapper for subprocess, which in turn aims to replace the older modules and functions. Envoy is subprocess for humans.

Example usage from the README:

>>> r = envoy.run('git config', data='data to pipe in', timeout=2)

>>> r.status_code
129
>>> r.std_out
'usage: git config [options]'
>>> r.std_err
''

Pipe stuff around too:

>>> r = envoy.run('uptime | pbcopy')

>>> r.command
'pbcopy'
>>> r.status_code
0

>>> r.history
[<Response 'uptime'>]
15.11.2012 17:13
0
49

Use subprocess.

...or for a very simple command:

import os
os.system('cat testfile')
18.09.2008 01:43
1
38

os.system is OK, but kind of dated. It's also not very secure. Instead, try subprocess. subprocess does not call sh directly and is therefore more secure than os.system.

Get more information here.

18.09.2008 01:53
Comments
While I agree with the overall recommendation, subprocess does not remove all of the security problems, and has some pesky issues of its own. by tripleee, 03.12.2018 05:36
2
40

Calling an external command in Python

Simple, use subprocess.run, which returns a CompletedProcess object:

>>> import subprocess
>>> completed_process = subprocess.run('python --version')
Python 3.6.1 :: Anaconda 4.4.0 (64-bit)
>>> completed_process
CompletedProcess(args='python --version', returncode=0)

Why?

As of Python 3.5, the documentation recommends subprocess.run:

The recommended approach to invoking subprocesses is to use the run() function for all use cases it can handle. For more advanced use cases, the underlying Popen interface can be used directly.

Here's an example of the simplest possible usage - and it does exactly as asked:

>>> import subprocess
>>> completed_process = subprocess.run('python --version')
Python 3.6.1 :: Anaconda 4.4.0 (64-bit)
>>> completed_process
CompletedProcess(args='python --version', returncode=0)

run waits for the command to successfully finish, then returns a CompletedProcess object. It may instead raise TimeoutExpired (if you give it a timeout= argument) or CalledProcessError (if it fails and you pass check=True).

As you might infer from the above example, stdout and stderr both get piped to your own stdout and stderr by default.

We can inspect the returned object and see the command that was given and the returncode:

>>> completed_process.args
'python --version'
>>> completed_process.returncode
0

Capturing output

If you want to capture the output, you can pass subprocess.PIPE to the appropriate stderr or stdout:

>>> cp = subprocess.run('python --version', 
                        stderr=subprocess.PIPE, 
                        stdout=subprocess.PIPE)
>>> cp.stderr
b'Python 3.6.1 :: Anaconda 4.4.0 (64-bit)\r\n'
>>> cp.stdout
b''

(I find it interesting and slightly counterintuitive that the version info gets put to stderr instead of stdout.)

Pass a command list

One might easily move from manually providing a command string (like the question suggests) to providing a string built programmatically. Don't build strings programmatically. This is a potential security issue. It's better to assume you don't trust the input.

>>> import textwrap
>>> args = ['python', textwrap.__file__]
>>> cp = subprocess.run(args, stdout=subprocess.PIPE)
>>> cp.stdout
b'Hello there.\r\n  This is indented.\r\n'

Note, only args should be passed positionally.

Full Signature

Here's the actual signature in the source and as shown by help(run):

def run(*popenargs, input=None, timeout=None, check=False, **kwargs):

The popenargs and kwargs are given to the Popen constructor. input can be a string of bytes (or unicode, if specify encoding or universal_newlines=True) that will be piped to the subprocess's stdin.

The documentation describes timeout= and check=True better than I could:

The timeout argument is passed to Popen.communicate(). If the timeout expires, the child process will be killed and waited for. The TimeoutExpired exception will be re-raised after the child process has terminated.

If check is true, and the process exits with a non-zero exit code, a CalledProcessError exception will be raised. Attributes of that exception hold the arguments, the exit code, and stdout and stderr if they were captured.

and this example for check=True is better than one I could come up with:

>>> subprocess.run("exit 1", shell=True, check=True)
Traceback (most recent call last):
  ...
subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1

Expanded Signature

Here's an expanded signature, as given in the documentation:

subprocess.run(args, *, stdin=None, input=None, stdout=None, stderr=None, 
shell=False, cwd=None, timeout=None, check=False, encoding=None, 
errors=None)

Note that this indicates that only the args list should be passed positionally. So pass the remaining arguments as keyword arguments.

