Check if a command is valid

[Kenny Meyer]

Chris Rebert (clp2 at rebertia.com) wrote:
> 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.

I'm glad you pointed that out, because you're right.  I subconciously
meant a file that is in the $PATH.

[snip]
> 
> 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?
>
I have tried this and found some unexpected issues with Python 2.6 which I
though I should point out:

Firstly, the function `check_output` in the `subprocess` module only comes with
Python 2.7, but I have found a similar function called `check_call` [1] which
seems is similar, but not the same.

[1] http://docs.python.org/library/subprocess.html#subprocess.check_call

The code now looks like this:

from shlex import split as shell_tokenize
from subprocess import check_call, CalledProcessError

def is_valid_command(command):
    try:
        executable = shell_tokenize(command)[0]
        check_call(['which', executable]) # Raises CalledProcessError if
										  # something went wrong
        return True                              
    except (ValueError, IndexError, CalledProcessError): # Catch exception if there
														 # was an error calling the process
        return False

The idea with `which` is really great one.

Thanks a lot, for your time and your input.

-- 
Onward and upwards,
Kenny Meyer
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 198 bytes
Desc: Digital signature
URL: <http://mail.python.org/pipermail/python-list/attachments/20100713/b4420cc7/attachment-0001.sig>