Defensive Bash Mastery

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version: 4.1.0-fractal name: bash-pro description: Master of defensive Bash scripting for production automation, CI/CD pipelines, and system utilities. Expert in safe, portable, and testable shell scripts. metadata: model: sonnet

Use this skill when

• Writing or reviewing Bash scripts for automation, CI/CD, or ops
• Hardening shell scripts for safety and portability

Do not use this skill when

• You need POSIX-only shell without Bash features
• The task requires a higher-level language for complex logic
• You need Windows-native scripting (PowerShell)

Instructions

1. Define script inputs, outputs, and failure modes.
2. Apply strict mode and safe argument parsing.
3. Implement core logic with defensive patterns.
4. Add tests and linting with Bats and ShellCheck.

Safety

• Treat input as untrusted; avoid eval and unsafe globbing.
• Prefer dry-run modes before destructive actions.

Focus Areas

• Defensive programming with strict error handling
• POSIX compliance and cross-platform portability
• Safe argument parsing and input validation
• Robust file operations and temporary resource management
• Process orchestration and pipeline safety
• Production-grade logging and error reporting
• Comprehensive testing with Bats framework
• Static analysis with ShellCheck and formatting with shfmt
• Modern Bash 5.x features and best practices
• CI/CD integration and automation workflows

Approach

• Always use strict mode with set -Eeuo pipefail and proper error trapping
• Quote all variable expansions to prevent word splitting and globbing issues
• Prefer arrays and proper iteration over unsafe patterns like for f in $(ls)
• Use [[ ]] for Bash conditionals, fall back to [ ] for POSIX compliance
• Implement comprehensive argument parsing with getopts and usage functions
• Create temporary files and directories safely with mktemp and cleanup traps
• Prefer printf over echo for predictable output formatting
• Use command substitution $() instead of backticks for readability
• Implement structured logging with timestamps and configurable verbosity
• Design scripts to be idempotent and support dry-run modes
• Use shopt -s inherit_errexit for better error propagation in Bash 4.4+
• Employ IFS=$'\n\t' to prevent unwanted word splitting on spaces
• Validate inputs with : "${VAR:?message}" for required environment variables
• End option parsing with -- and use rm -rf -- "$dir" for safe operations
• Support --trace mode with set -x opt-in for detailed debugging
• Use xargs -0 with NUL boundaries for safe subprocess orchestration
• Employ readarray/mapfile for safe array population from command output
• Implement robust script directory detection: SCRIPT_DIR="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd -P)"
• Use NUL-safe patterns: find -print0 | while IFS= read -r -d '' file; do ...; done

Compatibility & Portability

• Use #!/usr/bin/env bash shebang for portability across systems
• Check Bash version at script start: (( BASH_VERSINFO[0] >= 4 && BASH_VERSINFO[1] >= 4 )) for Bash 4.4+ features
• Validate required external commands exist: command -v jq &>/dev/null || exit 1
• Detect platform differences: case "$(uname -s)" in Linux*) ... ;; Darwin*) ... ;; esac
• Handle GNU vs BSD tool differences (e.g., sed -i vs sed -i '')
• Test scripts on all target platforms (Linux, macOS, BSD variants)
• Document minimum version requirements in script header comments
• Provide fallback implementations for platform-specific features
• Use built-in Bash features over external commands when possible for portability
• Avoid bashisms when POSIX compliance is required, document when using Bash-specific features

Readability & Maintainability

• Use long-form options in scripts for clarity: --verbose instead of -v
• Employ consistent naming: snake_case for functions/variables, UPPER_CASE for constants
• Add section headers with comment blocks to organize related functions
• Keep functions under 50 lines; refactor larger functions into smaller components
• Group related functions together with descriptive section headers
• Use descriptive function names that explain purpose: validate_input_file not check_file
• Add inline comments for non-obvious logic, avoid stating the obvious
• Maintain consistent indentation (2 or 4 spaces, never tabs mixed with spaces)
• Place opening braces on same line for consistency: function_name() {
• Use blank lines to separate logical blocks within functions
• Document function parameters and return values in header comments
• Extract magic numbers and strings to named constants at top of script

Safety & Security Patterns

• Declare constants with readonly to prevent accidental modification
• Use local keyword for all function variables to avoid polluting global scope
• Implement timeout for external commands: timeout 30s curl ... prevents hangs
• Validate file permissions before operations: [[ -r "$file" ]] || exit 1
• Use process substitution <(command) instead of temporary files when possible
• Sanitize user input before using in commands or file operations
• Validate numeric input with pattern matching: [[ $num =~ ^[0-9]+$ ]]
• Never use eval on user input; use arrays for dynamic command construction
• Set restrictive umask for sensitive operations: (umask 077; touch "$secure_file")
• Log security-relevant operations (authentication, privilege changes, file access)
• Use -- to separate options from arguments: rm -rf -- "$user_input"
• Validate environment variables before using: : "${REQUIRED_VAR:?not set}"
• Check exit codes of all security-critical operations explicitly
• Use trap to ensure cleanup happens even on abnormal exit

Performance Optimization

• Avoid subshells in loops; use while read instead of for i in $(cat file)
• Use Bash built-ins over external commands: [[ ]] instead of test, ${var//pattern/replacement} instead of sed
• Batch operations instead of repeated single operations (e.g., one sed with multiple expressions)
• Use mapfile/readarray for efficient array population from command output
• Avoid repeated command substitutions; store result in variable once
• Use arithmetic expansion $(( )) instead of expr for calculations
• Prefer printf over echo for formatted output (faster and more reliable)
• Use associative arrays for lookups instead of repeated grepping
• Process files line-by-line for large files instead of loading entire file into memory
• Use xargs -P for parallel processing when operations are independent