Popen

When use Popen instead? I would struggle to find use-case based on the arguments alone. Direct usage of Popen would, however, give you access to its methods, including poll, 'send_signal', 'terminate', and 'wait'.

Here's the Popen signature as given in the source. I think this is the most precise encapsulation of the information (as opposed to help(Popen)):

def __init__(self, args, bufsize=-1, executable=None,
             stdin=None, stdout=None, stderr=None,
             preexec_fn=None, close_fds=_PLATFORM_DEFAULT_CLOSE_FDS,
             shell=False, cwd=None, env=None, universal_newlines=False,
             startupinfo=None, creationflags=0,
             restore_signals=True, start_new_session=False,
             pass_fds=(), *, encoding=None, errors=None):

But more informative is the Popen documentation:

subprocess.Popen(args, bufsize=-1, executable=None, stdin=None,
                 stdout=None, stderr=None, preexec_fn=None, close_fds=True,
                 shell=False, cwd=None, env=None, universal_newlines=False,
                 startupinfo=None, creationflags=0, restore_signals=True,
                 start_new_session=False, pass_fds=(), *, encoding=None, errors=None)

Execute a child program in a new process. On POSIX, the class uses os.execvp()-like behavior to execute the child program. On Windows, the class uses the Windows CreateProcess() function. The arguments to Popen are as follows.

Understanding the remaining documentation on Popen will be left as an exercise for the reader.

18.10.2017 16:37
Comments
A simple example of two-way communication between a primary process and a subprocess can be found here: stackoverflow.com/a/52841475/1349673 by James Hirschorn, 16.10.2018 18:05
The first example should probably have shell=True or (better yet) pass the command as a list. by tripleee, 03.12.2018 05:16
1
35

There is also Plumbum

>>> from plumbum import local
>>> ls = local["ls"]
>>> ls
LocalCommand(<LocalPath /bin/ls>)
>>> ls()
u'build.py\ndist\ndocs\nLICENSE\nplumbum\nREADME.rst\nsetup.py\ntests\ntodo.txt\n'
>>> notepad = local["c:\\windows\\notepad.exe"]
>>> notepad()                                   # Notepad window pops up
u''                                             # Notepad window is closed by user, command returns
10.10.2014 17:41
Comments
An explanation would be in order. by Peter Mortensen, 07.04.2021 17:29
1
29

Use:

import os

cmd = 'ls -al'

os.system(cmd)

os - This module provides a portable way of using operating system-dependent functionality.

For the more os functions, here is the documentation.

29.06.2015 11:34
Comments
it's also deprecated. use subprocess by Corey Goldberg, 09.12.2015 18:13
1
30

It can be this simple:

import os
cmd = "your command"
os.system(cmd)
30.04.2018 13:47
Comments
This fails to point out the drawbacks, which are explained in much more detail in PEP-324. The documentation for os.system explicitly recommends avoiding it in favor of subprocess. by tripleee, 03.12.2018 05:02
0
28

I quite like shell_command for its simplicity. It's built on top of the subprocess module.

Here's an example from the documentation:

>>> from shell_command import shell_call
>>> shell_call("ls *.py")
setup.py  shell_command.py  test_shell_command.py
0
>>> shell_call("ls -l *.py")
-rw-r--r-- 1 ncoghlan ncoghlan  391 2011-12-11 12:07 setup.py
-rw-r--r-- 1 ncoghlan ncoghlan 7855 2011-12-11 16:16 shell_command.py
-rwxr-xr-x 1 ncoghlan ncoghlan 8463 2011-12-11 16:17 test_shell_command.py
0
13.08.2012 18:36
5
27

As of Python 3.7.0 released on June 27th 2018 (https://docs.python.org/3/whatsnew/3.7.html), you can achieve your desired result in the most powerful while equally simple way. This answer intends to show you the essential summary of various options in a short manner. For in-depth answers, please see the other ones.


TL;DR in 2021

The big advantage of os.system(...) was its simplicity. subprocess is better and still easy to use, especially as of Python 3.5.

import subprocess
subprocess.run("ls -a", shell=True)

Note: This is the exact answer to your question - running a command

like in a shell


Preferred Way

If possible, remove the shell overhead and run the command directly (requires a list).

import subprocess
subprocess.run(["help"])
subprocess.run(["ls", "-a"])

Pass program arguments in a list. Don't include \"-escaping for arguments containing spaces.


