Shell Scripts Best Practices

Comprehensive best practices guide for shell scripting, designed for AI agents and LLMs. Contains 48 rules across 9 categories, prioritized by impact from critical (safety, portability) to incremental (style). Each rule includes detailed explanations, real-world examples comparing incorrect vs. correct implementations, and specific impact metrics.

When to Apply

Reference these guidelines when:

• Writing new bash or POSIX shell scripts
• Reviewing shell scripts for security vulnerabilities
• Debugging scripts that fail silently or behave unexpectedly
• Porting scripts between Linux, macOS, and containers
• Optimizing shell script performance
• Setting up CI/CD pipelines with shell scripts

Rule Categories by Priority

Priority	Category	Impact	Prefix	Rules
1	Safety & Security	CRITICAL	safety-	6
2	Portability	CRITICAL	port-	5
3	Error Handling	HIGH	err-	6
4	Variables & Data	HIGH	var-	5
5	Quoting & Expansion	MEDIUM-HIGH	quote-	6
6	Functions & Structure	MEDIUM	func-	5
7	Testing & Conditionals	MEDIUM	test-	5
8	Performance	LOW-MEDIUM	perf-	6
9	Style & Formatting	LOW	style-	4

Quick Reference

1. Safety & Security (CRITICAL)

• safety-command-injection - Prevent command injection from user input
• safety-eval-avoidance - Avoid eval for dynamic commands
• safety-absolute-paths - Use absolute paths for external commands
• safety-temp-files - Create secure temporary files
• safety-suid-forbidden - Never use SUID/SGID on shell scripts
• safety-argument-injection - Prevent argument injection with double dash

2. Portability (CRITICAL)

• port-shebang-selection - Choose shebang based on portability needs
• port-avoid-bashisms - Avoid bashisms in POSIX scripts
• port-printf-over-echo - Use printf instead of echo for portability
• port-export-syntax - Use portable export syntax
• port-test-portability - Use portable test constructs

3. Error Handling (HIGH)

• err-strict-mode - Use strict mode for error detection
• err-exit-codes - Use meaningful exit codes
• err-trap-cleanup - Use trap for cleanup on exit
• err-stderr-messages - Send error messages to stderr
• err-pipefail - Use pipefail to catch pipeline errors
• err-check-commands - Check command success explicitly

4. Variables & Data (HIGH)

• var-use-arrays - Use arrays for lists instead of strings
• var-local-scope - Use local for function variables
• var-naming-conventions - Follow variable naming conventions
• var-readonly-constants - Use readonly for constants
• var-default-values - Use parameter expansion for defaults

5. Quoting & Expansion (MEDIUM-HIGH)

• quote-always-quote-variables - Always quote variable expansions
• quote-dollar-at - Use "$@" for argument passing
• quote-command-substitution - Quote command substitutions
• quote-brace-expansion - Use braces for variable clarity
• quote-here-documents - Use here documents for multi-line strings
• quote-glob-safety - Control glob expansion explicitly

6. Functions & Structure (MEDIUM)

• func-main-pattern - Use main() function pattern
• func-single-purpose - Write single-purpose functions
• func-return-values - Use return values correctly
• func-documentation - Document functions with header comments
• func-avoid-aliases - Prefer functions over aliases

7. Testing & Conditionals (MEDIUM)

• test-double-brackets - Use [[ ]] for tests in bash
• test-arithmetic - Use (( )) for arithmetic comparisons
• test-explicit-empty - Use explicit empty/non-empty string tests
• test-file-operators - Use correct file test operators
• test-case-patterns - Use case for pattern matching

8. Performance (LOW-MEDIUM)

• perf-builtins-over-external - Use builtins over external commands
• perf-avoid-subshells - Avoid unnecessary subshells
• perf-process-substitution - Use process substitution for temp files
• perf-read-files - Read files efficiently
• perf-parameter-expansion - Use parameter expansion for string operations
• perf-batch-operations - Batch operations instead of loops

9. Style & Formatting (LOW)

• style-indentation - Use consistent indentation
• style-file-structure - Follow consistent file structure
• style-comments - Write useful comments
• style-shellcheck - Use ShellCheck for static analysis

How to Use

Read individual reference files for detailed explanations and code examples:

• Section definitions - Category structure and impact levels
• Rule template - Template for adding new rules

Reference Files

File	Description
AGENTS.md	Complete compiled guide with all rules
references/_sections.md	Category definitions and ordering
assets/templates/_template.md	Template for new rules
metadata.json	Version and reference information

Key Sources

• Google Shell Style Guide
• ShellCheck
• Greg's Wiki (wooledge.org)
• POSIX Shell Specification