Command-Line 101: sys.argv

By the end of this lesson you can:

• Read command-line arguments with sys.argv.
• Explain why sys.argv[0] is the script name, not the first real argument.
• Convert string arguments to the types you need (and handle when they're missing).
• Write a script someone can run as python greet.py Aisha 3.

Every command you've typed in this course is a program plus arguments:

pip install requests      →  program "pip", args ["install", "requests"]
git commit -m "fix bug"   →  program "git", args ["commit", "-m", "fix bug"]
python tool.py data.csv   →  program "python", args ["tool.py", "data.csv"]

It's just a list of strings

import sys

print(sys.argv)

Run it as python show.py hello 42 file.txt and you get:

['show.py', 'hello', '42', 'file.txt']

• sys.argv[0] is always the script name (here show.py).
• sys.argv[1] onward are the real arguments the user typed.
• Everything is a string — even 42 arrives as '42'.

Counting and slicing

import sys

args = sys.argv[1:]          # drop the script name
print("you passed", len(args), "arguments")
for i, value in enumerate(args, start=1):
    print(f"  arg {i}: {value!r}")

Using sys.argv[1:] is the standard move: it gives you just the user's words, ignoring the script name.

Converting types

Because arguments are strings, you convert what you need:

import sys

name  = sys.argv[1]            # already a string — fine
times = int(sys.argv[2])      # convert "3" → 3

for _ in range(times):
    print(f"Hello, {name}!")

What if the argument is missing?

Reading sys.argv[1] when nothing was passed raises IndexError. A good tool checks first and prints a clear usage message:

import sys

if len(sys.argv) < 3:
    print("Usage: python greet.py <name> <times>")
    sys.exit(1)               # non-zero = "something went wrong"

name  = sys.argv[1]
times = int(sys.argv[2])
print("\n".join(f"Hello, {name}!" for _ in range(times)))

sys.exit(1) stops the program and tells the shell it failed — important when other scripts run yours. sys.exit(0) means success.

Goal: python convert.py 100 km mi prints kilometres in miles. Let's build it piece by piece.

import sys

RATES = {                      # how many of "to" in one "from"
    ("km", "mi"): 0.621371,
    ("mi", "km"): 1.609344,
    ("kg", "lb"): 2.204623,
    ("lb", "kg"): 0.453592,
}

if len(sys.argv) != 4:
    print("Usage: python convert.py <amount> <from> <to>")
    sys.exit(1)

amount = float(sys.argv[1])
unit_from = sys.argv[2]
unit_to   = sys.argv[3]

rate = RATES.get((unit_from, unit_to))
if rate is None:
    print(f"Don't know how to convert {unit_from} → {unit_to}")
    sys.exit(1)

result = amount * rate
print(f"{amount} {unit_from} = {result:.2f} {unit_to}")

Run it:

$ python convert.py 100 km mi
100.0 km = 62.14 mi

$ python convert.py 5 kg lb
5.0 kg = 11.02 lb

$ python convert.py 100 km lightyears
Don't know how to convert km → lightyears

Read the code

Three defensive checks make this a real tool, not a demo: wrong argument count → usage message; bad number → float() raises a clear error; unknown units → friendly message instead of a crash. Each one exits non-zero so a calling script knows it failed. That care is the whole difference between "works on my machine" and "works."

sys.argv is the list of words the shell hands your script: index 0 is the script name, 1 onward are the user's arguments, and everything is a string. Slice with sys.argv[1:], convert types with int()/float(), check the count before you index, and exit non-zero on errors so other tools know you failed. It's the foundation under every command-line tool — and the moment you need flags or help text, you'll reach for argparse next lesson.