Bun CLI Development

Build powerful, production-grade CLI tools with Bun. Master argument parsing, output formatting, error handling, subcommands, and testing patterns proven in production across the SideQuest marketplace.

Quick Navigation

• Quick Start — Get a working CLI in 5 minutes
• Core Patterns — Argument parsing, output, usage, errors, subcommands
• Advanced Features — Dry-run, auto-commit, git integration
• Testing Your CLI — Unit and integration test patterns
• Reference — Comprehensive pattern guide + Para Obsidian example (9/10)

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Quick Start

Goal: Build a CLI tool that feels natural to use and is easy to maintain.

Minimal CLI Template

#!/usr/bin/env bun

import { color } from "@side-quest/core/formatters";

function printUsage(): void {
  console.log(color("cyan", "My CLI Tool v1.0"));
  console.log("Usage: my-cli <command> [options]");
  console.log("  config    Show configuration");
  console.log("  help      Show this help");
}

async function main(): Promise<void> {
  const [, , command] = process.argv;

  if (!command || command === "help") {
    printUsage();
    return;
  }

  try {
    switch (command) {
      case "config":
        console.log("Config: {...}");
        break;
      default:
        console.error(`Unknown command: ${command}`);
        process.exit(1);
    }
  } catch (error) {
    console.error("Error:", error instanceof Error ? error.message : error);
    process.exit(1);
  }
}

main();

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Core Patterns

1. Argument Parsing

The marketplace standard uses manual parsing (not external libraries). This keeps CLIs simple, dependency-light, and predictable.

Handle three flag formats:

• --flag value — Spaced syntax
• --flag=value — Equals syntax
• --flag — Boolean flag

function parseArgs(argv: string[]) {
  const positional: string[] = [];
  const flags: Record<string, string | boolean> = {};

  for (let i = 0; i < argv.length; i++) {
    const arg = argv[i];
    if (!arg) continue;

    if (arg.startsWith("--")) {
      const [keyRaw, value] = arg.split("=");
      const key = keyRaw?.slice(2);
      if (!key) continue;

      const next = argv[i + 1];
      if (value !== undefined) {
        flags[key] = value;
      } else if (next && !next.startsWith("--")) {
        flags[key] = next;
        i++;
      } else {
        flags[key] = true;
      }
    } else {
      positional.push(arg);
    }
  }

  const [command, subcommand, ...rest] = positional;
  return { command: command ?? "", subcommand, positional: rest, flags };
}

For detailed patterns and edge cases, see bun-cli-patterns.md § Argument Parsing.

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2. Output Formatting

Always support both markdown (human) and JSON (machine) formats.

import { OutputFormat, parseOutputFormat } from "@side-quest/core/formatters";

type Result = { title: string; items: string[] };

function formatMarkdown(result: Result): string {
  return `# ${result.title}\n\n${result.items.map(i => `- ${i}`).join("\n")}`;
}

function formatJson(result: Result): string {
  return JSON.stringify(result, null, 2);
}

function formatOutput(result: Result, format: OutputFormat): string {
  return format === "json" ? formatJson(result) : formatMarkdown(result);
}

// In main()
const format = parseOutputFormat(flags.format);
console.log(formatOutput(result, format));

Benefits: Humans read markdown (colored, readable), scripts parse JSON (structured, typeable).

For color palettes and advanced formatting, see bun-cli-patterns.md § Output Formatting.

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3. Usage Text

Make your CLI self-documenting with clear, scannable usage text.

function printUsage(): void {
  const lines = [
    color("cyan", "My CLI Tool"),
    "",
    "Usage:",
    "  my-cli config [--format md|json]",
    "  my-cli list [path] [--format md|json]",
    "  my-cli create --template <type> [options]",
    "",
    "Options:",
    "  --format md|json     Output format (default: md)",
    "  --dry-run            Show changes without applying",
    "  --help               Show this help",
    "",
    "Examples:",
    "  my-cli config --format json",
    "  my-cli list . --format md",
    "  my-cli create --template project --dry-run",
  ];

  console.log(lines.map(line => color("cyan", line)).join("\n"));
}

Key points:

• Colored headers (cyan)
• Real, copy-paste examples
• All three flag formats shown
• Structure: Usage → Options → Examples

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4. Error Handling

Be explicit and contextual with errors.

try {
  const config = loadConfig();

  if (!config.vault) {
    console.error("Error: VAULT environment variable required");
    process.exit(1);
  }

  // Do work...

} catch (error) {
  const message = error instanceof Error ? error.message : String(error);
  console.error(`Error: ${message}`);
  process.exit(1);
}

Conventions:

• Exit code 0 = success
• Exit code 1 = error
• Prefix errors with "Error:"
• Include contextual information (missing env, invalid file, etc.)
• Avoid stack traces in user output

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5. Subcommands

For CLIs with many operations, use two-level commands:

case "frontmatter": {
  const subcommand = args[0];
  switch (subcommand) {
    case "get":
      // ...
    case "validate":
      // ...
    case "migrate":
      // ...
    default:
      console.error(`Unknown subcommand: frontmatter ${subcommand}`);
      process.exit(1);
  }
  break;
}

Benefits:

• Flat namespace frontmatter get vs. frontmatter-get
• Easy to add subcommands
• Clear semantic grouping

---

Advanced Features

Dry-Run Support

Every write operation should support --dry-run:

const dryRun = flags["dry-run"] === true;
const result = await deleteFile(vault, file, { dryRun });

if (dryRun) {
  console.log("Would delete:", file);
} else {
  console.log("Deleted:", file);
}

