Add gemini extensions link command (#7241)

Co-authored-by: Bryan Morgan <bryanmorgan@google.com>

packages/cli/src/commands/extensions/new.ts

@@ -0,0 +1,86 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { access, cp, mkdir, readdir } from 'node:fs/promises';
+import { join, dirname } from 'node:path';
+import type { CommandModule } from 'yargs';
+import { fileURLToPath } from 'node:url';
+import { getErrorMessage } from '../../utils/errors.js';
+
+interface NewArgs {
+  path: string;
+  template: string;
+}
+
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = dirname(__filename);
+
+const EXAMPLES_PATH = join(__dirname, 'examples');
+
+async function pathExists(path: string) {
+  try {
+    await access(path);
+    return true;
+  } catch (_e) {
+    return false;
+  }
+}
+
+async function copyDirectory(template: string, path: string) {
+  if (await pathExists(path)) {
+    throw new Error(`Path already exists: ${path}`);
+  }
+
+  const examplePath = join(EXAMPLES_PATH, template);
+  await mkdir(path, { recursive: true });
+  await cp(examplePath, path, { recursive: true });
+}
+
+async function handleNew(args: NewArgs) {
+  try {
+    await copyDirectory(args.template, args.path);
+    console.log(
+      `Successfully created new extension from template "${args.template}" at ${args.path}.`,
+    );
+    console.log(
+      `You can install this using "gemini extensions link ${args.path}" to test it out.`,
+    );
+  } catch (error) {
+    console.error(getErrorMessage(error));
+    throw error;
+  }
+}
+
+async function getBoilerplateChoices() {
+  const entries = await readdir(EXAMPLES_PATH, { withFileTypes: true });
+  return entries
+    .filter((entry) => entry.isDirectory())
+    .map((entry) => entry.name);
+}
+
+export const newCommand: CommandModule = {
+  command: 'new <path> <template>',
+  describe: 'Create a new extension from a boilerplate example.',
+  builder: async (yargs) => {
+    const choices = await getBoilerplateChoices();
+    return yargs
+      .positional('path', {
+        describe: 'The path to create the extension in.',
+        type: 'string',
+      })
+      .positional('template', {
+        describe: 'The boilerplate template to use.',
+        type: 'string',
+        choices,
+      });
+  },
+  handler: async (args) => {
+    await handleNew({
+      path: args['path'] as string,
+      template: args['template'] as string,
+    });
+  },
+};