How to call an external program in python and retrieve the output and return code?
Look at the subprocess module: a simple example follows...
from subprocess import Popen, PIPE
process = Popen(["ls", "-la", "."], stdout=PIPE)
(output, err) = process.communicate()
exit_code = process.wait()
Call external program from python and get its output
To run an external program and get its output, use subprocess.check_output
on Python 2.7+. The example from the docs:
>>> subprocess.check_output(["ls", "-l", "/dev/null"])
'crw-rw-rw- 1 root root 1, 3 Oct 18 2007 /dev/null\n'
check_call
just returns the return code of the program, not the output.
Running shell command and capturing the output
In all officially maintained versions of Python, the simplest approach is to use the subprocess.check_output
function:
>>> subprocess.check_output(['ls', '-l'])
b'total 0\n-rw-r--r-- 1 memyself staff 0 Mar 14 11:04 files\n'
check_output
runs a single program that takes only arguments as input.1 It returns the result exactly as printed to stdout
. If you need to write input to stdin
, skip ahead to the run
or Popen
sections. If you want to execute complex shell commands, see the note on shell=True
at the end of this answer.
The check_output
function works in all officially maintained versions of Python. But for more recent versions, a more flexible approach is available.
Modern versions of Python (3.5 or higher): run
If you're using Python 3.5+, and do not need backwards compatibility, the new run
function is recommended by the official documentation for most tasks. It provides a very general, high-level API for the subprocess
module. To capture the output of a program, pass the subprocess.PIPE
flag to the stdout
keyword argument. Then access the stdout
attribute of the returned CompletedProcess
object:
>>> import subprocess
>>> result = subprocess.run(['ls', '-l'], stdout=subprocess.PIPE)
>>> result.stdout
b'total 0\n-rw-r--r-- 1 memyself staff 0 Mar 14 11:04 files\n'
The return value is a bytes
object, so if you want a proper string, you'll need to decode
it. Assuming the called process returns a UTF-8-encoded string:
>>> result.stdout.decode('utf-8')
'total 0\n-rw-r--r-- 1 memyself staff 0 Mar 14 11:04 files\n'
This can all be compressed to a one-liner if desired:
>>> subprocess.run(['ls', '-l'], stdout=subprocess.PIPE).stdout.decode('utf-8')
'total 0\n-rw-r--r-- 1 memyself staff 0 Mar 14 11:04 files\n'
If you want to pass input to the process's stdin
, you can pass a bytes
object to the input
keyword argument:
>>> cmd = ['awk', 'length($0) > 5']
>>> ip = 'foo\nfoofoo\n'.encode('utf-8')
>>> result = subprocess.run(cmd, stdout=subprocess.PIPE, input=ip)
>>> result.stdout.decode('utf-8')
'foofoo\n'
You can capture errors by passing stderr=subprocess.PIPE
(capture to result.stderr
) or stderr=subprocess.STDOUT
(capture to result.stdout
along with regular output). If you want run
to throw an exception when the process returns a nonzero exit code, you can pass check=True
. (Or you can check the returncode
attribute of result
above.) When security is not a concern, you can also run more complex shell commands by passing shell=True
as described at the end of this answer.
Later versions of Python streamline the above further. In Python 3.7+, the above one-liner can be spelled like this:
>>> subprocess.run(['ls', '-l'], capture_output=True, text=True).stdout
'total 0\n-rw-r--r-- 1 memyself staff 0 Mar 14 11:04 files\n'
Using run
this way adds just a bit of complexity, compared to the old way of doing things. But now you can do almost anything you need to do with the run
function alone.
Older versions of Python (3-3.4): more about check_output
If you are using an older version of Python, or need modest backwards compatibility, you can use the check_output
function as briefly described above. It has been available since Python 2.7.
subprocess.check_output(*popenargs, **kwargs)
It takes takes the same arguments as Popen
(see below), and returns a string containing the program's output. The beginning of this answer has a more detailed usage example. In Python 3.5+, check_output
is equivalent to executing run
with check=True
and stdout=PIPE
, and returning just the stdout
attribute.
You can pass stderr=subprocess.STDOUT
to ensure that error messages are included in the returned output. When security is not a concern, you can also run more complex shell commands by passing shell=True
as described at the end of this answer.
If you need to pipe from stderr
or pass input to the process, check_output
won't be up to the task. See the Popen
examples below in that case.
Complex applications and legacy versions of Python (2.6 and below): Popen
If you need deep backwards compatibility, or if you need more sophisticated functionality than check_output
or run
provide, you'll have to work directly with Popen
objects, which encapsulate the low-level API for subprocesses.
The Popen
constructor accepts either a single command without arguments, or a list containing a command as its first item, followed by any number of arguments, each as a separate item in the list. shlex.split
can help parse strings into appropriately formatted lists. Popen
objects also accept a host of different arguments for process IO management and low-level configuration.
To send input and capture output, communicate
is almost always the preferred method. As in:
output = subprocess.Popen(["mycmd", "myarg"],
stdout=subprocess.PIPE).communicate()[0]
Or
>>> import subprocess
>>> p = subprocess.Popen(['ls', '-a'], stdout=subprocess.PIPE,
... stderr=subprocess.PIPE)
>>> out, err = p.communicate()
>>> print out
.
..
foo
If you set stdin=PIPE
, communicate
also allows you to pass data to the process via stdin
:
>>> cmd = ['awk', 'length($0) > 5']
>>> p = subprocess.Popen(cmd, stdout=subprocess.PIPE,
... stderr=subprocess.PIPE,
... stdin=subprocess.PIPE)
>>> out, err = p.communicate('foo\nfoofoo\n')
>>> print out
foofoo
Note Aaron Hall's answer, which indicates that on some systems, you may need to set stdout
, stderr
, and stdin
all to PIPE
(or DEVNULL
) to get communicate
to work at all.
In some rare cases, you may need complex, real-time output capturing. Vartec's answer suggests a way forward, but methods other than communicate
are prone to deadlocks if not used carefully.
As with all the above functions, when security is not a concern, you can run more complex shell commands by passing shell=True
.
Notes
1. Running shell commands: the shell=True
argument
Normally, each call to run
, check_output
, or the Popen
constructor executes a single program. That means no fancy bash-style pipes. If you want to run complex shell commands, you can pass shell=True
, which all three functions support. For example:
>>> subprocess.check_output('cat books/* | wc', shell=True, text=True)
' 1299377 17005208 101299376\n'
However, doing this raises security concerns. If you're doing anything more than light scripting, you might be better off calling each process separately, and passing the output from each as an input to the next, via
run(cmd, [stdout=etc...], input=other_output)
Or
Popen(cmd, [stdout=etc...]).communicate(other_output)
The temptation to directly connect pipes is strong; resist it. Otherwise, you'll likely see deadlocks or have to do hacky things like this.
Call an external command in python and output to file
You can use system from os.
import os
os.system("ls -l > file.txt")
Writing the output of a command can be accomplished by ">". If you want to append instead of overwriting the file, you can use ">>".
How do I execute a program or call a system command?
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 thesubprocess
documentation for some helpful recipes.
On Python 3.4 and earlier, use subprocess.call
instead of .run
:
subprocess.call(["ls", "-l"])
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