Auto-Commit Integration

For tools that modify files, consider git integration:

if (flags["auto-commit"]) {
  const { isRepo, isClean } = await checkGitStatus(vault);
  if (!isRepo) throw new Error("Must be in a git repository");
  if (!isClean) throw new Error("Working tree must be clean");

  await autoCommitChanges(vault, changedFiles);
}

---

Testing Your CLI

Unit Testing with Bun

import { describe, expect, test } from "bun:test";
import { parseArgs } from "./args";

describe("CLI argument parsing", () => {
  test("parses --key value format", () => {
    const result = parseArgs(["command", "--name", "test"]);
    expect(result.flags.name).toBe("test");
  });

  test("parses --key=value format", () => {
    const result = parseArgs(["command", "--name=test"]);
    expect(result.flags.name).toBe("test");
  });

  test("handles boolean flags", () => {
    const result = parseArgs(["command", "--verbose"]);
    expect(result.flags.verbose).toBe(true);
  });
});

Integration Testing

# Test real CLI invocation
bun run src/cli.ts config --format json

# Verify output is valid JSON
bun run src/cli.ts config --format json | jq .

# Test error handling
bun run src/cli.ts unknown-command
echo $?  # Should be 1

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Reference

📚 Comprehensive Pattern Guide

See bun-cli-patterns.md for the complete, detailed reference:

• File structure — Project layout and organization
• Entry point — Shebang, imports, main flow
• Argument utilities — Parsing key=value, lists, type coercion
• Output utilities — Color palettes, formatting helpers
• Exit codes — Success (0), errors (1-3)
• Configuration & environment — Loading, validation
• Testing — Unit and integration test patterns
• Bun-specific patterns — Process I/O, file I/O, shell commands
• Command dispatch — Simple vs. complex CLI architectures
• Examples — Real implementations from marketplace
• Checklist — Implementation, testing, documentation verification
• Anti-patterns — Don't do these!
• Migration guide — Updating existing CLIs to standard

🔍 Example Implementation

See bun-cli-patterns.md § Para Obsidian CLI Review:

• Score: 9/10 — Exemplary reference implementation
• Real implementation analyzed against standard
• All patterns demonstrated in production code
• Subcommands, dry-run, auto-commit, error handling

Use Para Obsidian CLI as a template for:

• Argument parsing pattern
• Usage output structure
• Output formatting (md/json)
• Error handling
• Subcommand dispatch

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Common Pitfalls

❌ Don't

• Use external CLI libraries (oclif, yargs, commander) — Keep it simple
• Skip error handling — Users need clear feedback
• Ignore markdown output — Always support both markdown + JSON
• Create confusing flag names — Be explicit and consistent
• Forget the shebang — #!/usr/bin/env bun at the top

✅ Do

• Start with manual parsing — It's simpler than you think
• Test all three flag formats — Users will use all of them
• Provide real examples — Copy-paste examples in usage text
• Support --help — Make your CLI self-documenting
• Exit with proper codes — 0 for success, 1 for error

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Checklist: Building a CLI

• Shebang at top: #!/usr/bin/env bun
• JSDoc explaining CLI purpose
• Argument parsing (--flag value, --flag=value, --flag)
• Usage function with examples
• Subcommand dispatch (if needed)
• Try/catch error handling with contextual messages
• Support both markdown (default) and JSON output
• Exit codes: 0 for success, 1 for error
• Tests for argument parsing
• Tests for each command/subcommand
• README explaining usage
• Package.json bin entry (if applicable)

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Pro Tips

Tip 1: Progressive Disclosure in Help

// Basic help (what I do)
my-cli help
// Shows: command list + brief descriptions

// Advanced help (how to use me)
my-cli help create
// Shows: create command + all options + examples

Tip 2: Output to Stderr for Errors

// Use console.error for errors (goes to stderr)
console.error("Error:", message);  // ✅ Correct

// Avoid using console.log for errors
console.log("Error:", message);   // ❌ Goes to stdout

Tip 3: Use Color Strategically

// Color headers and important info
console.log(color("green", "✅ Success"));
console.log(color("yellow", "⚠️  Warning"));
console.error(color("red", "❌ Error"));

// Don't color everything — readers get fatigued

Tip 4: Validate at Boundaries

// Validate user input (flags, args) immediately
if (!flags.name || typeof flags.name !== "string") {
  console.error("Error: --name flag required");
  process.exit(1);
}

// Trust internal functions (already validated)
function processName(name: string) {
  // name is guaranteed to be a non-empty string
}

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FAQ

Q: Should I use oclif or similar frameworks? A: No. Manual parsing is simpler and keeps CLIs lean. The marketplace standard uses manual parsing across all CLIs.

Q: How do I handle secrets in CLIs? A: Use environment variables. Never accept secrets as flags (they'd appear in shell history).

Q: Should subcommands have their own help? A: Yes. my-cli subcommand --help should show help for that subcommand specifically.

Q: When should I add colors? A: For headers, success messages, and errors. Don't color everything — let contrast do the work.

Q: How do I test CLIs effectively? A: Unit test argument parsing. Integration test actual CLI invocations with real files.

Q: Why manual parsing instead of libraries? A: Zero dependencies, explicit and predictable, easy to extend, familiar across all marketplace CLIs.

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Last Updated: 2025-12-05 Status: Reference Implementation Related: bun-cli-patterns.md (comprehensive reference + example)