Code Formatter

Automatically format code files using language-specific formatters with comprehensive error handling.

Overview

This skill formats code files by detecting the programming language and applying the appropriate formatter (Prettier for JS/TS, Black for Python, rustfmt for Rust). It provides clear feedback on changes and handles edge cases systematically.

When to Use This Skill

Use when you need to format code before commits, ensure consistent style across projects, or apply language-specific formatting standards automatically.

Instructions for Claude

When this skill is activated, follow these steps to format code files.

Step 1: Validate Input File

Action: Verify that the specified file exists and is accessible.

Implementation:

# Check file exists
if [ ! -f "$FILE_PATH" ]; then
    echo "Error: File '$FILE_PATH' not found. Check path and try again."
    exit 1
fi

# Verify file is readable
if [ ! -r "$FILE_PATH" ]; then
    echo "Error: Cannot read '$FILE_PATH'. Fix with: chmod +r '$FILE_PATH'"
    exit 2
fi

# Check file size (max 10MB)
file_size=$(stat -c%s "$FILE_PATH" 2>/dev/null || stat -f%z "$FILE_PATH")
if [ $file_size -gt 10485760 ]; then
    echo "Warning: File is $(($file_size / 1024 / 1024))MB (max: 10MB). Continue? (y/n)"
    read -r response
    if [[ "$response" != "y" ]]; then
        exit 0
    fi
fi

Success Criteria:

• ✓ File exists at specified path
• ✓ File is readable (not a permissions error)
• ✓ File size ≤ 10MB or user confirms proceed

Error Handling:

• If file not found (exit 1) → Display error with path, abort
• If permissions denied (exit 2) → Display error with chmod fix, abort
• If file too large → Warn user, allow proceed or abort

---

Step 2: Detect File Language and Formatter

Action: Determine programming language from file extension and select appropriate formatter.

Implementation:

# Detect language by extension
case "$FILE_PATH" in
    *.js|*.jsx|*.ts|*.tsx|*.json)
        FORMATTER="prettier"
        FORMATTER_CMD="prettier --write"
        CHECK_CMD="prettier --check"
        ;;
    *.py)
        FORMATTER="black"
        FORMATTER_CMD="black"
        CHECK_CMD="black --check"
        ;;
    *.rs)
        FORMATTER="rustfmt"
        FORMATTER_CMD="rustfmt"
        CHECK_CMD="rustfmt --check"
        ;;
    *)
        echo "Error: Unsupported file type '${FILE_PATH##*.}'"
        echo "Supported: .js, .jsx, .ts, .tsx, .json (Prettier), .py (Black), .rs (rustfmt)"
        exit 3
        ;;
esac

echo "Detected language: ${FILE_PATH##*.} → Using $FORMATTER"

Success Criteria:

• ✓ File extension recognized
• ✓ Appropriate formatter selected
• ✓ Formatter choice logged

Error Handling:

• If unsupported extension (exit 3) → Display error with supported types, abort

---

Step 3: Check Formatter Installation

Action: Verify the required formatter is installed before attempting to run.

Implementation:

# Check if formatter exists
if ! command -v $FORMATTER &> /dev/null; then
    echo "Error: $FORMATTER is not installed."

    # Provide installation instructions
    case "$FORMATTER" in
        prettier)
            echo "Install with: npm install -g prettier"
            ;;
        black)
            echo "Install with: pip install black"
            ;;
        rustfmt)
            echo "Install with: rustup component add rustfmt"
            ;;
    esac

    echo "Install now and retry? (y/n)"
    read -r response
    if [[ "$response" == "y" ]]; then
        # User can install manually, then we retry
        exit 4
    else
        exit 4
    fi
fi

Success Criteria:

• ✓ Formatter found in PATH
• ✓ Formatter version logged (optional)

Error Handling:

• If formatter not found (exit 4) → Display installation instructions, offer retry

---

Step 4: Check for Syntax Errors

Action: Run formatter in check mode to detect syntax errors before modifying file.

Implementation:

# Create backup before checking
cp "$FILE_PATH" "${FILE_PATH}.backup"

# Check for syntax errors
$CHECK_CMD "$FILE_PATH" > /tmp/format-check.txt 2>&1
check_exit=$?

if [ $check_exit -ne 0 ]; then
    echo "Syntax errors detected:"
    cat /tmp/format-check.txt
    echo ""
    echo "Fix syntax errors first? (y/n)"
    read -r response
    if [[ "$response" != "y" ]]; then
        rm "${FILE_PATH}.backup"
        exit 0
    else
        # User will fix manually
        rm "${FILE_PATH}.backup"
        exit 5
    fi
fi

Success Criteria:

• ✓ Formatter check completes without errors
• ✓ Backup created successfully

Error Handling:

• If syntax errors (exit 5) → Display errors with line numbers, ask user to fix first

---

Step 5: Run Formatter and Report Changes

Action: Execute formatter with timeout and report what changed.

