Claude Skill Bash

Bash Best Practices Skill

This skill ensures all bash scripts follow enterprise-grade best practices for maintainability, reliability, and user-friendliness.

When This Skill Applies

This skill should be automatically invoked when:

• Creating new bash scripts or shell scripts
• Editing existing .sh or .bash files
• User requests scripts for automation, deployment, backup, or system tasks
• Reviewing or refactoring bash code
• User mentions: bash, shell script, main function, argument parsing, script structure

Script Complexity Guidelines

This skill recognizes two categories of scripts with different structural requirements:

Simple Scripts

Scripts that do one simple thing well without unnecessary ceremony.

Characteristics:

• Less than ~30 lines of actual logic (excluding comments and whitespace)
• No command-line arguments required
• Single, focused purpose that's self-evident
• Linear flow or minimal branching
• Output is often consumed by other programs or scripts

Examples: Version detection, environment checks, data format conversion, simple calculations

Structural Requirements:

• Main function and guard clause: Optional
• Usage function: Not needed (no arguments to document)
• Argument parsing: Not applicable
• Color output: Optional (often unnecessary)
• Can use direct execution without function wrapping

Ordinary Scripts

Scripts with broader scope requiring full structure for maintainability.

Characteristics:

• More than ~30 lines of logic OR
• Takes command-line arguments OR
• Multiple functions or complex branching OR
• Interactive use by humans OR
• Requires detailed documentation

Examples: Deployment tools, backup utilities, system maintenance scripts, development tools

Structural Requirements:

• Main function and guard clause: Required
• Usage function: Required
• Argument parsing: Required (if taking arguments)
• Full error handling: Required
• User-friendly output: Required (for interactive scripts)

Core Principles to Apply

For Ordinary Scripts

When writing ordinary scripts, ensure:

1. Main function pattern with guard clause - Every script must have a main() function
2. Usage function - Clear help documentation
3. Argument parsing - Structured option handling in main
4. Dependency checking - Explicit validation of required tools
5. Explicit error handling - Never use set -e, handle errors explicitly
6. Function organization - Correct ordering and single responsibility
7. User-friendly output - Colored, structured output with status indicators

For Simple Scripts

When writing simple scripts, ensure:

1. Clear purpose comment - Explain what the script does at the top
2. Explicit error handling - Never use set -e, handle errors explicitly
3. Proper exit codes - 0 for success, non-zero for failure
4. Error output to stderr - Use >&2 for error messages
5. Variable safety - Quote variables, handle undefined cases

Script Structure Templates

Template for Ordinary Scripts

Every new bash script should follow this structure:

#!/usr/bin/env bash

# Global declarations
DEPENDENCIES=(jq curl git)  # List external tools needed
SCRIPT_NAME=$(basename "$0")
VERSION="1.0.0"

# Color definitions (if outputting to terminal)
RED='\033[0;31m'
GREEN='\033[0;32m'
YELLOW='\033[1;33m'
BLUE='\033[0;34m'
NC='\033[0m' # No Color

# Allow users to disable colors
if [[ -n "${NO_COLOR:-}" ]] || [[ "${TERM:-}" == "dumb" ]]; then
    RED=""
    GREEN=""
    YELLOW=""
    BLUE=""
    NC=""
fi

function usage() {
    cat <<EOM

Brief description of what this script does.

usage: ${SCRIPT_NAME} [options]

options:
    -i|--input        <file>     Input file to process (required)
    -o|--output       <file>     Output file (optional, defaults to stdout)
    -v|--verbose                 Enable verbose output
    -h|--help                    Show this help message
    --version                    Show version information

dependencies: ${DEPENDENCIES[@]}

examples:
    ${SCRIPT_NAME} -i data.txt -o report.json
    ${SCRIPT_NAME} --input data.txt --verbose

EOM
    exit 1
}

function main() {
    # Default values
    local input_file=""
    local output_file=""
    local verbose=false

    # Parse arguments
    while [ "$1" != "" ]; do
        case $1 in
        -i | --input)
            shift
            input_file="$1"
            ;;
        -o | --output)
            shift
            output_file="$1"
            ;;
        -v | --verbose)
            verbose=true
            ;;
        --version)
            echo "${SCRIPT_NAME} version ${VERSION}"
            exit 0
            ;;
        -h | --help)
            usage
            ;;
        *)
            echo "Error: Unknown option '$1'"
            usage
            ;;
        esac
        shift
    done

    # Validate required arguments
    if [ -z "$input_file" ]; then
        echo "Error: Input file is required" >&2
        usage
    fi

    # Check dependencies
    exit_on_missing_tools "${DEPENDENCIES[@]}"

    # Main script logic
    process_file "$input_file" "$output_file" "$verbose"
}

# Business logic functions
function process_file() {
    local input="$1"
    local output="$2"
    local verbose="$3"

    # Implementation here
    echo "Processing ${input}..."
}

# Utility functions
function exit_on_missing_tools() {
    for cmd in "$@"; do
        if command -v "$cmd" &>/dev/null; then
            continue
        fi
        printf "Error: Required tool '%s' is not installed or not in PATH\n" "$cmd"
        exit 1
    done
}

