A Command Line Interface (CLI) application allows users to interact with programs through a terminal or command prompt instead of a graphical interface. Python is one of the best languages for building CLI applications due to its simplicity and powerful libraries.
1. Why Build CLI Applications?
Lightweight & Fast – No need for a GUI, making it efficient.
Automated Workflows – Easily script tasks for automation.
Remote Accessibility – Run programs on servers without a GUI.
Flexible Input Handling – Accepts command-line arguments, interactive input, or configuration files.
2. Basic CLI Application Using sys.argv
The sys module allows handling command-line arguments.
Example: Simple CLI Tool
import sys
def greet():
if len(sys.argv) > 1:
name = sys.argv[1]
print(f"Hello, {name}!")
else:
print("Usage: python script.py <name>")
greet()
How to Run
python script.py Alice
Output:
Hello, Alice!
sys.argv[0] is always the script name.
sys.argv[1] is the first argument passed.
3. Using argparse for Advanced CLI Arguments
The argparse module makes handling command-line arguments more user-friendly.
Example: CLI with Named Arguments
import argparse
def main():
parser = argparse.ArgumentParser(description="A simple CLI calculator.")
parser.add_argument("num1", type=float, help="First number")
parser.add_argument("num2", type=float, help="Second number")
parser.add_argument("--operation", choices=["add", "sub", "mul", "div"], default="add", help="Operation to perform")
args = parser.parse_args()
if args.operation == "add":
result = args.num1 + args.num2
elif args.operation == "sub":
result = args.num1 - args.num2
elif args.operation == "mul":
result = args.num1 * args.num2
elif args.operation == "div":
result = args.num1 / args.num2
print(f"Result: {result}")
if __name__ == "__main__":
main()
How to Run
python calculator.py 10 5 --operation mul
Output:
Result: 50.0
Improves error handling
Supports optional arguments
Automatically generates --help documentation
4. Interactive CLI Applications with input()
For CLI apps requiring user input, use input().
Example: Interactive To-Do List
def todo_list():
tasks = []
while True:
task = input("Enter a task (or type 'exit' to quit): ")
if task.lower() == "exit":
break
tasks.append(task)
print("\nYour To-Do List:")
for i, task in enumerate(tasks, 1):
print(f"{i}. {task}")
todo_list()
How to Run
python todo.py
Handles multiple inputs
Stores and displays user tasks
5. Building a CLI App with click
The click library simplifies CLI development and supports subcommands.
Installation
pip install click
Example: CLI with Click
import click
@click.command()
@click.argument("name")
@click.option("--greeting", default="Hello", help="Custom greeting message.")
def greet(name, greeting):
"""A simple CLI greeting app."""
click.echo(f"{greeting}, {name}!")
if __name__ == "__main__":
greet()
How to Run
python greet.py Alice --greeting "Hi"
Output:
Hi, Alice!
Easy to define commands and options
Handles input validation
6. Packaging CLI Applications as Executables
To distribute your CLI application without requiring Python installation, use PyInstaller.
Installation
pip install pyinstaller
Creating an Executable
pyinstaller --onefile my_cli_app.py
Generates a single-file executable in the dist/ folder.