shell-expert

---
created: 2025-12-16
modified: 2026-09-24
reviewed: 2026-04-25
name: shell-expert
description: "Shell scripting: bash, zsh, POSIX, CLI tools, cross-platform automation. Use when writing shell scripts, pipes, command-line automation, or portable shell code."
user-invocable: false
allowed-tools: Bash, Grep, Glob, Read, Write, Edit, TodoWrite
---

# Shell Expert

Expert knowledge for shell scripting, command-line tools, and automation with focus on robust, portable, and efficient solutions.

## When to Use This Skill

| Use this skill when... | Use justfile-expert instead when... |
|---|---|
| Authoring portable bash, zsh, or POSIX shell scripts | Wrapping commands as named recipes for a project task runner |
| Composing pipelines and one-off automation logic | Standardising entry points across a team or repo |
| Hardening scripts with `set -euo pipefail`, traps, and quoting | Defining cross-platform commands without writing shell glue |

| Use this skill when... | Use jq-json-processing instead when... |
|---|---|
| Glue logic that wires together CLI tools | The work is purely transforming JSON input |
| Writing reusable functions, argument parsing, or signal handling | A single jq expression can replace a chain of shell commands |

**Command-Line Tool Mastery**
- Expert knowledge of modern CLI tools (jq, yq, fd, rg, etc.)
- JSON/YAML processing and transformation
- File searching and text manipulation
- System automation and orchestration

**Shell Scripting Excellence**
- POSIX-compliant shell scripting for maximum portability
- Bash-specific features and best practices
- Error handling and defensive programming
- Cross-platform compatibility (Linux, macOS, BSD)

**Automation & Integration**
- CI/CD pipeline scripting
- System administration automation
- Tool integration and workflow automation
- Performance optimization for shell operations

## Key Capabilities

**JSON/YAML Processing**
- **jq**: Complex JSON queries, transformations, and filtering
- **yq**: YAML manipulation, in-place editing, format conversion
- **jd**: JSON diffing and patching for configuration management
- **Data pipeline construction**: Chaining tools for complex transformations

**File Operations & Search**
- **fd**: Fast, user-friendly file finding with intuitive syntax
- **rg (ripgrep)**: Lightning-fast recursive grep with gitignore support
- **lsd**: Modern ls replacement with visual enhancements
- **find/grep alternatives**: When and how to use modern replacements

**Shell Script Development**
- **Error Handling**: Proper trap usage, exit codes, error propagation
- **Input Validation**: Argument parsing, option handling, user input sanitization
- **Debugging**: Set options (-x, -e, -u, -o pipefail), debug output strategies
- **Performance**: Process substitution, parallel execution, efficient loops

**Cross-Platform Scripting**
- **Platform Detection**: OS-specific behavior handling
- **Path Management**: Portable path construction and manipulation
- **Tool Availability**: Checking for and handling missing dependencies
- **Compatibility Layers**: Writing scripts that work everywhere

**Automation Patterns**
- **Idempotent Operations**: Scripts that can run multiple times safely
- **Atomic Operations**: Ensuring all-or-nothing execution
- **Progress Reporting**: User-friendly output and status updates
- **Logging & Monitoring**: Structured logging for automated systems

## Essential Commands

**jq - JSON Processing**
```bash
jq . data.json                                    # Pretty-print
jq -r '.key.subkey' data.json                     # Extract value
jq '.items[] | select(.status == "active")'       # Filter
```

**yq - YAML Processing**
```bash
yq '.services.web.image' docker-compose.yml       # Read value
yq -i '.version = "2.1.0"' config.yml            # Update in-place
yq -o json config.yml                             # Convert to JSON
```

**fd - Fast File Finding**
```bash
fd 'pattern'                 # Find by pattern
fd -e md                     # Find by extension
fd -e sh -x shellcheck {}    # Find and execute
```

**rg - Recursive Grep**
```bash
rg 'DATABASE_URL'            # Basic search
rg 'TODO' -t python          # Search specific file types
rg -C 3 'error'              # Search with context
```

## Best Practices

**Script Development Workflow**
1. **Requirements Analysis**: Understand automation need and target platforms
2. **Tool Selection**: Choose appropriate tools for the task
3. **Prototype Development**: Create initial script with core functionality
4. **Error Handling**: Add robust error handling and edge case management
5. **Cross-Platform Testing**: Verify script works on all target systems
6. **Performance Optimization**: Profile and optimize for efficiency
7. **Documentation**: Add clear usage instructions and inline comments

**Critical Guidelines**
- Always use shellcheck for linting
- Set strict mode: `set -euo pipefail`
- Under `pipefail`, never pipe into a reader that exits early (`grep -q`, `head`): the producer gets SIGPIPE and a pipeline that succeeded reports failure, intermittently. Use `grep -q … <<<"$var"` or `grep … >/dev/null` (see REFERENCE.md)
- Quote all variables: `"${var}"`
- Use functions for reusable code
- Implement proper cleanup with trap
- Provide helpful error messages
- Include --help and --version options
- Use meaningful variable names
- Comment complex logic
- Test with different shells when targeting POSIX

## Common Patterns

**Robust Script Template**
```bash
#!/usr/bin/env bash
set -euo pipefail
IFS=$'\n\t'

trap 'echo "Error on line $LINENO"' ERR
trap cleanup EXIT

cleanup() {
    rm -f "$TEMP_FILE" 2>/dev/null || true
}

main() {
    parse_args "$@"
    validate_environment
    execute_task
}

main "$@"
```

**Cross-Platform Detection**
```bash
detect_os() {
    case "$OSTYPE" in
        linux*)   OS="linux" ;;
        darwin*)  OS="macos" ;;
        msys*)    OS="windows" ;;
        *)        OS="unknown" ;;
    esac
}
```

For detailed command-line tools reference, advanced automation examples, and troubleshooting guidance, see REFERENCE.md.

• REFERENCE.md 11 KB
• SKILL.md 6 KB