Arrays: Multiple Values in One Variable

Arrays are just variables that hold multiple values. That's it.

But here's the thing: Arrays make scripts powerful. You can process lists. You can iterate over data. You can organize information.

What Arrays Actually Are

Think of arrays like a list. One variable. Multiple values.

fruits=("apple" "banana" "orange")

That's an array. Three values. One variable.

Creating Arrays

# Basic
fruits=("apple" "banana" "orange")

# Explicit
declare -a fruits=("apple" "banana" "orange")

# One at a time
fruits[0]="apple"
fruits[1]="banana"
fruits[2]="orange"

All three work. Use the first one. It's simplest.

Important: Arrays start at 0. First element is [0], not [1]. This confuses everyone at first.

Accessing Elements

fruits=("apple" "banana" "orange")
echo "${fruits[0]}"               # apple
echo "${fruits[1]}"               # banana
echo "${fruits[2]}"               # orange

The braces are important. `${fruits[0]}` not $fruits[0]. Bash needs the braces.

Accessing All Elements

fruits=("apple" "banana" "orange")
echo "${fruits[@]}"              # All elements (separate)
echo "${fruits[*]}"              # All elements (one string)

The difference: @ gives you separate words. * gives you one string.

My take: Use @. Always. It's what you want 99% of the time.

Real example:

$ fruits=("apple" "banana" "orange")
$ for fruit in "${fruits[@]}"; do echo "$fruit"; done
apple
banana
orange

Array Length

fruits=("apple" "banana" "orange")
echo "${#fruits[@]}"             # 3

The # gives you the length. `${#array[@]}` = number of elements.

Array Indices

echo "${!fruits[@]}"             # 0 1 2

The ! gives you the indices. Useful when you need to know which indices exist.

Looping Through Arrays

# By index
for i in "${!fruits[@]}"; do
    echo "$i: ${fruits[$i]}"
done

# By value
for fruit in "${fruits[@]}"; do
    echo "$fruit"
done

My take: Loop by value most of the time. Simpler. Loop by index when you need the index.

Adding Elements

fruits=("apple" "banana")
fruits+=("orange")               # Add to end
fruits[3]="grape"                # Add at index

The += adds to the end. Simple.

Removing Elements

unset fruits[1]                  # Remove element
fruits=("${fruits[@]}")          # Reindex (optional)

unset removes an element. The reindexing is optional. Usually you don't need it.

Real-World Examples

Process Multiple Files

files=("file1.txt" "file2.txt" "file3.txt")

for file in "${files[@]}"; do
    if [ -f "$file" ]; then
        echo "Processing $file"
        # Process file
    fi
done

Server List

servers=("server1.example.com" "server2.example.com" "server3.example.com")

for server in "${servers[@]}"; do
    echo "Checking $server"
    ping -c 1 "$server" > /dev/null && echo "$server is up" || echo "$server is down"
done

Configuration Values

config=("nginx" "8080" "/var/www/html")

service="${config[0]}"
port="${config[1]}"
directory="${config[2]}"

echo "Service: $service, Port: $port, Directory: $directory"

Common Mistakes (I've Made These)

1. Forgetting braces: $fruits[0] doesn't work. Use `${fruits[0]}`.

2. Using * instead of @: * gives one string. @ gives separate words. Use @.

3. Not quoting: "${fruits[@]}" not ${fruits[@]}. Quotes matter.

4. Starting at 1: Arrays start at 0. Not 1. This confuses everyone.

5. Forgetting # for length: `${#array[@]}` not ${array[@]} for length.

What's Next?

Now that you understand arrays, let's talk about File Operations. We'll cover working with files programmatically.

Or practice with arrays. Create some. Loop through them. Get comfortable.

---

Personal note: When I started, I avoided arrays. I'd use multiple variables. file1, file2, file3. Then I learned arrays. Now I use them constantly. They make scripts so much cleaner. Learn arrays. They're worth it.