# Guard clause - only execute main if script is run directly
if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
    main "$@"
    exit 0
fi

Template for Simple Scripts

For scripts that do one simple thing without arguments:

#!/usr/bin/env bash
# Purpose: Determine version for CI builds based on git state
# Usage: determine-version.sh
# Output: Version string to stdout

# For simple scripts, direct execution is fine - no main() required

# Check if we're on a tag
TAG=$(git describe --exact-match --tags HEAD 2>/dev/null)

if [ -n "$TAG" ]; then
    # On a tag - use the tag as version
    echo "${TAG#v}"
else
    # Not on tag - determine from branch
    if [ -n "$GITHUB_HEAD_REF" ]; then
        # Pull request
        BRANCH="$GITHUB_HEAD_REF"
    elif [ -n "$GITHUB_REF_NAME" ]; then
        # Push to branch
        BRANCH="$GITHUB_REF_NAME"
    else
        # Fallback to current branch
        BRANCH=$(git symbolic-ref -q --short HEAD 2>/dev/null)
        if [ -z "$BRANCH" ]; then
            echo "Error: Unable to determine branch" >&2
            exit 1
        fi
    fi

    # Sanitize branch name for version string
    SANITIZED=$(echo "$BRANCH" | sed 's/[^a-zA-Z0-9._-]/_/g')
    echo "${SANITIZED}-SNAPSHOT"
fi

Key points for simple scripts:

• Clear purpose comment at the top
• Direct execution without main() wrapper
• Errors still go to stderr with >&2
• Meaningful exit codes (0 for success, non-zero for failure)
• Variables are still properly quoted
• No need for usage() function when there are no arguments
• No need for color output when output is data

Detailed Best Practices

1. Script Bootstrapping

Main Function Pattern

• ALWAYS use a main function as the entry point
• This enables safe sourcing by other scripts
• Provides clear structure and testability

The Guard Clause

if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
    main "$@"
    exit 0
fi

• Must be at the very end of the script
• Enables the script to be sourced without execution
• Always include explicit exit 0 after main

2. Usage Function and Argument Parsing

Usage Function Requirements

• Must be defined before main() function
• Use heredoc for clean formatting
• Include: description, options, dependencies, examples
• Exit with status 1

Argument Parsing Best Practices

function main() {
    # Set defaults first
    local input_file=""
    local verbose=false

    # Parse with while loop and case
    while [ "$1" != "" ]; do
        case $1 in
        -i | --input)
            shift
            input_file="$1"
            ;;
        -v | --verbose)
            verbose=true
            ;;
        -h | --help)
            usage
            ;;
        *)
            echo "Error: Unknown option '$1'"
            usage
            ;;
        esac
        shift
    done

    # Validate after parsing
    if [ -z "$input_file" ]; then
        echo "Error: Input file is required" >&2
        usage
    fi
}

3. Dependency Declaration and Checking

Always Declare Dependencies

DEPENDENCIES=(jq curl git docker)

Check Before Use

function exit_on_missing_tools() {
    for cmd in "$@"; do
        if command -v "$cmd" &>/dev/null; then
            continue
        fi
        printf "Error: Required tool '%s' is not installed or not in PATH\n" "$cmd"
        exit 1
    done
}

Include in Usage

Show dependencies in the usage function so users know requirements upfront.

4. Function Organization

Correct Function Order (MANDATORY)

1. usage() - First after global declarations
2. main() - Immediately after usage
3. Business logic functions - Core functionality
4. Utility functions - Generic helpers

NEVER add section comment headers like "# UTILITY FUNCTIONS". The code structure speaks for itself.

Function Design Principles

• Single Responsibility: Each function does one thing
• Clear Inputs/Outputs: Use local variables, return meaningful status
• Descriptive Names: Use verb-noun combinations (create_backup, validate_input)
• Keep Small: Break down functions longer than 50 lines
• Use Local Variables: Always declare with local

5. Error Handling

AVOID set -e

Never use set -e. Instead, use explicit error handling:

# Good - Explicit handling
if ! command; then
    echo "Error: Command failed" >&2
    exit 1
fi

# Good - Chain with ||
cd /some/directory || {
    echo "Error: Cannot change directory" >&2
    exit 1
}

# Good - Capture and check
output=$(command 2>&1)
if [ $? -ne 0 ]; then
    echo "Error: Command failed: $output" >&2
    exit 1
fi

Error Handling Rules

• Always redirect errors to stderr with >&2
• Provide context about what failed
• Use meaningful exit codes
• Clean up temporary files on failure

6. User-Friendly Output

Color Output (When Appropriate)

# Define colors at script top
RED='\033[0;31m'
GREEN='\033[0;32m'
YELLOW='\033[1;33m'
BLUE='\033[0;34m'
NC='\033[0m'

# Respect NO_COLOR environment
if [[ -n "${NO_COLOR:-}" ]] || [[ "${TERM:-}" == "dumb" ]]; then
    RED="" GREEN="" YELLOW="" BLUE="" NC=""
fi

Output Functions

function print_header() {
    echo -e "${BLUE}=======================${NC}"
    echo -e "${BLUE}$1${NC}"
    echo -e "${BLUE}=======================${NC}"
}

function print_success() {
    echo -e "${GREEN}✅ $1${NC}"
}

function print_error() {
    echo -e "${RED}❌ Error: $1${NC}" >&2
}

function print_warning() {
    echo -e "${YELLOW}⚠️  Warning: $1${NC}"
}

function print_step() {
    local current="$1"
    local total="$2"
    local message="$3"
    echo -e "${YELLOW}[${current}/${total}] ${message}${NC}"
}

7. Keep Main Function Lean

Main should only:

1. Parse arguments
2. Validate inputs
3. Check dependencies
4. Call business functions
5. Handle high-level errors

Business logic belongs in separate functions.

