A simple Python script that encrypts a string using the Caesar cipher.

caesar_encrypt.py

#!/usr/bin/env python3
"""
caesar_encrypt.py

Encrypt a string with the Caesar cipher.

Usage:
    # 1. Pass both text and shift on the command line
    $ python caesar_encrypt.py "Hello, World!" 5

    # 2. Only pass the text – uses default shift of 3
    $ python caesar_encrypt.py "Attack at dawn!"

    # 3. No arguments – script will prompt interactively
    $ python caesar_encrypt.py
"""
import sys

def caesar_shift_char(ch: str, shift: int) -> str:
    if ch.isupper():
        base = ord('A')
    elif ch.islower():
        base = ord('a')
    else:
        return ch
    offset = ord(ch) - base
    new_offset = (offset + shift) % 26
    return chr(base + new_offset)

def caesar_encrypt(text: str, shift: int) -> str:
    return ''.join(caesar_shift_char(ch, shift) for ch in text)

def main() -> None:
    args = sys.argv[1:]
    if not args:
        plaintext = input("Enter the text to encrypt: ")
        try:
            shift = int(input("Enter the shift amount [default 3]: ") or "3")
        except ValueError:
            print("Invalid shift value. Using default shift of 3.")
            shift = 3
    else:
        plaintext = args[0]
        if len(args) > 1:
            try:
                shift = int(args[1])
            except ValueError:
                print(f"Invalid shift '{args[1]}'. Using default shift of 3.")
                shift = 3
        else:
            shift = 3
    ciphertext = caesar_encrypt(plaintext, shift)
    print("\nCiphertext:\n")
    print(ciphertext)

if __name__ == "__main__":
    main()