Shell Scripting — Arrays

# ── Declaration methods ───────────────────────────────────

# Method 1 — literal list
servers=("web-01" "web-02" "db-01" "cache-01")

# Method 2 — explicit declare
declare -a databases
databases=("myapp_prod" "myapp_stage" "analytics")

# Method 3 — one element at a time
ports[0]=3306
ports[1]=5432
ports[2]=6379

# Method 4 — from command output
log_files=($(ls /var/log/*.log))       # Word-split on spaces — risky
mapfile -t log_files < <(ls /var/log/*.log)  # Safe — one per line

# Method 5 — mapfile (readarray) from file
mapfile -t hosts < /etc/hosts_list    # Each line → one element

# Method 6 — split a string into array
csv="alpha,beta,gamma,delta"
IFS=',' read -ra parts <<< "${csv}"
echo "${parts[1]}"                       # beta

servers=("web-01" "web-02" "db-01" "cache-01")

# Access single element
echo "${servers[0]}"       # web-01  (first)
echo "${servers[2]}"       # db-01   (third)
echo "${servers[-1]}"      # cache-01 (last — bash 4.3+)
echo "${servers[-2]}"      # db-01   (second from end)

# All elements
echo "${servers[@]}"       # web-01 web-02 db-01 cache-01  (individually quoted)
echo "${servers[*]}"       # web-01 web-02 db-01 cache-01  (joined with IFS)

# Length
echo "${#servers[@]}"      # 4  (element count)
echo "${#servers[0]}"      # 6  (length of element 0 string)

# All indices
echo "${!servers[@]}"      # 0 1 2 3

# Slice — ${arr[@]:start:length}
echo "${servers[@]:1:2}"   # web-02 db-01  (2 elements from index 1)
echo "${servers[@]:2}"     # db-01 cache-01 (from index 2 to end)

fruits=("apple" "banana" "cherry")

# Append one element
fruits+=("date")
echo "${fruits[@]}"        # apple banana cherry date

# Append multiple
fruits+=("elderberry" "fig")

# Modify an element
fruits[1]="blueberry"      # banana → blueberry

# Delete an element (leaves sparse gap)
unset 'fruits[2]'          # Removes cherry; indices: 0,1,3,4,5

# Re-index after deletion (compact the array)
fruits=("${fruits[@]}")    # Rebuild as 0-indexed: 0,1,2,3,4

# Delete entire array
unset fruits

# Concatenate two arrays
a=("one" "two")
b=("three" "four")
c=("${a[@]}" "${b[@]}")    # one two three four

# Copy an array
original=("x" "y" "z")
copy=("${original[@]}")

servers=("web-01" "web-02" "db-01 primary" "cache-01")

# ── Method 1: for-in with "$@" — handles spaces correctly
for server in "${servers[@]}"; do
  echo "Checking: ${server}"
done

# ── Method 2: by index — useful when you need position
for i in "${!servers[@]}"; do
  echo "[${i}] ${servers[$i]}"
done

# ── Method 3: C-style for loop
for (( i=0; i<${#servers[@]}; i++ )); do
  echo "${servers[$i]}"
done

# ── Real-world: check all servers in parallel
check_server() {
  local host="${1}"
  if ping -c1 -W2 "${host}" >/dev/null 2>&1; then
    echo "  ✔ ${host} — reachable"
  else
    echo "  ✘ ${host} — UNREACHABLE"
  fi
}

hosts=("8.8.8.8" "1.1.1.1" "192.168.1.1")
echo "Checking connectivity..."
for host in "${hosts[@]}"; do
  check_server "${host}" &    # Run in parallel
done
wait                          # Wait for all background jobs

nums=(5 2 8 1 9 3)
words=("banana" "apple" "cherry" "apple" "date")

# Sort array (pipe through sort, rebuild)
mapfile -t sorted_nums  < <(printf "%s\n" "${nums[@]}"  | sort -n)
mapfile -t sorted_words < <(printf "%s\n" "${words[@]}" | sort)

echo "Sorted nums : ${sorted_nums[*]}"    # 1 2 3 5 8 9
echo "Sorted words: ${sorted_words[*]}"   # apple apple banana cherry date

# Deduplicate (sort -u)
mapfile -t unique < <(printf "%s\n" "${words[@]}" | sort -u)
echo "Unique: ${unique[*]}"               # apple banana cherry date

# Search — check if element exists
contains() {
  local target="${1}"; shift
  local elem
  for elem in "$@"; do
    [[ "${elem}" == "${target}" ]] && return 0
  done
  return 1
}

if contains "cherry" "${words[@]}"; then
  echo "Found cherry!"
fi

# Find index of element
find_index() {
  local target="${1}"; local arr=("${@:2}")
  for i in "${!arr[@]}"; do
    [[ "${arr[$i]}" == "${target}" ]] && { echo "${i}"; return 0; }
  done
  echo "-1"; return 1
}
find_index "db-01" "${servers[@]}"    # Returns index of "db-01"

# Bash cannot pass arrays by value — use nameref (bash 4.3+) or expansion

# Method 1 — expand elements as arguments (most common)
print_all() {
  echo "Got ${#} items:"
  for item in "$@"; do
    echo "  - ${item}"
  done
}
my_arr=("alpha" "beta" "gamma")
print_all "${my_arr[@]}"     # Passes elements as arguments

# Method 2 — nameref (bash 4.3+) — pass array by name
process_array() {
  local -n arr_ref="${1}"   # nameref — arr_ref IS the array
  echo "Array has ${#arr_ref[@]} elements"
  for item in "${arr_ref[@]}"; do
    echo "  ${item}"
  done
}
process_array my_arr         # Pass array NAME not expansion

# Method 3 — return array from function via global or nameref
get_active_dbs() {
  local -n result="${1}"   # Output array by nameref
  result=()
  while IFS= read -r db; do
    result+=("${db}")
  done < <(mysql -se "SHOW DATABASES" 2>/dev/null)
}
declare -a active_dbs
get_active_dbs active_dbs
echo "Found: ${active_dbs[*]}"

#!/usr/bin/env bash
# multi_db_backup.sh — Back up multiple databases using arrays

set -euo pipefail

readonly BACKUP_DIR="/var/backups/mysql"
readonly DATE=$(date '+%Y%m%d_%H%M')
readonly RETENTION_DAYS=7

DATABASES=("myapp_prod" "analytics" "user_data" "audit_logs")

success=()
failed=()

backup_db() {
  local db="${1}"
  local outfile="${BACKUP_DIR}/${db}_${DATE}.sql.gz"

  if mysqldump "${db}" | gzip > "${outfile}"; then
    echo "  ✔ ${db} → $(du -sh "${outfile}" | cut -f1)"
    return 0
  else
    echo "  ✘ ${db} — FAILED"
    return 1
  fi
}

mkdir -p "${BACKUP_DIR}"
echo "Starting backup of ${#DATABASES[@]} databases..."

for db in "${DATABASES[@]}"; do
  if backup_db "${db}"; then
    success+=("${db}")
  else
    failed+=("${db}")
  fi
done

echo "────────────────────────────────"
echo "✔ Succeeded: ${#success[@]}  — ${success[*]:-none}"
echo "✘ Failed   : ${#failed[@]}   — ${failed[*]:-none}"

# Cleanup old backups
find "${BACKUP_DIR}" -name "*.sql.gz" -mtime +"${RETENTION_DAYS}" -delete

[[ ${#failed[@]} -gt 0 ]] && exit 1
exit 0