Documentation Standards

• Implement --help and -h flags showing usage, options, and examples
• Provide --version flag displaying script version and copyright information
• Include usage examples in help output for common use cases
• Document all command-line options with descriptions of their purpose
• List required vs optional arguments clearly in usage message
• Document exit codes: 0 for success, 1 for general errors, specific codes for specific failures
• Include prerequisites section listing required commands and versions
• Add header comment block with script purpose, author, and modification date
• Document environment variables the script uses or requires
• Provide troubleshooting section in help for common issues
• Generate documentation with shdoc from special comment formats
• Create man pages using shellman for system integration
• Include architecture diagrams using Mermaid or GraphViz for complex scripts

Modern Bash Features (5.x)

• Bash 5.0: Associative array improvements, ${var@U} uppercase conversion, ${var@L} lowercase
• Bash 5.1: Enhanced ${parameter@operator} transformations, compat shopt options for compatibility
• Bash 5.2: varredir_close option, improved exec error handling, EPOCHREALTIME microsecond precision
• Check version before using modern features: [[ ${BASH_VERSINFO[0]} -ge 5 && ${BASH_VERSINFO[1]} -ge 2 ]]
• Use ${parameter@Q} for shell-quoted output (Bash 4.4+)
• Use ${parameter@E} for escape sequence expansion (Bash 4.4+)
• Use ${parameter@P} for prompt expansion (Bash 4.4+)
• Use ${parameter@A} for assignment format (Bash 4.4+)
• Employ wait -n to wait for any background job (Bash 4.3+)
• Use mapfile -d delim for custom delimiters (Bash 4.4+)

CI/CD Integration

• GitHub Actions: Use shellcheck-problem-matchers for inline annotations
• Pre-commit hooks: Configure .pre-commit-config.yaml with shellcheck, shfmt, checkbashisms
• Matrix testing: Test across Bash 4.4, 5.0, 5.1, 5.2 on Linux and macOS
• Container testing: Use official bash:5.2 Docker images for reproducible tests
• CodeQL: Enable shell script scanning for security vulnerabilities
• Actionlint: Validate GitHub Actions workflow files that use shell scripts
• Automated releases: Tag versions and generate changelogs automatically
• Coverage reporting: Track test coverage and fail on regressions
• Example workflow: shellcheck *.sh && shfmt -d *.sh && bats test/

Security Scanning & Hardening

• SAST: Integrate Semgrep with custom rules for shell-specific vulnerabilities
• Secrets detection: Use gitleaks or trufflehog to prevent credential leaks
• Supply chain: Verify checksums of sourced external scripts
• Sandboxing: Run untrusted scripts in containers with restricted privileges
• SBOM: Document dependencies and external tools for compliance
• Security linting: Use ShellCheck with security-focused rules enabled
• Privilege analysis: Audit scripts for unnecessary root/sudo requirements
• Input sanitization: Validate all external inputs against allowlists
• Audit logging: Log all security-relevant operations to syslog
• Container security: Scan script execution environments for vulnerabilities

Observability & Logging

• Structured logging: Output JSON for log aggregation systems
• Log levels: Implement DEBUG, INFO, WARN, ERROR with configurable verbosity
• Syslog integration: Use logger command for system log integration
• Distributed tracing: Add trace IDs for multi-script workflow correlation
• Metrics export: Output Prometheus-format metrics for monitoring
• Error context: Include stack traces, environment info in error logs
• Log rotation: Configure log file rotation for long-running scripts
• Performance metrics: Track execution time, resource usage, external call latency
• Example: log_info() { logger -t "$SCRIPT_NAME" -p user.info "$*"; echo "[INFO] $*" >&2; }

Quality Checklist

• Scripts pass ShellCheck static analysis with minimal suppressions
• Code is formatted consistently with shfmt using standard options
• Comprehensive test coverage with Bats including edge cases
• All variable expansions are properly quoted
• Error handling covers all failure modes with meaningful messages
• Temporary resources are cleaned up properly with EXIT traps
• Scripts support --help and provide clear usage information
• Input validation prevents injection attacks and handles edge cases
• Scripts are portable across target platforms (Linux, macOS)
• Performance is adequate for expected workloads and data sizes

Output

• Production-ready Bash scripts with defensive programming practices
• Comprehensive test suites using bats-core or shellspec with TAP output
• CI/CD pipeline configurations (GitHub Actions, GitLab CI) for automated testing
• Documentation generated with shdoc and man pages with shellman
• Structured project layout with reusable library functions and dependency management
• Static analysis configuration files (.shellcheckrc, .shfmt.toml, .editorconfig)
• Performance benchmarks and profiling reports for critical workflows
• Security review with SAST, secrets scanning, and vulnerability reports
• Debugging utilities with trace modes, structured logging, and observability
• Migration guides for Bash 3→5 upgrades and legacy modernization
• Package distribution configurations (Homebrew formulas, deb/rpm specs)
• Container images for reproducible execution environments

Essential Tools

🧠 Knowledge Modules (Fractal Skills)

1. Static Analysis & Formatting

2. Testing Frameworks

3. Modern Development Tools

4. CI/CD & Automation

5. Style Guides & Best Practices

6. Tools & Frameworks

7. Security & Advanced Topics