Advanced Use Cases

Checking The Output

The following code speaks for itself:

import subprocess
result = subprocess.run(["ls", "-a"], capture_output=True, text=True)
if "stackoverflow-logo.png" in result.stdout:
    print("You're a fan!")
else:
    print("You're not a fan?")

result.stdout is all normal program output excluding errors. Read result.stderr to get them.

capture_output=True - turns capturing on. Otherwise result.stderr and result.stdout would be None. Available from Python 3.7.

text=True - a convenience argument added in Python 3.7 which converts the received binary data to Python strings you can easily work with.

Checking the returncode

Do

if result.returncode == 127: print("The program failed for some weird reason")
elif result.returncode == 0: print("The program succeeded")
else: print("The program failed unexpectedly")

If you just want to check if the program succeeded (returncode == 0) and otherwise throw an Exception, there is a more convenient function:

result.check_returncode()

But it's Python, so there's an even more convenient argument check which does the same thing automatically for you:

result = subprocess.run(..., check=True)

stderr should be inside stdout

You might want to have all program output inside stdout, even errors. To accomplish this, run

result = subprocess.run(..., stderr=subprocess.STDOUT)

result.stderr will then be None and result.stdout will contain everything.

Using shell=False with an argument string

shell=False expects a list of arguments. You might however, split an argument string on your own using shlex.

import subprocess
import shlex
subprocess.run(shlex.split("ls -a"))

That's it.

Common Problems

Chances are high you just started using Python when you come across this question. Let's look at some common problems.

FileNotFoundError: [Errno 2] No such file or directory: 'ls -a': 'ls -a'

You're running a subprocess without shell=True . Either use a list (["ls", "-a"]) or set shell=True.

TypeError: [...] NoneType [...]

Check that you've set capture_output=True.

TypeError: a bytes-like object is required, not [...]

You always receive byte results from your program. If you want to work with it like a normal string, set text=True.

subprocess.CalledProcessError: Command '[...]' returned non-zero exit status 1.

Your command didn't run successfully. You could disable returncode checking or check your actual program's validity.

TypeError: init() got an unexpected keyword argument [...]

You're likely using a version of Python older than 3.7.0; update it to the most recent one available. Otherwise there are other answers in this Stack Overflow post showing you older alternative solutions.

13.10.2020 19:20
Comments
"The big advantage of os.system(...) was its simplicity. subprocess is better" - how subprocess is better? I am happily using os.system, not sure how switching to subprocess and remembering extra shell=True benefits me. What kind of thing is better in subprocess? by reducing activity, 26.03.2021 09:14
You're right in that os.system(...) is a reasonable choice for executing commands in terms of simple "blind" execution. However, the use cases are rather limited - as soon as you want to capture the output, you have to use a whole other library and then you start having both - subprocess and os for similar use cases in your code. I prefer to keep the code clean and use only one of them. Second, and I would have put that section at the top but the TL;DR has to answer the question exactly, you should not use shell=True, but instead what I've written in the Preferred Way section. by fameman, 27.03.2021 11:30
The problem with os.system(...) and shell=True is that you're spawning a new shell process, just to execute your command. This means, you have to do manual escaping which is not as simple as you might think - especially when targeting both POSIX and Windows. For user-supplied input, this is a no-go (just imagine the user entered something with " quotes - you'd have to escape them as well). Also, the shell process itself could load code you don't need - not only does it delay the program, but it could also lead to unexpected side effects, ending with a wrong return code. by fameman, 27.03.2021 11:34
Summing up, os.system(...) is valid to use, indeed. But as soon as you're writing more than a quick python helper script, I'd recommend you to go for subprocess.run without shell=True. For more information about the drawbacks of os.system, I'd like to propose you a read through this SO answer: stackoverflow.com/a/44731082/6685358 by fameman, 27.03.2021 11:35
thanks! I wanted to edit "better" to include that link, but I got error about full edit queue. by reducing activity, 27.03.2021 15:01
1
24

There is another difference here which is not mentioned previously.

subprocess.Popen executes the <command> as a subprocess. In my case, I need to execute file <a> which needs to communicate with another program, <b>.

