Shell Scripting — Functions

# ── Syntax 1: function keyword (bash-specific) ────────────
function greet {
  echo "Hello, ${1}!"
}

# ── Syntax 2: POSIX style (recommended — more portable) ───
greet() {
  echo "Hello, ${1}!"
}

# Both work identically in bash. Use syntax 2.

# ── Calling a function ────────────────────────────────────
greet "Vriddh"        # Hello, Vriddh!
greet "World"         # Hello, World!

# ── Functions must be defined BEFORE they are called ──────
main() {
  setup    # Fine — setup is defined below
  run
}

setup() { echo "Setting up..."; }
run()   { echo "Running..."; }

main    # Called at the bottom — setup and run already defined

#!/usr/bin/env bash

check_port() {
  # Arguments accessed as $1, $2 ... inside function
  local host="${1:?check_port requires a host}"
  local port="${2:?check_port requires a port}"
  local timeout="${3:-3}"   # optional, default 3 seconds

  if nc -z -w "${timeout}" "${host}" "${port}" >/dev/null 2>&1; then
    echo "  ✔ ${host}:${port} is open"
    return 0   # success exit code
  else
    echo "  ✘ ${host}:${port} is closed" >&2
    return 1   # failure exit code
  fi
}

# ── Calling with return value check ───────────────────────
if check_port "prod-db-01" "3306"; then
  echo "DB accessible — proceeding"
else
  echo "DB not accessible — aborting"
  exit 1
fi

# ── Capturing function output ─────────────────────────────
get_db_version() {
  local host="${1}"
  mysql -h "${host}" -Nse "SELECT VERSION()" 2>/dev/null
}

version=$(get_db_version "prod-db-01")
echo "MySQL version: ${version}"   # e.g. 8.0.36

# ── Pattern 1: echo and capture (most common) ─────────────
get_timestamp() {
  date '+%Y-%m-%d %H:%M:%S'
}
ts=$(get_timestamp)
echo "Timestamp: ${ts}"

# ── Pattern 2: global variable (fast, no subshell) ────────
get_free_space() {
  # Sets a global variable instead of echoing
  __free_space=$(df -BG "${1}" | awk 'NR==2{print $4}')
}
get_free_space "/"
echo "Free: ${__free_space}"   # Use __ prefix to avoid name clashes

# ── Pattern 3: nameref output variable (bash 4.3+) ────────
get_hostname() {
  local -n _out="${1}"   # nameref to caller's variable
  _out=$(hostname -f)
}
get_hostname myhost
echo "Host: ${myhost}"   # Populated by function via nameref

# ── Returning multiple values via nameref ─────────────────
parse_dsn() {
  # parse_dsn "mysql://user:pass@host:3306/dbname" user_var host_var port_var
  local dsn="${1}"
  local -n _user="${2}" _host="${3}" _port="${4}"

  _user=$(echo "${dsn}" | sed 's|.*://\([^:]*\):.*|\1|')
  _host=$(echo "${dsn}" | sed 's|.*@\([^:]*\):.*|\1|')
  _port=$(echo "${dsn}" | sed 's|.*:\([0-9]*\)/.*|\1|')
}

parse_dsn "mysql://appuser:secret@prod-db-01:3306/mydb" db_user db_host db_port
echo "User: ${db_user}, Host: ${db_host}, Port: ${db_port}"

backup_db() {
  # Named parameter style with defaults
  local host="${1:?ERROR: host is required}"
  local db="${2:?ERROR: database name is required}"
  local output_dir="${3:-/backups}"          # default /backups
  local compress="${4:-true}"                # default compress
  local timestamp
  timestamp=$(date '+%Y%m%d_%H%M%S')
  local filename="${output_dir}/${db}_${timestamp}.sql"

  echo "  Backing up ${db} from ${host}..."
  mysqldump -h "${host}" "${db}" > "${filename}" || {
    echo "  ERROR: Backup failed" >&2
    return 1
  }

  if [[ "${compress}" == "true" ]]; then
    gzip "${filename}"
    echo "  ✔ Saved: ${filename}.gz"
  else
    echo "  ✔ Saved: ${filename}"
  fi
}

# Call with all args
backup_db "prod-db-01" "myapp" "/backups/daily" "true"

# Call with defaults for 3rd and 4th args
backup_db "prod-db-01" "myapp"

# Missing required arg — exits with error
backup_db "prod-db-01"   # ERROR: database name is required

# ── Factorial ─────────────────────────────────────────────
factorial() {
  local n="${1}"
  (( n <= 1 )) && { echo 1; return; }
  echo $(( n * $(factorial $(( n - 1 ))) ))
}
echo "5! = $(factorial 5)"   # 120

# ── Directory tree traversal ─────────────────────────────
tree_size() {
  local dir="${1}"
  local depth="${2:-0}"
  local indent
  indent=$(printf '%*s' $(( depth * 2 )) '')

  printf "%s%s/\n" "${indent}" "$(basename "${dir}")"

  for item in "${dir}"/*; do
    [[ -d "${item}" ]] && tree_size "${item}" $(( depth + 1 ))
    [[ -f "${item}" ]] && printf "%s  %s\n" \
      "${indent}" "$(basename "${item}")"
  done
}
tree_size "/etc/nginx"

#!/usr/bin/env bash
# Standard logging and error functions used in every production script

readonly LOG_FILE="/var/log/myapp/deploy.log"
readonly TIMESTAMP=$(date '+%Y-%m-%d %H:%M:%S')

log()     { printf "[%s] INFO  %s\n"  "${TIMESTAMP}" "$*" | tee -a "${LOG_FILE}"; }
log_ok()  { printf "[%s] OK    %s\n"  "${TIMESTAMP}" "$*" | tee -a "${LOG_FILE}"; }
log_warn(){ printf "[%s] WARN  %s\n"  "${TIMESTAMP}" "$*" | tee -a "${LOG_FILE}"; }
log_err() { printf "[%s] ERROR %s\n"  "${TIMESTAMP}" "$*" | tee -a "${LOG_FILE}" >&2; }
die()     { log_err "$*"; exit 1; }

require_cmd() {
  for cmd in "$@"; do
    command -v "${cmd}" >/dev/null 2>&1 \
      || die "Required command not found: ${cmd}"
  done
}

require_env() {
  for var in "$@"; do
    [[ -n "${!var:-}" ]] \
      || die "Required env var not set: ${var}"
  done
}

# Usage
require_cmd mysql curl jq
require_env DB_HOST DB_PASS APP_ENV

log "Deployment started"
log_ok "All prerequisites met"
log_warn "Running in ${APP_ENV} — double check targets"