backstage-cli: add config:schema command.

Signed-off-by: Andreas Stenius <git@astekk.se>

.changeset/short-geese-double.md

@@ -0,0 +1,5 @@
+---
+'@backstage/cli': minor
+---
+
+New config command to export the configuration schema. When running backstage-cli with yarn, consider using `yarn --silent backstage-cli config:schema` to get a clean output on `stdout`.

packages/cli/src/commands/config/schema.ts

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2021 Spotify AB
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { Command } from 'commander';
+import { stringify as stringifyYaml } from 'yaml';
+import { loadCliConfig } from '../../lib/config';
+
+export default async (cmd: Command) => {
+  const { schema } = await loadCliConfig({
+    args: [],
+    fromPackage: cmd.package,
+    mockEnv: true,
+  });
+
+  const data = schema.serialize();
+
+  if (cmd.format === 'json') {
+    process.stdout.write(`${JSON.stringify(data, null, 2)}\n`);
+  } else {
+    process.stdout.write(`${stringifyYaml(data)}\n`);
+  }
+};

packages/cli/src/commands/index.ts

@@ -177,6 +177,15 @@ export function registerCommands(program: CommanderStatic) {
     )
     .action(lazy(() => import('./config/validate').then(m => m.default)));
 
+  program
+    .command('config:schema')
+    .option(
+      '--package <name>',
+      'Only output config schema that applies to the given package',
+    )
+    .description('Print configuration schema')
+    .action(lazy(() => import('./config/schema').then(m => m.default)));
+
   program
     .command('versions:bump')
     .description('Bump Backstage packages to the latest versions')

packages/cli/src/lib/config.ts

@@ -48,8 +48,10 @@ export async function loadCliConfig(options: Options) {
     configPaths,
   });
 
-  console.log(
-    `Loaded config from ${appConfigs.map(c => c.context).join(', ')}`,
+  // printing to stderr to not clobber stdout in case the cli command
+  // outputs structured data (e.g. as config:schema does)
+  process.stderr.write(
+    `Loaded config from ${appConfigs.map(c => c.context).join(', ')}\n`,
   );
 
   try {