Arrays
When you need to store multiple values - file lists, server names, configuration options - arrays are your tool.
Creating Arrays
hljs bash
#!/bin/bash
# Method 1: Direct assignment
fruits=("apple" "banana" "cherry")
# Method 2: One at a time
colors[0]="red"
colors[1]="green"
colors[2]="blue"
# Method 3: From command output
files=($(ls *.txt))
Accessing Elements
hljs bash
#!/bin/bash
fruits=("apple" "banana" "cherry")
echo "${fruits[0]}" # apple (first element)
echo "${fruits[1]}" # banana
echo "${fruits[2]}" # cherry
echo "${fruits[-1]}" # cherry (last element)
Use Braces
$fruits[0] doesn't work as expected. Always use ${fruits[0]}.
All Elements
hljs bash
#!/bin/bash
fruits=("apple" "banana" "cherry")
echo "${fruits[@]}" # apple banana cherry (as separate words)
echo "${fruits[*]}" # apple banana cherry (as single string)
Array Length
hljs bash
#!/bin/bash
fruits=("apple" "banana" "cherry")
echo "${#fruits[@]}" # 3 (number of elements)
echo "${#fruits[0]}" # 5 (length of first element "apple")
Looping Through Arrays
hljs bash
#!/bin/bash
servers=("web1" "web2" "db1" "db2")
for server in "${servers[@]}"; do
echo "Checking $server..."
ping -c 1 "$server" > /dev/null && echo " UP" || echo " DOWN"
done
Quote the Array
Always use "${array[@]}" with quotes. Without quotes, elements with spaces break into multiple items.
Adding Elements
hljs bash
#!/bin/bash
fruits=("apple" "banana")
# Append
fruits+=("cherry")
fruits+=("date" "elderberry")
echo "${fruits[@]}" # apple banana cherry date elderberry
Removing Elements
hljs bash
#!/bin/bash
fruits=("apple" "banana" "cherry" "date")
# Remove by index
unset 'fruits[1]' # Removes "banana"
echo "${fruits[@]}" # apple cherry date
# Note: This leaves a gap! Index 1 is now empty
echo "${fruits[1]}" # (empty)
echo "${fruits[2]}" # cherry
To reindex after removal:
hljs bash
fruits=("${fruits[@]}") # Repack array
Slicing Arrays
hljs bash
#!/bin/bash
letters=("a" "b" "c" "d" "e" "f")
echo "${letters[@]:2}" # c d e f (from index 2)
echo "${letters[@]:2:3}" # c d e (3 elements starting at index 2)
Array Indices
hljs bash
#!/bin/bash
fruits=("apple" "banana" "cherry")
# Get all indices
echo "${!fruits[@]}" # 0 1 2
# Loop with indices
for i in "${!fruits[@]}"; do
echo "$i: ${fruits[$i]}"
done
Associative Arrays (Dictionaries)
hljs bash
#!/bin/bash
# Must declare as associative
declare -A user
user[name]="John"
user[email]="john@example.com"
user[role]="admin"
echo "${user[name]}" # John
echo "${user[email]}" # john@example.com
# All keys
echo "${!user[@]}" # name email role
# All values
echo "${user[@]}" # John john@example.com admin
Practical Examples
Config Array
hljs bash
#!/bin/bash
declare -A config
config[host]="localhost"
config[port]="8080"
config[debug]="true"
# Use config
curl "http://${config[host]}:${config[port]}/api"
Process File List
hljs bash
#!/bin/bash
# Collect files
backup_files=()
for f in /etc/*.conf; do
[[ -f "$f" ]] && backup_files+=("$f")
done
echo "Found ${#backup_files[@]} config files"
# Process them
for file in "${backup_files[@]}"; do
cp "$file" /backup/
done
Multiple Servers
hljs bash
#!/bin/bash
declare -A servers=(
[web]="192.168.1.10"
[db]="192.168.1.20"
[cache]="192.168.1.30"
)
for name in "${!servers[@]}"; do
ip="${servers[$name]}"
echo "Checking $name ($ip)..."
done
Knowledge Check
How do you get the number of elements in an array?
Quick Reference
| Syntax | Purpose |
|---|---|
arr=("a" "b" "c") | Create array |
${arr[0]} | Access element |
${arr[@]} | All elements |
${#arr[@]} | Array length |
${!arr[@]} | All indices |
arr+=("d") | Append |
unset 'arr[1]' | Remove element |
declare -A arr | Associative array |
Key Takeaways
- Create arrays with
arr=("a" "b" "c") - Access elements with
${arr[index]} - Always quote
"${arr[@]}"when expanding ${#arr[@]}gives array length- Use
declare -Afor key-value pairs - Arrays are great for file lists, configs, server names
Next: string manipulation techniques.