Check if a command is valid

[Chris Rebert]

On Mon, Jul 12, 2010 at 6:29 PM, Kenny Meyer <knny.myer at gmail.com> wrote:
> Hello,
>
> I have to figure out if a string is callable on a Linux system. I'm

"callable" seems vague. Is a command string with invalid arguments but
a valid executable "callable"? If no, then there's no general way to
test "callability" without actually running the command.

> actually doing this:
>
>    def is_valid_command(command):
>        retcode = 100 # initialize
>        if command:
>            retcode = subprocess.call(command, shell=True)
>        if retcode is 0:

That should be `== 0`, not `is 0`. The fact that `is 0` just so
happens to work is an implementation detail.

>            print "Valid command."
>        else:
>            print "Looks not so good..."
>
>    is_valid_command("ls")
>
> Never mind the code, because this is not the original.
> The side effect of subprocess.call() is that it *actually* executes
> it, but I just need the return code.

Well, you're not gonna be able to get the command's return code
without actually running it (unless perhaps you're referring to a
return code from the shell itself?).

> What are better ways of doing this?

One idea:

from shlex import split as shell_tokenize
from subprocess import check_output

def is_valid_command(command):
    try:
        executable = shell_tokenize(command)[0]
    except (ValueError, IndexError):# invalid shell syntax
        return False
    return bool(check_output(['which', executable]))# on the PATH?

Cheers,
Chris
--
http://blog.rebertia.com