8. Code Comments

Avoid Excessive Comments

• Well-named functions are self-documenting
• Only comment the "why", not the "what"
• Use comments for:
  • Complex algorithms
  • Non-obvious workarounds
  • Business logic rules
  • External API quirks

# Bad - Obvious comment
# Increment counter
counter=$((counter + 1))

# Good - Explains why
# Exponential backoff with jitter to prevent thundering herd
delay=$((2 ** attempt * 1000 + RANDOM % 1000))

Script Generation Checklist

When generating a new script, verify:

• Shebang line: #!/usr/bin/env bash
• DEPENDENCIES array declared
• usage() function defined first
• main() function handles argument parsing
• Dependency checking with exit_on_missing_tools
• Guard clause at script end
• Explicit error handling (no set -e)
• Functions properly ordered
• Local variables in functions
• Errors redirected to stderr
• Meaningful exit codes
• Color output with NO_COLOR support (if applicable)

Common Patterns

Interactive Prompts

# Yes/no confirmation
read -p "Continue? (y/n): " -n 1 -r
echo
if [[ ! $REPLY =~ ^[Yy]$ ]]; then
    echo "Operation cancelled"
    exit 0
fi

# Menu selection
echo "Select an option:"
echo "1) Full backup"
echo "2) Incremental backup"
read -p "Enter choice (1-2): " choice

# Password (hidden)
read -s -p "Enter password: " password
echo  # Add newline

Temporary Files

# Create temp file safely
temp_file=$(mktemp)
trap "rm -f '$temp_file'" EXIT

# Use temp directory
temp_dir=$(mktemp -d)
trap "rm -rf '$temp_dir'" EXIT

Pipeline Error Handling

# Without set -e, check PIPESTATUS
command1 | command2 | command3
if [ "${PIPESTATUS[0]}" -ne 0 ]; then
    echo "Error: command1 failed" >&2
    exit 1
fi

Examples to Follow

When users request scripts for common tasks, apply these patterns:

Backup Script

• Include timestamp in backup names
• Verify source exists before backup
• Check available disk space
• Provide progress feedback
• Clean up old backups based on retention

Deployment Script

• Validate environment first
• Show clear step progression
• Roll back on failure
• Log all operations
• Provide summary at end

Data Processing Script

• Validate input format
• Handle large files efficiently
• Provide progress indicators
• Support resume on failure
• Generate detailed reports

Choosing Between Simple and Ordinary Scripts

Decision Tree

Ask these questions to determine which pattern to use:

1. Does the script take command-line arguments?

   • YES → Use ordinary script pattern (needs usage() and parsing)
   • NO → Continue to question 2

2. Is the logic more than ~30 lines (excluding comments)?

   • YES → Use ordinary script pattern (needs structure)
   • NO → Continue to question 3

3. Does it have multiple functions or complex branching?

   • YES → Use ordinary script pattern (needs organization)
   • NO → Continue to question 4

4. Will it be run interactively by humans who need help?

   • YES → Use ordinary script pattern (needs user-friendly features)
   • NO → Continue to question 5

5. Is it doing one simple, self-evident task?

   • YES → Use simple script pattern
   • NO → Use ordinary script pattern

When in doubt, use the ordinary pattern. It's better to have structure you don't need than to need structure you don't have.

Examples of Simple Scripts

• determine-version.sh - Outputs version string based on git state
• check-env.sh - Verifies environment variables are set
• format-json.sh - Reformats JSON from stdin to stdout
• count-lines.sh - Counts lines in specified file types
• is-production.sh - Returns 0 if on production branch

Examples of Ordinary Scripts

• deploy.sh - Multi-step deployment with rollback
• backup.sh - Database backup with retention management
• setup-dev.sh - Developer environment initialization
• test-runner.sh - Configurable test execution
• migrate-data.sh - Data migration with validation

Testing Scripts

Always test generated scripts for:

1. Syntax: bash -n script.sh
2. Undefined variables: Run with set -u temporarily
3. Dependencies: Run without required tools installed
4. Error paths: Test with invalid inputs
5. Sourcing: Verify guard clause works (for ordinary scripts)

Remember

Every script you generate or modify should be production-ready:

• Maintainable by others
• Self-documenting through good structure
• Gracefully handles errors
• Provides helpful output
• Follows consistent patterns

These practices ensure scripts are professional, reliable, and user-friendly.