Bash arrays and how to iterate them correctly

Arrays in Bash

Bash has two array types: indexed (numeric keys) and associative (string keys, requires Bash 4+). Arrays are zero-indexed.

# Declare and populate
servers=("web-01" "web-02" "db-01")

# Access by index
echo ${servers[0]}    # web-01
echo ${servers[-1]}   # db-01 (last element)

# All elements
echo ${servers[@]}    # web-01 web-02 db-01

# Array length
echo ${#servers[@]}   # 3

# Append
servers+=("cache-01")

# Slice: elements 1 and 2
echo ${servers[@]:1:2}

Iterate an Array

for server in "${servers[@]}"; do
  echo "Pinging $server..."
  ping -c 1 "$server" > /dev/null && echo "OK" || echo "FAIL"
done

Always quote "${array[@]}" — without quotes, elements with spaces split into separate words.

Associative Arrays

declare -A config
config["host"]="localhost"
config["port"]="5432"
config["db"]="myapp"

echo ${config["host"]}   # localhost

# Iterate key-value pairs
for key in "${!config[@]}"; do
  echo "$key = ${config[$key]}"
done

Associative arrays require declare -A. Use them to avoid parallel arrays — instead of separate names and values arrays that must stay in sync.