Functions and a Real Backup Script

Functions keep scripts tidy by grouping reusable logic. To finish, we will build a small backup script that uses everything so far.

Defining a function

#!/bin/bash

greet() {
    echo "Hello, $1"
}

greet Ada
greet Grace

• greet() defines the function
• $1 inside the function is the first argument passed to the function
• Call it simply by name, with or without arguments

Functions with return values

#!/bin/bash

is_file() {
    [ -f "$1" ]
}

if is_file "notes.txt"; then
    echo "Yes, it is a file"
fi

The function's [ -f "$1" ] returns 0 (success) or non-zero, which if understands directly.

The backup script

Create backup.sh:

#!/bin/bash
# Simple timestamped backup of a directory

backup_dir() {
    local source=$1
    local dest=$2

    if [ ! -d "$source" ]; then
        echo "Error: $source is not a directory"
        return 1
    fi

    mkdir -p "$dest"
    cp -r "$source" "$dest/$(basename "$source")-$(date +%Y%m%d-%H%M%S)"
    echo "Backed up $source to $dest"
}

if [ -z "$1" ]; then
    echo "Usage: $0 <source-dir> [destination-dir]"
    exit 1
fi

backup_dir "$1" "${2:-$HOME/backups}"

• local makes a variable local to the function
• $(basename "$source") strips the path to just the folder name
• return 1 exits the function with an error code

Try it

1. chmod +x backup.sh
2. Create a test folder with a few files: mkdir -p demo && touch demo/a demo/b
3. Run ./backup.sh demo — then ls ~/backups and inspect the timestamped copy
4. Run ./backup.sh with no argument — see the usage message

Next steps

You now have the core of shell scripting: shebang, variables, conditionals, loops and functions. Combine them to automate backups, rename batches of files, or monitor logs — the terminal is your tool.