Posted By: Anonymous
I want to write a function that will execute a shell command and return its output as a string, no matter, is it an error or success message. I just want to get the same result that I would have gotten with the command line.
What would be a code example that would do such a thing?
def run_command(cmd): # ?????? print run_command('mysqladmin create test -uroot -pmysqladmin12') # Should output something like: # mysqladmin: CREATE DATABASE failed; error: 'Can't create database 'test'; database exists'
In all officially maintained versions of Python, the simplest approach is to use the
>>> subprocess.check_output(['ls', '-l']) b'total 0n-rw-r--r-- 1 memyself staff 0 Mar 14 11:04 filesn'
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
Popen sections. If you want to execute complex shell commands, see the note on
shell=True at the end of this answer.
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):
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
>>> import subprocess >>> result = subprocess.run(['ls', '-l'], stdout=subprocess.PIPE) >>> result.stdout b'total 0n-rw-r--r-- 1 memyself staff 0 Mar 14 11:04 filesn'
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 0n-rw-r--r-- 1 memyself staff 0 Mar 14 11:04 filesn'
This can all be compressed to a one-liner if desired:
>>> subprocess.run(['ls', '-l'], stdout=subprocess.PIPE).stdout.decode('utf-8') 'total 0n-rw-r--r-- 1 memyself staff 0 Mar 14 11:04 filesn'
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 = 'foonfoofoon'.encode('utf-8') >>> result = subprocess.run(cmd, stdout=subprocess.PIPE, input=ip) >>> result.stdout.decode('utf-8') 'foofoon'
You can capture errors by passing
stderr=subprocess.PIPE (capture to
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 0n-rw-r--r-- 1 memyself staff 0 Mar 14 11:04 filesn'
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
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.
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
stdout=PIPE, and returning just the
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):
If you need deep backwards compatibility, or if you need more sophisticated functionality than
run provide, you’ll have to work directly with
Popen objects, which encapsulate the low-level API for subprocesses.
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()
>>> import subprocess >>> p = subprocess.Popen(['ls', '-a'], stdout=subprocess.PIPE, ... stderr=subprocess.PIPE) >>> out, err = p.communicate() >>> print out . .. foo
If you set
communicate also allows you to pass data to the process via
>>> cmd = ['awk', 'length($0) > 5'] >>> p = subprocess.Popen(cmd, stdout=subprocess.PIPE, ... stderr=subprocess.PIPE, ... stdin=subprocess.PIPE) >>> out, err = p.communicate('foonfoofoon') >>> print out foofoo
Note Aaron Hall’s answer, which indicates that on some systems, you may need to set
stdin all to
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
1. Running shell commands: the
Normally, each call to
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 101299376n'
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)
The temptation to directly connect pipes is strong; resist it. Otherwise, you’ll likely see deadlocks or have to do hacky things like this.