I tried subprocess, and execution was successful. However <b> could not communicate with <a>. Everything is normal when I run both from the terminal.

One more: (NOTE: kwrite behaves different from other applications. If you try the below with Firefox, the results will not be the same.)

If you try os.system("kwrite"), program flow freezes until the user closes kwrite. To overcome that I tried instead os.system(konsole -e kwrite). This time program continued to flow, but kwrite became the subprocess of the console.

Anyone runs the kwrite not being a subprocess (i.e. in the system monitor it must appear at the leftmost edge of the tree).

08.01.2010 21:11
Comments
What do you mean by "Anyone runs the kwrite not being a subprocess"? by Peter Mortensen, 03.06.2018 20:14
0
24

os.system does not allow you to store results, so if you want to store results in some list or something, a subprocess.call works.

11.06.2012 22:28
0
22

I tend to use subprocess together with shlex (to handle escaping of quoted strings):

>>> import subprocess, shlex
>>> command = 'ls -l "/your/path/with spaces/"'
>>> call_params = shlex.split(command)
>>> print call_params
["ls", "-l", "/your/path/with spaces/"]
>>> subprocess.call(call_params)
30.04.2014 14:37
0
22

subprocess.check_call is convenient if you don't want to test return values. It throws an exception on any error.

18.01.2011 19:21
0
17

In Windows you can just import the subprocess module and run external commands by calling subprocess.Popen(), subprocess.Popen().communicate() and subprocess.Popen().wait() as below:

# Python script to run a command line
import subprocess

def execute(cmd):
    """
        Purpose  : To execute a command and return exit status
        Argument : cmd - command to execute
        Return   : exit_code
    """
    process = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    (result, error) = process.communicate()

    rc = process.wait()

    if rc != 0:
        print "Error: failed to execute command:", cmd
        print error
    return result
# def

command = "tasklist | grep python"
print "This process detail: \n", execute(command)

Output:

This process detail:
python.exe                     604 RDP-Tcp#0                  4      5,660 K
17.06.2016 09:14
0
17

I wrote a library for this, shell.py.

It's basically a wrapper for popen and shlex for now. It also supports piping commands, so you can chain commands easier in Python. So you can do things like:

ex('echo hello shell.py') | "awk '{print $2}'"
01.05.2014 20:49
0
16

You can use Popen, and then you can check the procedure's status:

from subprocess import Popen

proc = Popen(['ls', '-l'])
if proc.poll() is None:
    proc.kill()

Check out subprocess.Popen.

16.07.2012 15:16
1
16

To fetch the network id from the OpenStack Neutron:

#!/usr/bin/python
import os
netid = "nova net-list | awk '/ External / { print $2 }'"
temp = os.popen(netid).read()  /* Here temp also contains new line (\n) */
networkId = temp.rstrip()
print(networkId)

Output of nova net-list

+--------------------------------------+------------+------+
| ID                                   | Label      | CIDR |
+--------------------------------------+------------+------+
| 431c9014-5b5d-4b51-a357-66020ffbb123 | test1      | None |
| 27a74fcd-37c0-4789-9414-9531b7e3f126 | External   | None |
| 5a2712e9-70dc-4b0e-9281-17e02f4684c9 | management | None |
| 7aa697f5-0e60-4c15-b4cc-9cb659698512 | Internal   | None |
+--------------------------------------+------------+------+

Output of print(networkId)

27a74fcd-37c0-4789-9414-9531b7e3f126
20.07.2016 09:50
Comments
You should not recommend os.popen() in 2016. The Awk script could easily be replaced with native Python code. by tripleee, 03.12.2018 05:49
1
15

Under Linux, in case you would like to call an external command that will execute independently (will keep running after the Python script terminates), you can use a simple queue as task spooler or the at command.

An example with task spooler:

import os
os.system('ts <your-command>')

Notes about task spooler (ts):

  1. You could set the number of concurrent processes to be run ("slots") with:

    ts -S <number-of-slots>

  2. Installing ts doesn't requires admin privileges. You can download and compile it from source with a simple make, add it to your path and you're done.

