Exit early from refresh-code-excerpts if wrong diff version

README.md

@@ -93,13 +93,13 @@ if you already have the required packages installed.
       **Note**: Unless you're editing files under `site-shared`, you can safely
       ignore `ERROR: directory is already being watched` messages.
       For details, see [#1363](https://github.com/flutter/website/issues/1363).
-      
+
       **Note**: The first time you run either one of these commands,
       jekyll takes anywhere between 10 - 20 seconds to generate static
       content inside the `_sites` directory. If you try to verify the
       site locally but aren't able to see the content right away,
-      wait 20 seconds before stopping the 
-      server or concluding that something is wrong. 
+      wait 20 seconds before stopping the
+      server or concluding that something is wrong.
  1. Prior to submitting, validate site links:<br>
     `./tool/shared/check-links.sh`
 
@@ -108,14 +108,14 @@ if you already have the required packages installed.
 > To fix Jekyll, stop the `serve.sh` script and remove the generated site files:
 > hand, and then restart the `serve.sh` script:
 
-> `npm run clean` 
+> `npm run clean`
 > OR
 > `rm -Rf ./_site/* ./.jekyll*`
 
-> Next, restart the `serve.sh` script: 
+> Next, restart the `serve.sh` script:
 
 > `npm run start`
-> OR 
+> OR
 > `./tool/serve.sh`
 
 ## Creating and/or editing DartPad example code
@@ -140,7 +140,7 @@ This `iframe` depends on the following GitHub gist url:
 For detailed instructions on how to use this approach to DartPad examples, see the [DartPad embedding guide].
 
 ### DartPad example code in this repo - `src/_packages/dartpad_picker`
-Some DartPad example code remains in `.dart` files in this repo, and must be compiled via `src/_packages/dartpad_picker/compile.sh`. For an example, consult `src/_packages/dartpad_picker/web/dartpad_picker_main.dart`. 
+Some DartPad example code remains in `.dart` files in this repo, and must be compiled via `src/_packages/dartpad_picker/compile.sh`. For an example, consult `src/_packages/dartpad_picker/web/dartpad_picker_main.dart`.
 
 In order to create or change example code using `dartpad_picker`, you must regenerate the JavaScript:
 
@@ -172,13 +172,13 @@ You can deploy your local edits to a personal staging site as follows
     $ npx firebase use my-foo
     Now using alias staging (<your project name>)
     ```
- 
+
  1. Tell Firebase to execute deployment of your project:
- 
+
    ```console
    $ npx firebase deploy
    ```
-   
+
    Your personal version of the Flutter website is now deployed to Firebase.
 
 Alternatively, you can skip the previous steps and just use the deploy script:

tool/refresh-code-excerpts.sh

@@ -4,6 +4,14 @@ set -e -o pipefail
 
 readonly rootDir="$(cd "$(dirname "$0")/.." && pwd)"
 
+readonly diffVersion="$(diff -v 2>&1 | head -n 1 | awk '{print $NF}')"
+
+if [[ ${diffVersion:0:1} != "3" ]]; then
+  echo "Unsupported diff version ${diffVersion}. Please install diffutils version 3.6 or higher."
+  echo "On macOS, you can run 'brew install diffutils'."
+  exit 1
+fi
+
 function usage() {
   echo $1; echo
   echo "Usage: $(basename $0) [--help] [-k|--keep-dart-tool] [--legacy] [path-to-src-file-or-folder]"