Question

I have a Python script and I was wondering how I can make it executable; in other words how can I run it by using a shell like bash.

I know the first thing is to stick on the first line #! /usr/bin/env python but then do I need for example the functions to be in a specific order (i.e., the main one at the top or the bottom). What’s more do I need to keep the extension .py for my python file (can I just call the function Dosomething?).

To be short, could you provide a simple guide, the important points someone has to take into account to make a Python file executable?

Answer 1

This is how I make an executable script. It doesn’t take eggs or anything like that into account. It’s just a simple script that I want to be able to execute. I’m assuming you are using linux.

#! /usr/bin/env python
import sys


def main():
    #
    # Do something ... Whatever processing you need to do, make it happen here.
    # Don't shove everything into main, break it up into testable functions!
    #
    # Whatever this function returns, is what the exit code of the interpreter,
    # i.e. your script, will be.  Because main is called by sys.exit(), it will
    # behave differently depending on what you return.
    # 
    # So, if you return None, 0 is returned.  If you return integer, that 
    # return code is used.  Anything else is printed to the console and 1 (error) 
    # is returned.
    #
    if an_error_occurred:
        return 'I\'m returning a string, it will be printed and 1 returned'

    # Otherwise 0, success is returned.
    return 0

# This is true if the script is run by the interpreter, not imported by another
# module.
if __name__ == '__main__':
    # main should return 0 for success, something else (usually 1) for error.
    sys.exit(main())

Now, if you’re permissions are set correctly, you can execute this script.

One thing to realize is as your script is processed each line is executed in the interpreter. This is true, regardless of how the processor “gets it”. That is importing a script as a module and executing it as a script essentially both work the same, in that they both execute each line of the module.

Once you realize your script is simply executing as it runs, you realize that the order of functions don’t matter. A function declaration is a function declaration. It’s when you call the function that matters.

So, in general, the layout of your script looks like this

def func1():
    pass
def func2():
    pass
def main():
    return 0

if __name__ == '__main__':
    sys.exit(main())

You create the functions you want to use first, then you use them. Hope it helps.