27.11.2016 00:15
Comments
ts is not standard on any distro I know of, though the pointer to at is mildly useful. You should probably also mention batch. As elsewhere, the os.system() recommendation should probably at least mention that subprocess is its recommended replacement. by tripleee, 03.12.2018 05:43
0
14

A simple way is to use the os module:

import os
os.system('ls')

Alternatively, you can also use the subprocess module:

import subprocess
subprocess.check_call('ls')

If you want the result to be stored in a variable try:

import subprocess
r = subprocess.check_output('ls')
24.08.2014 21:46
2
14

Very simplest way to run any command and get the result back:

from commands import getstatusoutput

try:
    return getstatusoutput("ls -ltr")
except Exception, e:
    return None
25.07.2012 06:51
Comments
Is this going to be deprecated in python 3.0? by 719016, 04.03.2014 14:16
Indeed, the commands documentation from Python 2.7 says it was deprecated in 2.6 and will be removed in 3.0. by tripleee, 03.12.2018 05:06
1
15

Invoke is a Python (2.7 and 3.4+) task execution tool and library. It provides a clean, high-level API for running shell commands:

>>> from invoke import run
>>> cmd = "pip install -r requirements.txt"
>>> result = run(cmd, hide=True, warn=True)
>>> print(result.ok)
True
>>> print(result.stdout.splitlines()[-1])
Successfully installed invocations-0.13.0 pep8-1.5.7 spec-1.3.1
14.09.2018 22:20
Comments
This is a great library. I was trying to explain it to a coworker the other day adn described it like this: invoke is to subprocess as requests is to urllib3. by user9074332, 12.03.2019 02:00
2
11

Here are my two cents: In my view, this is the best practice when dealing with external commands...

These are the return values from the execute method...

pass, stdout, stderr = execute(["ls","-la"],"/home/user/desktop")

This is the execute method...

def execute(cmdArray,workingDir):

    stdout = ''
    stderr = ''

    try:
        try:
            process = subprocess.Popen(cmdArray,cwd=workingDir, stdout=subprocess.PIPE, stderr=subprocess.PIPE, bufsize=1)
        except OSError:
            return [False, '', 'ERROR : command(' + ' '.join(cmdArray) + ') could not get executed!']

        for line in iter(process.stdout.readline, b''):

            try:
                echoLine = line.decode("utf-8")
            except:
                echoLine = str(line)

            stdout += echoLine

        for line in iter(process.stderr.readline, b''):

            try:
                echoLine = line.decode("utf-8")
            except:
                echoLine = str(line)

            stderr += echoLine

    except (KeyboardInterrupt,SystemExit) as err:
        return [False,'',str(err)]

    process.stdout.close()

    returnCode = process.wait()
    if returnCode != 0 or stderr != '':
        return [False, stdout, stderr]
    else:
        return [True, stdout, stderr]
14.10.2015 07:12
Comments
Deadlock potential: use the .communicate method instead by pppery, 07.07.2016 02:15
Better yet, avoid Popen() and use the higher-level API which is now collected into the single function subprocess.run() by tripleee, 03.12.2018 05:27
0
13

MOST OF THE CASES:

For the most of cases, a short snippet of code like this is all you are going to need:

import subprocess
import shlex

source = "test.txt"
destination = "test_copy.txt"

base = "cp {source} {destination}'"
cmd = base.format(source=source, destination=destination)
subprocess.check_call(shlex.split(cmd))

It is clean and simple.

subprocess.check_call run command with arguments and wait for command to complete.

shlex.split split the string cmd using shell-like syntax

REST OF THE CASES:

If this do not work for some specific command, most probably you have a problem with command-line interpreters. The operating system chose the default one which is not suitable for your type of program or could not found an adequate one on the system executable path.

Example:

Using the redirection operator on a Unix system

input_1 = "input_1.txt"
input_2 = "input_2.txt"
output = "merged.txt"
base_command = "/bin/bash -c 'cat {input} >> {output}'"

base_command.format(input_1, output=output)
subprocess.check_call(shlex.split(base_command))

base_command.format(input_2, output=output)
subprocess.check_call(shlex.split(base_command))

As it is stated in The Zen of Python: Explicit is better than implicit

So if using a Python >=3.6 function, it would look something like this:

import subprocess
import shlex

def run_command(cmd_interpreter: str, command: str) -> None:
    base_command = f"{cmd_interpreter} -c '{command}'"
    subprocess.check_call(shlex.split(base_command)

28.11.2019 13:40
2
12

Often, I use the following function for external commands, and this is especially handy for long running processes. The below method tails process output while it is running and returns the output, raises an exception if process fails.

