diff --git a/src/commands/update-build-index.ts b/src/commands/update-build-index.ts
new file mode 100644
index 0000000..c7b7df9
--- /dev/null
+++ b/src/commands/update-build-index.ts
@@ -0,0 +1,33 @@
+import { Command } from '@oclif/command'
+import { readFileSync } from 'fs'
+import { writeFile } from 'fs/promises'
+import { YAMLMap } from 'yaml/types'
+
+import { DEVICE_CONFIG_FLAGS, loadDeviceConfigs } from '../config/device'
+import { ADEVTOOL_DIR, MAIN_BUILD_INDEX_PART } from '../config/paths'
+import { fetchBuildIndex } from '../images/build-index'
+import { showGitDiff } from '../util/cli'
+import { yamlStringifyNoFold } from '../util/yaml'
+
+export default class UpdateBuildIndex extends Command {
+  static description = 'fetch main (non-beta) build index and if it has changed, update build-index-main.yml file in-place and show git diff'
+
+  static flags = DEVICE_CONFIG_FLAGS
+
+  async run() {
+    let { flags } = this.parse(UpdateBuildIndex)
+    let devices = await loadDeviceConfigs(flags.devices)
+
+    let index: YAMLMap = await fetchBuildIndex(devices)
+
+    let yaml = yamlStringifyNoFold(index)
+
+    if (readFileSync(MAIN_BUILD_INDEX_PART).toString() === yaml) {
+      this.log('main build index is up-to-date')
+      process.exit(1)
+    }
+
+    await writeFile(MAIN_BUILD_INDEX_PART, yaml)
+    showGitDiff(ADEVTOOL_DIR, MAIN_BUILD_INDEX_PART)
+  }
+}
diff --git a/src/util/cli.ts b/src/util/cli.ts
index c81c7b6..1d59670 100644
--- a/src/util/cli.ts
+++ b/src/util/cli.ts
@@ -1,5 +1,8 @@
-import ora from 'ora'
+import assert from 'assert'
 import chalk from 'chalk'
+import child_proc from 'child_process'
+import ora from 'ora'
+import path from 'path'
 
 export type ProgressCallback = (progress: string) => void
 
@@ -25,3 +28,13 @@ export async function withSpinner<Return>(action: string, callback: (spinner: or
 
   return ret
 }
+
+export function showGitDiff(repoPath: string, filePath?: string) {
+  let args = ['-C', repoPath, `diff`]
+  if (filePath !== undefined) {
+    args.push(path.relative(repoPath, filePath))
+  }
+
+  let ret = child_proc.spawnSync('git', args, { stdio: 'inherit' })
+  assert(ret.status === 0)
+}