Finally learn how to use command line apps… by making one!

Zack Akil

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May 21, 2018 · 5 min read

At the risk of alienating a lot of readers… I grew up with GUI’s, so I never needed to learn the ways of the terminal! This is socially acceptable in todays society of friendly user interfaces, except maybe if you’re in software engineering… whoops!

Getting back my software engineering street cred’

I’ve gotten pretty far just through copy and pasting full command-line operations from Stack-Overflow but have never become comfortable enough to use a command line app “properly”. What finally caused it to click for me was when I stumbled across a very handy python library called argparse that allows you to build a nice robust command line interface for your python scripts.

This tutorial is great in-depth explanation about how to use argparse, but I’ll go over the key eye openers:

argparse is part of the standard python library so pop open your code editor and follow along (you don’t need to install anything)!

HELP

The most useful part of every command line app!

# inside a file called my_app.pyimport argparse
parser = argparse.ArgumentParser(description="Nice little CL app!")
parser.parse_args()

The code above will do nothing, except by default you have the help flag!

In the command line you can run:

python my_app.py --help

or

python my_app.py -h

and you’ll get an output like this:

usage: my_app.py [-h]
Nice little CL app!
optional arguments:-h, --help  show this help message and exit

Seems pretty cool right? but wait, when you use argparse to add more functions (see below) this help output will automatically fill up with all of the instructions of how you can use your app!

REQUIRED ARGUMENTS

Lets say you want to have your app take in some variable, we just use the parser.add_argument() function and give our argument some label (in this case “name”):

import argparse
parser = argparse.ArgumentParser(description="Nice little CL app!")
parser.add_argument("name", help="Just your name, nothing special")
args = parser.parse_args()
print("Your name is what? " + args.name)

Notice how we added the help text! Now when we run python my_app.y -h we get all of the app details:

usage: my_app.py [-h] name
Nice little CL app!
positional arguments:name        Just your name, nothing special
optional arguments:-h, --help  show this help message and exit

Pretty cool, but let’s run our app with python my_app.py

usage: my_app.py [-h] namemy_app.py: error: too few arguments

That’s right! Automatic input checking!

Now lets run python my_app.py "Slim Shady"

Your name is what? Slim Shady

Pretty slick!

OPTIONAL ARGUMENTS

Maybe you want to give someone the option of telling you little more about themselves? Adding a double dash when using parser.add_argument() function will make that argument optional!

import argparse
parser = argparse.ArgumentParser(description=”Nice little CL app!”)
parser.add_argument(“name”, help=”Just your name, nothing special”)
parser.add_argument("--profession”, help=”Your nobel profession”)
args = parser.parse_args()
print(“Your name is what? “ + args.name)if args.profession:
    print(“What is your profession!? a “ + args.profession)

If you want to pass in a variable for that argument you just need to specify the double-dashed argument name before the variable you want to pass:

python my_app.py "Slim Shady" --profession "gift wrapper"

which gives you :

Your name is what? Slim ShadyWhat is your profession!? a gift wrapper

Or you don’t have to, it is optional after all!

python my_app.py "Slim Shady"

still gives you:

Your name is what? Slim Shady

and the magic once again when running python my_app.py -h :

usage: my_app.py [-h] [--profession PROFESSION] name
Nice little CL app!
positional arguments:name                      Just your name, nothing specialoptional arguments:-h, --help                show this help message and exit--profession PROFESSION   Your nobel profession

FLAGS

Maybe you just want to enable something cool to happen. Add action="store_true" to your parser.add_argument() function and you have yourself a flag argument :

import argparseparser = argparse.ArgumentParser(description="Nice little CL app!")
parser.add_argument("name", help="Just your name, nothing special")
parser.add_argument("--profession", help="Your nobel profession")
parser.add_argument("--cool", action="store_true", help="Add a little cool")
args = parser.parse_args()
print("Your name is what? " + args.name)cool_addition = " and dragon tamer" if args.cool else ""if args.profession:
    print("What is your profession!? a " + args.profession + cool_addition)

It’s pretty neat, you just plop the flag name into your command like so:

python my_app.py "Slim Shady" --profession "gift wrapper" --cool

and presto!

Your name is what? Slim ShadyWhat is your profession!? a gift wrapper and dragon tamer

and remember, you don’t have to use it, it’s just a flag:

python my_app.py "Slim Shady" --profession "gift wrapper"

will still give:

Your name is what? Slim ShadyWhat is your profession!? a gift wrapper

look though at the help command python my_app.py -h :

usage: my_app.py [-h] [--profession PROFESSION] [--cool] name
Nice little CL app!
positional arguments:name                      Just your name, nothing specialoptional arguments:-h, --help                show this help message and exit--profession PROFESSION   Your nobel profession--cool                    Add a little cool

I’m just going to assume you’re as satisfied with that as I am from now on.

SHORT FORM

Unveiling the mysterious one character arguments that confused me for so long. Just by adding a single letter prefixed with a single dash to your parser.add_argument() functions you have super short versions of the same arguments:

import argparseparser = argparse.ArgumentParser(description="Nice little CL app!")
parser.add_argument("name", help="Just your name, nothing special")parser.add_argument("-p", "--profession", help="Your nobel profession")parser.add_argument("-c", "--cool", action="store_true", help="Add a little cool")args = parser.parse_args()
print("Your name is what? " + args.name)
cool_addition = " and dragon tamer" if args.cool else ""
if args.profession:
    print("What is your profession!? a " + args.profession + cool_addition)

So that instead of typing in :

python my_app.py "Slim Shady" --profession "gift wrapper" --cool

you can just type:

python my_app.py "Slim Shady" -p "gift wrapper" -c

and you’ll get the same output:

Your name is what? Slim ShadyWhat is your profession!? a gift wrapper and dragon tamer

And this is reflected in the help text (python my_app.py -h ):

usage: my_app.py [-h] [--profession PROFESSION] [--cool] nameNice little CL app!positional arguments:name                           Just your name, nothing specialoptional arguments:-h, --help                      show this help message and exit-p PROFESSION, --profession PROFESSION   Your nobel profession-c, --cool                      Add a little cool

You now have a perfect little command line app and are hopefully more comfortable with finding your way around command line apps!

Just remember --help !