How to check if a string is a valid shell command using Python?

user_10857616 0
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I am making a program that adds additional functionality to the standard command shell in Windows. For instance, typing google followed by keywords will open a new tab with Google search for those keywords, etc. Whenever the input doesn't refer to a custom function I've created, it gets processed as a shell command using subprocess.call(rawCommand, shell=True).

Since I'd like to anticipate when my input isn't a valid command and return something like f"Invalid command: {rawCommand}", how should I go about doing that?

So far I've tried subprocess.call(rawCommand) which also return the standard output as well as the exit code. So that looks like this:

>>> from subprocess import call
>>> a, b = call("echo hello!", shell=1), call("xyz arg1 arg2", shell=1)
hello!
'xyz' is not recognized as an internal or external command,
operable program or batch file.
>>> a
0
>>> b
1

I'd like to simply recieve that exit code. Any ideas on how I can do this?

subprocess cmd python
2 Answers

Should you one day want deal with encoding errors, get back the result of the command you're running, have a timeout or decide which exit codes other than 0 may not trigger errors (i'm looking at you, java runtime !), here's a complete function that does that job:

import os
from logging import getLogger
import subprocess

logger = getLogger()


def command_runner(command, valid_exit_codes=None, timeout=300, shell=False, encoding='utf-8',
                   windows_no_window=False, **kwargs):
    """
    Whenever we can, we need to avoid shell=True in order to preseve better security
    Runs system command, returns exit code and stdout/stderr output, and logs output on error
    valid_exit_codes is a list of codes that don't trigger an error
    windows_no_window will hide the command window (works with Microsoft Windows only)
    
    Accepts subprocess.check_output arguments
        
    """

    # Set default values for kwargs
    errors = kwargs.pop('errors', 'backslashreplace')  # Don't let encoding issues make you mad
    universal_newlines = kwargs.pop('universal_newlines', False)
    creationflags = kwargs.pop('creationflags', 0)
    if windows_no_window:
        creationflags = creationflags | subprocess.CREATE_NO_WINDOW

    try:
        # universal_newlines=True makes netstat command fail under windows
        # timeout does not work under Python 2.7 with subprocess32 < 3.5
        # decoder may be unicode_escape for dos commands or utf-8 for powershell
        output = subprocess.check_output(command, stderr=subprocess.STDOUT, shell=shell,
                                         timeout=timeout, universal_newlines=universal_newlines, encoding=encoding,
                                         errors=errors, creationflags=creationflags, **kwargs)

    except subprocess.CalledProcessError as exc:
        exit_code = exc.returncode
        try:
            output = exc.output
        except Exception:
            output = "command_runner: Could not obtain output from command."
        if exit_code in valid_exit_codes if valid_exit_codes is not None else [0]:
            logger.debug('Command [%s] returned with exit code [%s]. Command output was:' % (command, exit_code))
            if isinstance(output, str):
                logger.debug(output)
            return exc.returncode, output
        else:
            logger.error('Command [%s] failed with exit code [%s]. Command output was:' %
                         (command, exc.returncode))
            logger.error(output)
            return exc.returncode, output
    # OSError if not a valid executable
    except (OSError, IOError) as exc:
        logger.error('Command [%s] failed because of OS [%s].' % (command, exc))
        return None, exc
    except subprocess.TimeoutExpired:
        logger.error('Timeout [%s seconds] expired for command [%s] execution.' % (timeout, command))
        return None, 'Timeout of %s seconds expired.' % timeout
    except Exception as exc:
        logger.error('Command [%s] failed for unknown reasons [%s].' % (command, exc))
        logger.debug('Error:', exc_info=True)
        return None, exc
    else:
        logger.debug('Command [%s] returned with exit code [0]. Command output was:' % command)
        if output:
            logger.debug(output)
        return 0, output

Usage:

exit_code, output = command_runner('whoami', shell=True)
user_26354430

Some shells have a syntax-checking mode (e.g., bash -n), but that’s the only form of error that’s separable from “try to execute the commands and see what happens”. Defining a larger class of “immediate” errors is a fraught proposition: if echo hello; ./foo is invalid because foo can’t be found as a command, what about false && ./foo, which will never try to run it, or cp /bin/ls foo; ./foo, which may succeed (or might fail to copy)? What about eval $(configure_shell); foo which might or might not manipulate PATH so as to find foo? What about foo || install_foo, where the failure might be anticipated?

As such, anticipating failure is not possible in any meaningful sense: your only real option is to capture the command’s output/error (as mentioned in the comments) and report them in some useful way.

user_85862270
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