Calling an external command in Python


Answers

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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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 "my mama didnt love me && rm -rf /".

Question

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




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.




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')



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



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.




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.




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




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




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



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()



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 docs.

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'>]



If what you need is 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).




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



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.




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 docs 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 (win xp), the parent process will not finish until the longtask.py has finished its work. It is not what you want in CGI-script. The problem is not specific to Python, in PHP community the problems are the same.

The solution is to pass DETACHED_PROCESS Process Creation Flag to the underlying CreateProcess function in win 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 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.




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