diff --git a/src/cli/help.ts b/src/cli/help.ts
new file mode 100644
index 0000000..fa7e900
--- /dev/null
+++ b/src/cli/help.ts
@@ -0,0 +1,33 @@
+export const help_text = `
+
+pkg-fence
+
+  Reading the NPM lockfile from stdin and filtering pkg name by rules.
+  Exit code: 0 for empty results, 1 for anything matched.
+
+
+
+Usage:
+
+                      cat   package-lock.json | pkg-fence <cmd>
+
+    npm shrinkwrap && cat npm-shrinkwrap.json | pkg-fence <cmd>
+
+
+
+Builtin Presets:
+
+    --lodash
+
+    --nolyfill
+
+
+
+Options:
+
+    --extra  foo    --extra  bar     --extra=abc,def
+
+    --ignore foo    --ignore bar    --ignore=abc,def
+
+`;
+
diff --git a/src/cli/main.ts b/src/cli/main.ts
index 4520d11..6909fe6 100644
--- a/src/cli/main.ts
+++ b/src/cli/main.ts
@@ -4,6 +4,7 @@ import { argv, stdin, exit } from 'node:process';
 
 import type { Fn } from '../common.ts';
 import { collect, type Flags } from '../collect.ts';
+import { help_text } from './help.ts';
 
 
 
@@ -52,20 +53,35 @@ export function parse (args: Iterable<string>): Flags {
 export async function main ({
 
         args = argv.slice(2),
-        lines = createInterface(stdin),
+        input = stdin,
+        lines: optional_lines,
         print = console.log,
         quit = exit,
 
 }: {
 
         args?: Iterable<string>,
+        input?: NodeJS.ReadableStream,
         lines?: AsyncIterable<string>,
-        print?: Fn<unknown, void>,
+        // deno-lint-ignore no-explicit-any
+        print?: Fn<any, void>,
         quit?: Fn<number, void>,
 
 } = {}): Promise<void> {
 
+    { // -h, --help
+
+        const [ cmd ] = Array.from(args);
+
+        if (cmd == null || cmd === '-h' || cmd === '--help') {
+            print(help_text);
+            return quit(0);
+        }
+
+    }
+
     const flags = parse(args);
+    const lines = optional_lines ?? createInterface({ input });
 
     let code = 0;
 
diff --git a/tests/cli/main.test.ts b/tests/cli/main.test.ts
index ee63793..28b2e66 100644
--- a/tests/cli/main.test.ts
+++ b/tests/cli/main.test.ts
@@ -1,6 +1,9 @@
 import { describe, it } from '@std/testing/bdd';
+import * as asserts from '@std/assert';
 import * as mock from '@std/testing/mock';
 
+import { Readable } from 'node:stream';
+
 import { make_lines } from '../utils.ts';
 
 import {
@@ -15,6 +18,55 @@ import {
 
 describe('main', function () {
 
+    for (const cmd of [ null, '-h', '--help' ]) {
+
+        it(`print help message on ${ cmd }`, async function () {
+
+            const args = cmd == null ? [] : [ cmd ];
+
+            const print = mock.spy(function (help: string) {
+                asserts.assertMatch(help, /Reading the NPM lockfile/);
+            });
+
+            const quit = mock.spy(() => {});
+
+            await main({ args, print, quit });
+
+            mock.assertSpyCalls(print, 1);
+
+            mock.assertSpyCallArg(quit, 0, 0, 0);
+            mock.assertSpyCalls(quit, 1);
+
+        });
+
+    }
+
+    it('ok in optional lines with input of ReadableStream', async function () {
+
+        const args = [ '--extra', 'acorn-jsx' ];
+
+        const input = Readable.from(`
+  "version": "1.2.3",
+  "lockfileVersion": 2,
+    "node_modules/lodash.memoize": {
+    "node_modules/acorn-jsx": {
+    "node_modules/side-channel": {
+        `);
+
+        const print = mock.spy(() => {});
+
+        const quit = mock.spy(() => {});
+
+        await main({ args, input, print, quit });
+
+        mock.assertSpyCallArg(print, 0, 0, 'acorn-jsx');
+        mock.assertSpyCalls(print, 1);
+
+        mock.assertSpyCallArg(quit, 0, 0, 1);
+        mock.assertSpyCalls(quit, 1);
+
+    });
+
     it('exit by 0 with no matches', async function () {
 
         const args = [ '--extra', 'wat' ];