Functions and Script ArchitectureLesson 3.3

Bash script structure and sourcing files

script organization pattern, sourcing with dot operator, source vs execute, library scripts, guard against re-sourcing, __dirname equivalent, relative paths in scripts

Structuring Scripts for Real Projects

Professional Bash scripts follow a consistent structure. This layout makes scripts readable, testable, and safe to source.

#!/usr/bin/env bash
set -euo pipefail

# 1. Resolve script directory (portable)
SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"

# 2. Source libraries relative to script, not CWD
source "${SCRIPT_DIR}/lib/logging.sh"
source "${SCRIPT_DIR}/lib/utils.sh"

# 3. Constants
readonly APP_NAME="myapp"
readonly LOG_FILE="/var/log/${APP_NAME}.log"

# 4. Functions
run_backup() {
  log INFO "Starting backup..."
  # ...
}

# 5. Main entrypoint
main() {
  parse_args "$@"
  run_backup
}

main "$@"

Source vs Execute

# Execute: runs in a subshell, no access to caller's scope
bash lib/helpers.sh

# Source: runs in current shell, shares scope
source lib/helpers.sh
. lib/helpers.sh  # POSIX equivalent

Guard Against Double-Sourcing

# In lib/logging.sh:
[[ -n "${_LOGGING_SH_LOADED:-}" ]] && return 0
_LOGGING_SH_LOADED=1

log() {
  echo "[$(date +%T)] $*" >&2
}

Use ${BASH_SOURCE[0]} instead of $0 when resolving paths — $0 returns the calling script's name when the file is sourced, not the library's name.