Shell Scripting — Function Libraries & Sourcing

# ── source vs . (dot) — identical behaviour ───────────────
source /path/to/library.sh   # bash-specific, more readable
. /path/to/library.sh         # POSIX, works in sh too

# ── Source relative to script location ───────────────────
readonly SCRIPT_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
source "${SCRIPT_DIR}/lib/logging.sh"
source "${SCRIPT_DIR}/lib/db.sh"
source "${SCRIPT_DIR}/lib/network.sh"

# ── Source with existence check ───────────────────────────
LIB="${SCRIPT_DIR}/lib/utils.sh"
[[ -f "${LIB}" ]] || { echo "ERROR: Missing library: ${LIB}" >&2; exit 1; }
source "${LIB}"

# ── Source config files ───────────────────────────────────
# Config files are just shell files with variable assignments
source /etc/myapp/config.env
source ~/.myapp/local.env   # user overrides (optional)

#!/usr/bin/env bash
# lib/logging.sh — Logging library with guard against double-source

# Guard: if already sourced, return immediately
[[ -n "${_LIB_LOGGING_LOADED:-}" ]] && return 0
readonly _LIB_LOGGING_LOADED=1

# ── Colours ───────────────────────────────────────────────
readonly RED=$'\033[0;31m'
readonly GREEN=$'\033[0;32m'
readonly YELLOW=$'\033[1;33m'
readonly BLUE=$'\033[0;34m'
readonly RESET=$'\033[0m'

# ── Log file (can be overridden before sourcing) ──────────
LOG_FILE="${LOG_FILE:-/var/log/app.log}"

_log() {
  local level="$1"; shift
  local colour="$1"; shift
  local ts
  ts=$(date '+%Y-%m-%d %H:%M:%S')
  printf "%b[%s] %-5s%b %s\n" "${colour}" "${ts}" "${level}" "${RESET}" "$*" \
    | tee -a "${LOG_FILE}"
}

log()  { _log "INFO"  "${BLUE}"   "$@"; }
ok()   { _log "OK"    "${GREEN}"  "$@"; }
warn() { _log "WARN"  "${YELLOW}" "$@"; }
err()  { _log "ERROR" "${RED}"    "$@" >&2; }
die()  { err "$@"; exit 1; }

#!/usr/bin/env bash
# lib/db.sh — MySQL database utility library

[[ -n "${_LIB_DB_LOADED:-}" ]] && return 0
readonly _LIB_DB_LOADED=1

# Requires lib/logging.sh to be sourced first
declare -f log >/dev/null 2>&1 || { echo "db.sh requires logging.sh" >&2; exit 1; }

db_query() {
  local host="${DB_HOST:?DB_HOST not set}"
  local user="${DB_USER:?DB_USER not set}"
  local pass="${DB_PASS:?DB_PASS not set}"
  local db="${1:?db_query: database required}"
  local sql="${2:?db_query: SQL required}"

  mysql -h "${host}" -u "${user}" -p"${pass}" \
        -D "${db}" -Nse "${sql}" 2>/dev/null
}

db_ping() {
  local host="${1:-${DB_HOST}}"
  mysql -h "${host}" -u "${DB_USER}" -p"${DB_PASS}" \
        -e "SELECT 1" >/dev/null 2>&1
}

db_table_exists() {
  local db="${1}" table="${2}"
  local result
  result=$(db_query "${db}" \
    "SELECT COUNT(*) FROM information_schema.tables
     WHERE table_schema='${db}' AND table_name='${table}'")
  [[ "${result}" -gt 0 ]]
}

db_row_count() {
  local db="${1}" table="${2}"
  db_query "${db}" "SELECT COUNT(*) FROM \`${table}\`"
}

#!/usr/bin/env bash
# deploy.sh — Main deployment script using libraries

set -euo pipefail

readonly SCRIPT_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)

# ── Load libraries in order ───────────────────────────────
source "${SCRIPT_DIR}/lib/logging.sh"
source "${SCRIPT_DIR}/lib/db.sh"
source "${SCRIPT_DIR}/lib/network.sh"

# ── Load config ───────────────────────────────────────────
source "${SCRIPT_DIR}/config/${APP_ENV:-production}.env"

main() {
  log  "Deployment starting — env: ${APP_ENV}"

  # Check DB reachable
  db_ping || die "Cannot reach database at ${DB_HOST}"
  ok   "Database is reachable"

  # Check table exists
  db_table_exists "myapp" "users" || die "users table missing!"
  ok   "Schema validated"

  # Get current row counts
  local users
  users=$(db_row_count "myapp" "users")
  log  "Current user count: ${users}"

  ok   "Deployment complete"
}

main "$@"

# Recommended directory layout for a bash project
myapp-scripts/
├── bin/                 # Executable scripts (added to PATH)
│   ├── deploy.sh
│   ├── backup.sh
│   └── health-check.sh
├── lib/                 # Libraries (sourced, not executed directly)
│   ├── logging.sh       # Logging + colour functions
│   ├── db.sh            # Database utilities
│   ├── network.sh       # Network checks
│   └── utils.sh         # Generic helpers
├── config/              # Environment config files
│   ├── development.env
│   ├── staging.env
│   └── production.env
├── tests/               # BATS test files
│   ├── test_logging.bats
│   └── test_db.bats
└── README.md

# Each script in bin/ starts with:
#!/usr/bin/env bash
readonly SCRIPT_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)
source "${SCRIPT_DIR}/lib/logging.sh"
source "${SCRIPT_DIR}/lib/utils.sh"