It comes out if the process is done using the poll() method on the process.

import subprocess,sys

def exec_long_running_proc(command, args):
    cmd = "{} {}".format(command, " ".join(str(arg) if ' ' not in arg else arg.replace(' ','\ ') for arg in args))
    print(cmd)
    process = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)

    # Poll process for new output until finished
    while True:
        nextline = process.stdout.readline().decode('UTF-8')
        if nextline == '' and process.poll() is not None:
            break
        sys.stdout.write(nextline)
        sys.stdout.flush()

    output = process.communicate()[0]
    exitCode = process.returncode

    if (exitCode == 0):
        return output
    else:
        raise Exception(command, exitCode, output)

You can invoke it like this:

exec_long_running_proc(command = "hive", args=["-f", hql_path])
23.03.2018 02:30
Comments
You'll get unexpected results passing an arg with space. Using repr(arg) instead of str(arg) might help by the mere coincidence that python and sh escape quotes the same way by sbk, 17.05.2018 12:08
@sbk repr(arg) didn't really help, the above code handles spaces as well. Now the following works exec_long_running_proc(command = "ls", args=["-l", "~/test file*"]) by am5, 17.11.2018 00:07
0
9

I wrote a small library to help with this use case:

https://pypi.org/project/citizenshell/

It can be installed using

pip install citizenshell

And then used as follows:

from citizenshell import sh
assert sh("echo Hello World") == "Hello World"

You can separate standard output from standard error and extract the exit code as follows:

result = sh(">&2 echo error && echo output && exit 13")
assert result.stdout() == ["output"]
assert result.stderr() == ["error"]
assert result.exit_code() == 13

And the cool thing is that you don't have to wait for the underlying shell to exit before starting processing the output:

for line in sh("for i in 1 2 3 4; do echo -n 'It is '; date +%H:%M:%S; sleep 1; done", wait=False)
    print ">>>", line + "!"

will print the lines as they are available thanks to the wait=False

>>> It is 14:24:52!
>>> It is 14:24:53!
>>> It is 14:24:54!
>>> It is 14:24:55!

More examples can be found at https://github.com/meuter/citizenshell

30.10.2018 11:40
0
9

Just to add to the discussion, if you include using a Python console, you can call external commands from IPython. While in the IPython prompt, you can call shell commands by prefixing '!'. You can also combine Python code with the shell, and assign the output of shell scripts to Python variables.

For instance:

In [9]: mylist = !ls

In [10]: mylist
Out[10]:
['file1',
 'file2',
 'file3',]
19.06.2013 23:18
0
9

Calling an external command in Python

A simple way to call an external command is using os.system(...). And this function returns the exit value of the command. But the drawback is we won't get stdout and stderr.

ret = os.system('some_cmd.sh')
if ret != 0 :
    print 'some_cmd.sh execution returned failure'

Calling an external command in Python in background

subprocess.Popen provides more flexibility for running an external command rather than using os.system. We can start a command in the background and wait for it to finish. And after that we can get the stdout and stderr.

proc = subprocess.Popen(["./some_cmd.sh"], stdout=subprocess.PIPE)
print 'waiting for ' + str(proc.pid)
proc.wait()
print 'some_cmd.sh execution finished'
(out, err) = proc.communicate()
print 'some_cmd.sh output : ' + out

Calling a long running external command in Python in the background and stop after some time

We can even start a long running process in the background using subprocess.Popen and kill it after sometime once its task is done.

proc = subprocess.Popen(["./some_long_run_cmd.sh"], stdout=subprocess.PIPE)
# Do something else
# Now some_long_run_cmd.sh exeuction is no longer needed, so kill it
os.system('kill -15 ' + str(proc.pid))
print 'Output : ' proc.communicate()[0]
04.04.2018 06:57
0
8

Use:

import subprocess

p = subprocess.Popen("df -h", shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE).communicate()[0]
print p.split("\n")

It gives nice output which is easier to work with:

