Bash Variables and Arrays - Assignment, Export, Arrays, Scope

Variables and Arrays in Bash

Variables store data for use throughout a script. Bash supports simple variables, indexed arrays, and associative arrays (bash 4+).

Variable Assignment

# Basic assignment (no spaces around =)
NAME="Alice"
COUNT=42
TODAY=$(date +%Y-%m-%d)

# Read-only variables
readonly VERSION="2.0.0"
VERSION="3.0.0"  # Error: readonly variable

# Unset a variable
unset TEMP_VAR

Environment Variables

Variables are local to the current shell by default. Use export to make them available to child processes:

export DATABASE_URL="postgres://localhost/mydb"
export PATH="$PATH:/usr/local/bin"

# Set for a single command
NODE_ENV=production node server.js

Special Variables

$0    # script name
$1-$9 # positional arguments
$#    # number of arguments
$@    # all arguments (individually quoted)
$?    # exit status of last command
$$    # PID of current shell
$!    # PID of last background process

Indexed Arrays

# Declaration
FRUITS=("apple" "banana" "cherry" "date")

# Access
echo ${FRUITS[0]}      # apple
echo ${FRUITS[@]}      # all elements
echo ${#FRUITS[@]}     # array length (4)

# Append
FRUITS+=("elderberry")

# Iterate
for fruit in "${FRUITS[@]}"; do
  echo "- $fruit"
done

# Slice
echo ${FRUITS[@]:1:2}  # banana cherry

Associative Arrays (Bash 4+)

declare -A CONFIG
CONFIG[host]="localhost"
CONFIG[port]="3000"
CONFIG[env]="production"

# Access
echo ${CONFIG[host]}

# All keys
echo ${!CONFIG[@]}     # host port env

# Iterate
for key in "${!CONFIG[@]}"; do
  echo "$key = ${CONFIG[$key]}"
done

# Check if key exists
if [[ -v CONFIG[host] ]]; then
  echo "host is set"
fi

Variable Scope

# Global (default)
GLOBAL_VAR="I'm global"

my_function() {
  local LOCAL_VAR="I'm local"
  echo $GLOBAL_VAR    # accessible
  echo $LOCAL_VAR     # accessible
}

my_function
echo $LOCAL_VAR       # empty (not accessible outside function)

Practical Patterns

# Configuration with defaults
DB_HOST=${DB_HOST:-localhost}
DB_PORT=${DB_PORT:-5432}
DB_NAME=${DB_NAME:?"DB_NAME is required"}

# Build connection string
CONNECTION="postgresql://${DB_HOST}:${DB_PORT}/${DB_NAME}"