Python-os. Command Execution

Stephen Hansen me+list/python at ixokai.io
Sat Feb 19 17:17:38 CET 2011


On 2/19/11 1:44 AM, Ganesh Kumar wrote:
> os.system("ls -l") & root.destroy

"&" here doesn't do what you think it does. Its a bitwise AND operator,
which is not the same thing as you may be expecting from other languages.

In Python, you'd do something more like:

   os.system("ls -l") and root.destroy()

But that's bad Python. First, let's look at what os.system actually returns:

    >>> import os
    >>> help(os.system)
    system(...)
        system(command) -> exit_status

        Execute the command (a string) in a subshell.

Now, an exit status of 0 is actually generally considered a success,
while 1-200odd is a failure.

Now, if you want to run 'root.destroy()' if and only if the 'ls -l'
command fails, you could do the above with an 'and'. Python DOES
short-circuit its logical AND's, so the root.destroy() will NOT be run
if ls -l succeeds (returns 0)... but really.

That's _very_ cryptic.

Its much better to do:

   if os.system("ls -l") != 0:
        root.destroy()

(You could simply say 'if not os.system("ls -l")', but in this context
where the more unusual behavior of '0 is true, >0 is false' which is
opposite of what is normal in Python, I prefer to explicitly spell it out)

If you instead mean the "&" to simply separate the statements, so that
after the os.system is done, then regardless of the outcome root.destroy
is called-- then... just separate the statements.

    os.system("ls -l")
    root.destroy()

You can use them on the same line with a semicolon if you really must.
But don't do that. :-)

Now, all of that said -- I'm not sure what exactly is going WRONG with
your program. You said GUI, and perhaps that's the problem? If you are
calling a unix interactive command line program from within a GUI
context, things are quite likely to go wrong unless you do a lot of
extra work. Are you expecting a new console window to pop up for 'top'
and for that to run on its own? If so -- that won't happen on its own or
with a single function call sort of easy way.

What OS are you on? What are you actually trying to do here?

-- 

   Stephen Hansen
   ... Also: Ixokai
   ... Mail: me+list/python (AT) ixokai (DOT) io
   ... Blog: http://meh.ixokai.io/

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