['Filesystem      Size  Used Avail Use% Mounted on',
 '/dev/sda6        32G   21G   11G  67% /',
 'none            4.0K     0  4.0K   0% /sys/fs/cgroup',
 'udev            1.9G  4.0K  1.9G   1% /dev',
 'tmpfs           387M  1.4M  386M   1% /run',
 'none            5.0M     0  5.0M   0% /run/lock',
 'none            1.9G   58M  1.9G   3% /run/shm',
 'none            100M   32K  100M   1% /run/user',
 '/dev/sda5       340G  222G  100G  69% /home',
 '']
24.06.2016 11:29
0
7

Use subprocess.call:

from subprocess import call

# Using list
call(["echo", "Hello", "world"])

# Single string argument varies across platforms so better split it
call("echo Hello world".split(" "))
12.04.2014 11:58
3
8

As an example (in Linux):

import subprocess
subprocess.run('mkdir test.dir', shell=True)

This creates test.dir in the current directory. Note that this also works:

import subprocess
subprocess.call('mkdir test.dir', shell=True)

The equivalent code using os.system is:

import os
os.system('mkdir test.dir')

Best practice would be to use subprocess instead of os, with .run favored over .call. All you need to know about subprocess is here. Also, note that all Python documentation is available for download from here. I downloaded the PDF packed as .zip. I mention this because there's a nice overview of the os module in tutorial.pdf (page 81). Besides, it's an authoritative resource for Python coders.

31.01.2018 17:42
Comments
According to docs.python.org/2/library/…, "shell=True" may raise a security concern. by Nick Predey, 20.03.2018 18:54
@Nick Predley: noted, but "shell=False" doesn't perform the desired function. What specifically are the security concerns and what's the alternative? Please let me know asap: I do not wish to post anything which may cause problems for anyone viewing this. by User, 21.03.2018 19:49
The basic warning is in the documentation but this question explains it in more detail: stackoverflow.com/questions/3172470/… by tripleee, 03.12.2018 05:14
4
3

The subprocess module described previously by Eli is very powerful, but the syntax to make a bog-standard system call and inspect its output, is unnecessarily prolix.

The easiest way to make a system call is with the commands module (Linux only).

> import commands
> commands.getstatusoutput("grep matter alice-in-wonderland.txt")
(0, "'Then it doesn't matter which way you go,' said the Cat.")

The first item in the tuple is the return code of the process. The second item is its standard output (and standard error, merged).

Python 2.6 (2008) deprecated the commands module, but that doesn't mean you shouldn't use it. Only that they're not developing it any more, which is okay, because it's already perfect (at it's a small, but important, function).


Update 2015: Python 3.5 added subprocess.run which is much easier to use than the older subprocess API https://docs.python.org/3/library/subprocess.html#subprocess.run . I recommend that.

18.04.2013 17:39
Comments
Deprecated doesn't only mean "isn't developed anymore" but also "you are discouraged from using this". Deprecated features may break anytime, may be removed anytime, or may dangerous. You should never use this in important code. Deprecation is merely a better way than removing a feature immediately, because it gives programmers the time to adapt and replace their deprecated functions. by Misch, 19.04.2013 08:07
Just to prove my point: "Deprecated since version 2.6: The commands module has been removed in Python 3. Use the subprocess module instead." by Misch, 19.04.2013 08:14
It's not dangerous! The Python devs are careful only to break features between major releases (ie. between 2.x and 3.x). I've been using the commands module since 2004's Python 2.4. It works the same today in Python 2.7. by Colonel Panic, 23.04.2013 16:09
With dangerous, I didn't mean that it may be removed anytime (that's a different problem), neither did I say that it is dangerous to use this specific module. However it may become dangerous if a security vulnerability is discovered but the module isn't further developed or maintained. (I don't want to say that this module is or isn't vulnerable to security issues, just talking about deprecated stuff in general) by Misch, 23.04.2013 16:23
1
6

After some research, I have the following code which works very well for me. It basically prints both standard output and standard error in real time.