Implementation:

# Run formatter with 60s timeout
timeout 60s $FORMATTER_CMD "$FILE_PATH" > /tmp/format-output.txt 2>&1
exit_code=$?

if [ $exit_code -eq 124 ]; then
    echo "Error: Formatter timed out after 60 seconds."
    mv "${FILE_PATH}.backup" "$FILE_PATH"  # Restore backup
    exit 6
elif [ $exit_code -ne 0 ]; then
    echo "Error: Formatter failed with exit code $exit_code"
    cat /tmp/format-output.txt
    mv "${FILE_PATH}.backup" "$FILE_PATH"  # Restore backup
    exit 7
fi

# Calculate changes
changes=$(diff -u "${FILE_PATH}.backup" "$FILE_PATH" | wc -l)

# Report results
if [ $changes -eq 0 ]; then
    echo "✓ No formatting changes needed for $FILE_PATH"
else
    echo "✓ Formatted $FILE_PATH with $FORMATTER"
    echo "  Changes: $(($changes / 2)) lines modified"
    echo "  Backup: ${FILE_PATH}.backup"
fi

# Cleanup
rm -f /tmp/format-check.txt /tmp/format-output.txt

exit 0

Success Criteria:

• ✓ Formatter completes within 60 seconds
• ✓ Formatter exits with code 0 (success)
• ✓ User receives clear feedback (X lines changed)
• ✓ Backup preserved for rollback

Error Handling:

• If timeout (exit 6) → Restore backup, display timeout message
• If formatter error (exit 7) → Restore backup, display formatter output

---

Edge Cases & Special Handling

Edge Case 1: File Has Mixed Line Endings

When: File contains both CRLF (Windows) and LF (Unix) line endings

Handling:

# Detect and normalize line endings before formatting
file "$FILE_PATH" | grep -q "CRLF"
if [ $? -eq 0 ]; then
    echo "Info: Normalizing line endings to LF (Unix style)"
    dos2unix "$FILE_PATH" 2>/dev/null || sed -i 's/\r$//' "$FILE_PATH"
fi

Success Criteria:

• ✓ Line endings detected and normalized
• ✓ User informed of normalization

---

Edge Case 2: Multiple Formatters Available

When: Multiple formatter versions installed (e.g., prettier in node_modules and global)

Handling:

# Use project-local formatter if available
if [ -f "./node_modules/.bin/$FORMATTER" ]; then
    FORMATTER_CMD="./node_modules/.bin/$FORMATTER --write"
    echo "Info: Using project-local $FORMATTER"
else
    echo "Info: Using global $FORMATTER"
fi

Success Criteria:

• ✓ Local formatter prioritized over global
• ✓ User informed which formatter used

---

Edge Case 3: Formatter Config File Present

When: .prettierrc, pyproject.toml, or rustfmt.toml exists

Handling:

# Formatters automatically detect config files, just inform user
if [ -f ".prettierrc" ] || [ -f "pyproject.toml" ] || [ -f "rustfmt.toml" ]; then
    echo "Info: Using custom formatter configuration"
fi

Success Criteria:

• ✓ Config file detected and used by formatter
• ✓ User informed of custom config

---

Error Codes & Recovery

Code	Error	User Message	Recovery Strategy
1	File not found	"Error: File '[PATH]' not found."	Check path, try again
2	Permissions denied	"Error: Cannot read '[PATH]'. Fix with: chmod +r"	Fix permissions, try again
3	Unsupported file type	"Error: Unsupported file type '.ext'. Supported: .js, .py, .rs"	Use supported file type
4	Formatter not installed	"Error: [FORMATTER] not installed. Install with: [CMD]"	Install formatter, try again
5	Syntax error	"Syntax errors detected: [ERRORS]"	Fix syntax, try again
6	Formatter timeout	"Error: Formatter timed out after 60s"	Use smaller file or fix infinite loop
7	Formatter failure	"Error: Formatter failed: [OUTPUT]"	Check formatter logs, fix issue

---

Success Verification Checklist

After execution, verify:

• ✓ File formatted according to language style guide
• ✓ Original file backed up before modification
• ✓ User received clear feedback on changes (X lines modified)
• ✓ No data loss or file corruption
• ✓ Exit code indicates success (0) or failure (1-7)

---

Performance Expectations

Metric	Target	Measurement
Execution Time	<5 seconds for typical file	Actual runtime
Max File Size	10MB	File size check
Timeout	60 seconds max	Timeout mechanism
Memory Usage	<100MB	Not measured (formatter-dependent)