Shell Scripting

Shell Scripting

Core Principles

1. Fail Explicitly

A script that encounters an error should stop, not continue with corrupted state. Use defensive coding: validate assumptions, check exit codes, and never assume success.

2. Clarity Over Cleverness

Shell scripting is already cryptic enough. Write scripts that are easy to read, not impressive one-liners. Your future self will thank you.

3. Portability By Default

Unless you have a specific reason to require Bash 4+, write for POSIX sh. Your script may need to run in a container, an embedded system, or a legacy environment.

4. Defensive Safety

Every variable could be empty. Every command could fail. Every file could be missing. Write scripts that survive these realities.

5. Principle of Least Surprise

Scripts should behave predictably. Use consistent exit codes, clear error messages, and help text. No silent failures, no hidden side effects.

Scripting Maturity Model

Level	Name	Description
0	Ad-hoc	One-off commands saved to a file. No error handling. Only the author understands it.
1	Functional	Has shebang and basic structure. Handles common success paths. Brittle.
2	Defensive	Uses set -euo pipefail, checks exit codes, validates arguments. Has basic error messages.
3	Robust	Uses functions, has help text, proper argument parsing with defaults. Handles cleanup with traps. Portable across environments.
4	Production	Comprehensive error handling, logging, structured output, CI-tested with shellcheck. Configuration via environment or config files.
5	Battle-tested	Unit-tested, documented, versioned, handles edge cases (race conditions, signals, resource limits). Used in critical production pipelines.

Target at least Level 3 for any script that runs unattended.

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Script Structure

Shebang and Set Flags

Every script begins with a shebang and safety flags:

#!/usr/bin/env bash
# ─────────────────────────────────────────────────────────────
# Script:     deploy.sh
# Author:     Your Name
# Description: Builds and deploys the application to staging
# Usage:      ./deploy.sh [--env staging|production] [--skip-tests]
# ─────────────────────────────────────────────────────────────

set -euo pipefail

# -e:  Exit immediately if any command exits with non-zero status
# -u:  Treat unset variables as an error (exit)
# -o pipefail: Pipeline's exit status is last non-zero command, or zero
# -x:  (debug) Print commands and their arguments as they execute