Agent Skills: CLI Application Code Review Patterns

CLI Application Code Review Patterns

Stack-specific rules loaded by dh:code-reviewer when CLI entrypoints are detected (argparse, click, typer, commander.js, or similar argument parsing libraries).

Exit Codes

• Exit code 0 must be returned only on success — any error condition must produce a non-zero exit code
• Returning exit code 0 after printing an error message is a blocking finding — tools in pipelines cannot detect the failure
• Common conventions: 1 for general errors, 2 for usage/argument errors, 3+ for application-specific codes documented in --help
• sys.exit(1) or process.exit(1) must be called on fatal errors, not just printing to stderr

# WRONG: exits 0 even on error
def main():
    try:
        run()
    except Exception as e:
        print(f"Error: {e}", file=sys.stderr)
    # implicit exit 0

# RIGHT: non-zero on error
def main():
    try:
        run()
    except Exception as e:
        print(f"Error: {e}", file=sys.stderr)
        sys.exit(1)

Help and Version Flags

• --help must be present and print usage information, option descriptions, and examples — the auto-generated help from argparse/click/typer is acceptable as a minimum
• --version must be present and print the version string matching pyproject.toml or package.json
• Help text must be consistent with actual behavior — stale help text is a blocking finding
• --help must exit 0; --version must exit 0

stdin / stdout / stderr Separation

• Progress indicators, status messages, and diagnostic output go to stderr
• Data output (results to be piped or redirected) goes to stdout
• Mixing diagnostic messages into stdout breaks pipe usage — this is a blocking finding for tools that produce structured output
• Error messages go to stderr with a non-zero exit code

Non-Interactive Operation

• Every interactive prompt (input(), readline, inquirer) must have a corresponding flag alternative (--flag value) for use in CI and scripts
• Interactive prompts that block in non-TTY environments (piped input, CI) are a blocking finding
• Detect TTY with sys.stdin.isatty() or equivalent and skip interactive prompts in non-TTY mode

Signal Handling

• SIGINT (Ctrl+C) must produce a clean exit — no stack traces, no partial file writes, exit code 130 by default
• SIGTERM must be handled in long-running processes — clean up resources and exit gracefully
• Temporary files created during execution must be cleaned up in signal handlers or atexit callbacks

Argument Validation

• Validate all arguments before beginning any work — do not fail halfway through a destructive operation due to a missing flag
• Report all validation errors at once rather than stopping at the first error
• File path arguments must be validated for existence and permissions before the operation begins, not after

ANSI Color Codes

• ANSI escape codes must not be emitted when NO_COLOR environment variable is set (any non-empty value)
• ANSI escape codes must not be emitted when stdout is not a TTY (piped output, file redirection)
• Check TTY with sys.stdout.isatty() or equivalent before colorizing output

Dry Run for Destructive Operations

• Any operation that deletes, modifies, or overwrites data must have a --dry-run flag that shows what would happen without doing it
• --dry-run output must clearly distinguish what would be changed and what would remain untouched

Anti-Patterns

# WRONG: no --dry-run for destructive command
@app.command()
def delete_records(pattern: str):
    records = find_records(pattern)
    for r in records:
        r.delete()
    print(f"Deleted {len(records)} records")

# RIGHT: dry-run support
@app.command()
def delete_records(pattern: str, dry_run: bool = typer.Option(False, "--dry-run")):
    records = find_records(pattern)
    if dry_run:
        for r in records:
            print(f"Would delete: {r.id}")
        print(f"Would delete {len(records)} records (dry run)")
        return
    for r in records:
        r.delete()
    print(f"Deleted {len(records)} records")

# WRONG: ANSI always on
print(f"\033[32mSuccess\033[0m")

# RIGHT: conditional color
import sys
USE_COLOR = sys.stdout.isatty() and not os.environ.get("NO_COLOR")
success = "\033[32mSuccess\033[0m" if USE_COLOR else "Success"
print(success)