Understanding Script vs Function Arguments in Bash

When writing shell scripts, one of the most common sources of confusion is positional parameters — the $1, $2, $3 variables that represent arguments passed to a script or function. Let’s walk through a real example to make this crystal clear.

The Script:

#!/bin/bash

backup(){
    backup_name="${1}_$(date +%F_%H-%M-%S).tar.gz"
    if tar -czf "$backup_name" "$1"; then
        echo "✅ Backup created: $backup_name"
    else
        echo "❌ Backup failed for $1"
    fi
}

list(){
    find . -type f -name "*.tar.gz"
}

case $1 in
    back)
        backup "$2"
        ;;
    list)
        list
        ;;
    *)
        echo "Usage:"
        echo "  $0 back <directory_or_file>"
        echo "  $0 list"
        ;;
esac

How It Works
Step 1: Script Arguments
When you run:
./backup-manager.sh back dir21
$0 → ./backup-manager.sh (the script name)
$1 → back
$2 → dir21
The case statement checks $1. Since it’s back, the script calls:
backup "$2"

Step 2: Function Arguments
Now the function backup() is called with one argument: dir21.
Inside the function:
$1 → dir21 (because $1 inside a function refers to the first argument passed to that function, not the script’s $1 anymore).
That’s why you see $2 in the script but $1 inside the function — they’re different scopes.

Example Run

$ ./backup-manager.sh back dir21
✅ Backup created: dir21_2026-04-03_09-25-00.tar.gz

$ ./backup-manager.sh list
./dir21_2026-04-03_09-25-00.tar.gz
Key Takeaways
Script arguments ($1, $2, …) are what the user types after the script name.
Function arguments ($1, $2, … inside the function) are what you pass when calling the function.
They are independent: $1 in the script is not the same as $1 inside the function.

Bonus Tip: Multiple Backups
You can extend the script to back up multiple directories at once:

case $1 in
    back)
        shift   # remove "back" from arguments
        for target in "$@"; do
            backup "$target"
        done
        ;;
    list)
        list
        ;;
    *)
        echo "Usage: $0 back <dir1> <dir2> ... | $0 list"
        ;;
esac

Now you can run: ./backup-manager.sh back dir21 dir22 dir23
and it will back up all three.

💡 This is a great example to blog about because it shows the layered nature of arguments in Bash — script level vs function level — and clears up a common beginner confusion.