Advanced Bash Scripting — Functions, Arrays, getopts, and trap

# Basic function
greet() {
  local name="$1"          # Restrict scope with local
  echo "Hello, $name"
}
greet "World"

# Return value (exit-code style)
is_root() {
  [[ $EUID -eq 0 ]]        # 0 is true, 1 is false
}
if is_root; then echo "running as root"; fi

# Return a value via stdout
get_date() {
  date +%Y%m%d
}
TODAY=$(get_date)
echo "$TODAY"

# Declaration and access
fruits=("apple" "banana" "cherry")
echo "${fruits[0]}"          # apple
echo "${fruits[@]}"          # all elements
echo "${#fruits[@]}"         # length: 3

# Append and delete
fruits+=("grape")
unset fruits[1]              # remove banana

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

# Associative array (Bash 4+)
declare -A config
config[host]="localhost"
config[port]="5432"
echo "${config[host]}:${config[port]}"

#!/usr/bin/env bash
usage() {
  echo "Usage: $0 [-h] [-n NAME] [-v]"
  exit 1
}

VERBOSE=false
NAME=""

while getopts ":hn:v" opt; do
  case $opt in
    h) usage ;;
    n) NAME="$OPTARG" ;;
    v) VERBOSE=true ;;
    :) echo "Option -$OPTARG requires an argument"; exit 1 ;;
    ?) echo "Unknown option: -$OPTARG"; exit 1 ;;
  esac
done
shift $((OPTIND - 1))       # remaining positional args

echo "Name: $NAME, Verbose: $VERBOSE"
echo "Remaining args: $*"

# Temp-file cleanup pattern
TMPFILE=$(mktemp)
trap 'rm -f "$TMPFILE"; echo "Cleaned up"; exit' EXIT INT TERM

# Do work
echo "data" > "$TMPFILE"
sleep 10      # TMPFILE is deleted even if you press Ctrl+C

# Debug trace
trap 'echo "Line $LINENO: $BASH_COMMAND"' DEBUG

# Strict mode (always recommended)
set -euo pipefail

# Colored output
RED='\033[0;31m'; GREEN='\033[0;32m'; NC='\033[0m'
echo -e "${GREEN}OK${NC} done"
echo -e "${RED}ERROR${NC} failed"

# Retry function
retry() {
  local n=0; local max=3; local delay=5
  until "$@"; do
    n=$((n+1))
    [[ $n -ge $max ]] && { echo "Failed after $max attempts"; return 1; }
    echo "Retry $n/$max in ${delay}s..."; sleep $delay
  done
}
retry curl -sf https://example.com

# Logging function
log() {
  echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*" | tee -a /var/log/myscript.log
}
log "Started"

cat > /etc/myapp/config.conf << EOF
host=${DB_HOST:-localhost}
port=${DB_PORT:-5432}
user=${DB_USER:-app}
EOF

# To write literally without variable expansion
cat > script.sh << 'EOF'
echo "$PATH"   # printed as-is (no expansion)
EOF