shell — Claude Code skill: safe bash and POSIX sh rules

---
name: shell
description: Shell scripting best practices for writing safe, portable, and maintainable bash/sh scripts. Use when writing, reviewing, or refactoring shell scripts, Dockerfile RUN commands, Makefile recipes, CI pipeline scripts, cron jobs, or systemd ExecStart directives. Triggers on bash, sh, POSIX, ShellCheck, error handling, quoting, variables, set -euo pipefail.
---

# Shell Scripts Best Practices (Community)

Comprehensive best practices guide for shell scripting, designed for AI agents and LLMs. Contains 49 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-` | 8 |
| 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-` | 3 |

## Quick Reference

### 1. Safety & Security (CRITICAL)

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

### 2. Portability (CRITICAL)

- [`port-shebang-selection`](references/port-shebang-selection.md) - Choose shebang based on portability needs
- [`port-avoid-bashisms`](references/port-avoid-bashisms.md) - Avoid bashisms in POSIX scripts
- [`port-printf-over-echo`](references/port-printf-over-echo.md) - Use printf instead of echo for portability
- [`port-export-syntax`](references/port-export-syntax.md) - Use portable export syntax
- [`port-test-portability`](references/port-test-portability.md) - Use portable test constructs

### 3. Error Handling (HIGH)

- [`err-strict-mode`](references/err-strict-mode.md) - Use strict mode for error detection
- [`err-exit-codes`](references/err-exit-codes.md) - Use meaningful exit codes
- [`err-trap-cleanup`](references/err-trap-cleanup.md) - Use trap for cleanup on exit
- [`err-stderr-messages`](references/err-stderr-messages.md) - Send error messages to stderr
- [`err-pipefail`](references/err-pipefail.md) - Use pipefail to catch pipeline errors
- [`err-check-commands`](references/err-check-commands.md) - Check command success explicitly
- [`err-shellcheck`](references/err-shellcheck.md) - Use ShellCheck for static analysis
- [`err-debug-tracing`](references/err-debug-tracing.md) - Use debug tracing with set -x and PS4

### 4. Variables & Data (HIGH)

- [`var-use-arrays`](references/var-use-arrays.md) - Use arrays for lists instead of strings
- [`var-local-scope`](references/var-local-scope.md) - Use local for function variables
- [`var-naming-conventions`](references/var-naming-conventions.md) - Follow variable naming conventions
- [`var-readonly-constants`](references/var-readonly-constants.md) - Use readonly for constants
- [`var-default-values`](references/var-default-values.md) - Use parameter expansion for defaults

### 5. Quoting & Expansion (MEDIUM-HIGH)

- [`quote-always-quote-variables`](references/quote-always-quote-variables.md) - Always quote variable expansions
- [`quote-dollar-at`](references/quote-dollar-at.md) - Use "$@" for argument passing
- [`quote-command-substitution`](references/quote-command-substitution.md) - Quote command substitutions
- [`quote-brace-expansion`](references/quote-brace-expansion.md) - Use braces for variable clarity
- [`quote-here-documents`](references/quote-here-documents.md) - Use here documents for multi-line strings
- [`quote-glob-safety`](references/quote-glob-safety.md) - Control glob expansion explicitly

### 6. Functions & Structure (MEDIUM)

- [`func-main-pattern`](references/func-main-pattern.md) - Use main() function pattern
- [`func-single-purpose`](references/func-single-purpose.md) - Write single-purpose functions
- [`func-return-values`](references/func-return-values.md) - Use return values correctly
- [`func-documentation`](references/func-documentation.md) - Document functions with header comments
- [`func-avoid-aliases`](references/func-avoid-aliases.md) - Prefer functions over aliases

### 7. Testing & Conditionals (MEDIUM)

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

### 8. Performance (LOW-MEDIUM)

- [`perf-builtins-over-external`](references/perf-builtins-over-external.md) - Use builtins over external commands
- [`perf-avoid-subshells`](references/perf-avoid-subshells.md) - Avoid unnecessary subshells
- [`perf-process-substitution`](references/perf-process-substitution.md) - Use process substitution for temp files
- [`perf-read-files`](references/perf-read-files.md) - Read files efficiently
- [`perf-parameter-expansion`](references/perf-parameter-expansion.md) - Use parameter expansion for string operations
- [`perf-batch-operations`](references/perf-batch-operations.md) - Batch operations instead of loops

### 9. Style & Formatting (LOW)

- [`style-indentation`](references/style-indentation.md) - Use consistent indentation
- [`style-file-structure`](references/style-file-structure.md) - Follow consistent file structure
- [`style-comments`](references/style-comments.md) - Write useful comments


## How to Use

Read individual reference files for detailed explanations and code examples:

- [Section definitions](references/_sections.md) - Category structure and impact levels
- [Rule template](assets/templates/_template.md) - Template for adding new rules

## Reference Files

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

## Key Sources

- [Google Shell Style Guide](https://google.github.io/styleguide/shellguide.html)
- [ShellCheck](https://www.shellcheck.net/)
- [Greg's Wiki (wooledge.org)](https://mywiki.wooledge.org/)
- [POSIX Shell Specification](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html)