PythonBeginner.help

Python Command Line Tool Example

A command line tool is a Python script that you run from the terminal and give input to when the script starts.

This beginner-friendly example shows:

  • how to build a small Python CLI tool
  • how to read command line arguments with sys.argv
  • how to handle missing or invalid input
  • how to run the script from the terminal

If you are new to this, the main idea is simple: the user runs a file like python tool.py Alice, and your script uses Alice as input.

Quick example #

Use this as the smallest possible command line tool example.

Save it as tool.py:

import sys

if len(sys.argv) < 2:
    print("Usage: python tool.py NAME")
    sys.exit(1)

name = sys.argv[1]
print(f"Hello, {name}!")

Run it like this:

python tool.py Alice

Expected output:

Hello, Alice!

This example teaches the basic pattern:

  • import sys
  • check whether an argument exists
  • read the argument
  • print a result

What this example builds #

This page builds a very small script that runs from the terminal.

It shows how to:

  • run a Python file from the command line
  • pass input as command line arguments
  • check whether the user gave enough input
  • print a useful result

The goal is to help you understand the basic pattern of a command line tool before moving on to more advanced tools like argparse.

When to use a command line tool #

A command line tool is useful when:

  • you want to run a task quickly without editing the code
  • the script should accept different values each time
  • you want to automate a small task
  • you are working with files, text, or simple calculations

This is a good first step before learning full argument parsers.

How command line arguments work #

Python stores command line arguments in sys.argv.

sys.argv is a list:

  • sys.argv[0] is the script filename
  • sys.argv[1] is the first value given by the user
  • sys.argv[2] is the second value, and so on

For example, if you run:

python tool.py Alice

then sys.argv looks like this:

['tool.py', 'Alice']

If you run:

python tool.py 3 5

then sys.argv looks like this:

['tool.py', '3', '5']

Notice that command line values are strings. If you need numbers, you must convert them. You can learn more on the int() function explained page.

Build the first working version #

Start with this script:

import sys

if len(sys.argv) < 2:
    print("Usage: python tool.py NAME")
    sys.exit(1)

name = sys.argv[1]
print(f"Hello, {name}!")

What each part does #

import sys

  • gives your script access to sys.argv

if len(sys.argv) < 2:

  • checks whether the user gave at least one real argument
  • remember: sys.argv[0] is only the filename

print("Usage: python tool.py NAME")

  • shows the user how to run the script correctly

sys.exit(1)

  • stops the program because the input is missing

name = sys.argv[1]

  • reads the first user argument

print(f"Hello, {name}!")

  • prints the result using an f-string

If you want to review printing output, see print() explained.

Improve the tool with a real task #

A greeting example is useful for learning, but a real tool often does a task.

Here is a second example that adds two numbers.

Save this as add_tool.py:

import sys

if len(sys.argv) < 3:
    print("Usage: python add_tool.py NUMBER1 NUMBER2")
    sys.exit(1)

try:
    num1 = int(sys.argv[1])
    num2 = int(sys.argv[2])
except ValueError:
    print("Error: both arguments must be whole numbers.")
    sys.exit(1)

total = num1 + num2
print(f"Result: {total}")

How this version works #

This script adds two improvements:

  • it expects two arguments instead of one
  • it handles bad input with try and except

Example runs #

Correct input:

python add_tool.py 3 5

Output:

Result: 8

Missing input:

python add_tool.py 3

Output:

Usage: python add_tool.py NUMBER1 NUMBER2

Invalid input:

python add_tool.py 3 hello

Output:

Error: both arguments must be whole numbers.

This is an important beginner pattern:

  1. check the number of arguments
  2. convert values if needed
  3. handle possible errors clearly

If you want more help with this part, see how to handle exceptions in Python and Python errors and exceptions explained.

How to run the script #

Follow these steps:

  1. Save the script in a file ending with .py
  2. Open a terminal in the same folder
  3. Run the script with python filename.py arguments

Example commands #

For the greeting tool:

python tool.py Alice

Expected output:

Hello, Alice!

For the add tool:

python add_tool.py 3 5

Expected output:

Result: 8

Useful terminal commands #

These commands can help when testing:

python tool.py
python tool.py Alice
python tool.py 3 5
python -V
pwd
cd path/to/script/folder
ls
dir

Notes:

  • python -V shows your Python version
  • pwd shows your current folder on macOS and Linux
  • ls lists files on macOS and Linux
  • dir lists files on Windows
  • cd changes to the folder that contains your script

Common beginner mistakes #

Here are some common problems when building a Python command line tool.

Forgetting that arguments are strings #

If you run:

python add_tool.py 3 5

the values "3" and "5" start as strings, not numbers.

That is why this fails to do numeric work unless you convert them:

num1 = int(sys.argv[1])
num2 = int(sys.argv[2])

Using sys.argv[1] without checking length first #

This is a common cause of IndexError.

Bad example:

import sys

name = sys.argv[1]
print(name)

If no argument is given, the script crashes.

Safer version:

import sys

if len(sys.argv) < 2:
    print("Usage: python tool.py NAME")
    sys.exit(1)

name = sys.argv[1]
print(name)

Running the script from the wrong folder #

If the terminal is not in the same folder as your file, Python may not find the script.

Use:

pwd
ls
dir
cd path/to/script/folder

to check where you are and move to the correct folder.

Misspelling the filename or command #

Make sure:

  • the filename is correct
  • the extension is .py
  • you use the correct Python command for your system

Next steps after this example #

After this example, a good path is:

A good exercise is to copy one of the scripts from this page and extend it into:

  • a mini calculator
  • a text formatter
  • a simple file-based tool

FAQ #

What is a command line tool in Python? #

It is a Python script that you run from the terminal and give input through command line arguments.

What is sys.argv? #

It is a list of values passed to the script from the command line. The first item is the script name.

Why does my script fail when no argument is given? #

You are probably reading sys.argv[1] before checking that the user actually passed an argument.

Can I use input() instead of sys.argv? #

Yes, but input() asks during program execution. sys.argv lets you pass values when starting the script.

What should I learn after this? #

Learn sys.argv first, then argparse, plus basic error handling with try-except.

See also #

Press Esc to close