stdout_result = 1
stderr_result = 1


def stdout_thread(pipe):
    global stdout_result
    while True:
        out = pipe.stdout.read(1)
        stdout_result = pipe.poll()
        if out == '' and stdout_result is not None:
            break

        if out != '':
            sys.stdout.write(out)
            sys.stdout.flush()


def stderr_thread(pipe):
    global stderr_result
    while True:
        err = pipe.stderr.read(1)
        stderr_result = pipe.poll()
        if err == '' and stderr_result is not None:
            break

        if err != '':
            sys.stdout.write(err)
            sys.stdout.flush()


def exec_command(command, cwd=None):
    if cwd is not None:
        print '[' + ' '.join(command) + '] in ' + cwd
    else:
        print '[' + ' '.join(command) + ']'

    p = subprocess.Popen(
        command, stdout=subprocess.PIPE, stderr=subprocess.PIPE, cwd=cwd
    )

    out_thread = threading.Thread(name='stdout_thread', target=stdout_thread, args=(p,))
    err_thread = threading.Thread(name='stderr_thread', target=stderr_thread, args=(p,))

    err_thread.start()
    out_thread.start()

    out_thread.join()
    err_thread.join()

    return stdout_result + stderr_result
14.03.2014 02:59
Comments
your code may lose data when the subprocess exits while there is some data is buffered. Read until EOF instead, see teed_call() by jfs, 13.07.2015 18:52
0
7

There are a lot of different ways to run external commands in Python, and all of them have their own plus sides and drawbacks.

My colleagues and me have been writing Python system administration tools, so we need to run a lot of external commands, and sometimes you want them to block or run asynchronously, time-out, update every second, etc.

There are also different ways of handling the return code and errors, and you might want to parse the output, and provide new input (in an expect kind of style). Or you will need to redirect standard input, standard output, and standard error to run in a different tty (e.g., when using GNU Screen).

So you will probably have to write a lot of wrappers around the external command. So here is a Python module which we have written which can handle almost anything you would want, and if not, it's very flexible so you can easily extend it:

https://github.com/hpcugent/vsc-base/blob/master/lib/vsc/utils/run.py

It doesn't work stand-alone and requires some of our other tools, and got a lot of specialised functionality over the years, so it might not be a drop-in replacement for you, but it can give you a lot of information on how the internals of Python for running commands work and ideas on how to handle certain situations.

17.04.2013 14:10
1
7

Here is calling an external command and return or print the command's output:

Python Subprocess check_output is good for

Run command with arguments and return its output as a byte string.

import subprocess
proc = subprocess.check_output('ipconfig /all')
print proc
11.10.2016 02:26
Comments
The argument should properly be tokenized into a list, or you should explicitly pass in shell=True. In Python 3.x (where x > 3 I think) you can retrieve the output as a proper string with universal_newlines=True and you probably want to switch to subproces.run() by tripleee, 03.12.2018 05:22
1
5

For Python 3.5+ it is recommended that you use the run function from the subprocess module. This returns a CompletedProcess object, from which you can easily obtain the output as well as return code.

from subprocess import PIPE, run

command = ['echo', 'hello']
result = run(command, stdout=PIPE, stderr=PIPE, universal_newlines=True)
print(result.returncode, result.stdout, result.stderr)
17.03.2016 10:48
Comments
answer with run function was added in 2015 year. You repeated it. I think it was a reason of down vote by Greg Eremeev, 11.03.2017 18:27
1
4

Using the Popen function of the subprocess Python module is the simplest way of running Linux commands. In that, the Popen.communicate() function will give your commands output. For example

import subprocess

..
process = subprocess.Popen(..)   # Pass command and arguments to the function
stdout, stderr = process.communicate()   # Get command output and error
..
24.07.2015 19:12
Comments
This is no longer true, and probably wasn't when this answer was posted. You should prefer subprocess.check_call() and friends unless you absolutely need the lower-level control of the more-complex Popen(). In recent Python versions, the go-to workhorse is subprocess.run() by tripleee, 03.12.2018 05:30
0
7

For using subprocess in Python 3.5+, the following did the trick for me on Linux:

import subprocess

# subprocess.run() returns a completed process object that can be inspected
c = subprocess.run(["ls", "-ltrh"], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
print(c.stdout.decode('utf-8'))

As mentioned in the documentation, PIPE values are byte sequences and for properly showing them decoding should be considered. For later versions of Python, text=True and encoding='utf-8' are added to kwargs of subprocess.run().

The output of the abovementioned code is:

total 113M
-rwxr-xr-x  1 farzad farzad  307 Jan 15  2018 vpnscript
-rwxrwxr-x  1 farzad farzad  204 Jan 15  2018 ex
drwxrwxr-x  4 farzad farzad 4.0K Jan 22  2018 scripts
.... # Some other lines
29.01.2019 05:11