Thursday, October 6, 2011

Command Line Applications

I'm old -- I admit it -- and I feel that command-line applications are still very, very important. Linux, for example, is packed full of almost innumerable command-line applications. In some cases, the Linux GUI tools are specifically just wrappers around the underlying command-line applications.

For many types of high-volume data processing, command-line applications are essential.

I've seen command-line applications done very badly.

Overusing Main

When writing OO programs, it's absolutely essential that the OS interface (public static void main in Java or the if __name__ == "__main__": block in Python) does as little as possible.

A good command-line program has the underlying tasks or actions defined in some easy-to-work with class hierarchy built on the Command design pattern. The actual main program part does just a few things: gather the relevant environment variables, parse command-line options and arguments, identify the configuration files, and initiate the appropriate commands. Nothing application-specific.

When the main method does application-specific work, that application functionality is buried in a method that's particularly hard to reuse. It's important to keep the application functionality away from the OS interface.

I'm finding that main programs should look something like this:

if __name__ == "__main__":
    logging.basicConfig( stream=sys.stderr )
    args= parse_args()
    logging.getLogger().setLevel( args.verbosity )
        for file in args.file:
            with open( file, "r" ) as source:
                process_file( source, args )
        status= 0
    except Exception as e:
        logging.exception( e )
        status= 3
    sys.exit( status )

That's it.  Nothing more in the top-level main program.  The process_file function becomes a reusable "command" and something that can be tested independently.


  1. Thank you for posting this. This is actually something I knew I was doing wrong but I didn't quite know exactly where I was making the wrong turn. This gets me much closer to the clean code I would like to be writing in python. Any chance you have a simple script you wrote you would like to share with us, that uses this method? I would love to see an example I can read over and use to learn from.

  2. Hi.

    I myself like to wrap this in a main function like this:
    def main(argv):

    if __name__ == '__main__':

    Also, the standard python fileinput module may come in handy.

  3. This is my standard main.

    if __name__ == '__main__' :
    if os.path.exists( "logging.conf" ) :
    else :

    fnName = sys.argv[ 1 ] '** function %s', fnName )
    n = locals()[ fnName ]( args = sys.argv[ 2: ] ) '** bye' )