Save the comment in an output variable.

  - Save the comment, as a JSON document, in an output variable.

README.md

@@ -108,7 +108,10 @@ the repository.
 ## Disabling Comments
 
 If you prefer to disable comments, you can set the `comment` input
-variable to `false` like so:
+variable to `false`.  The `comment` is also written to the `comment`
+output variable so it is possible to use it in a successive step.  The
+format is a JSON document with a single key, `body`.  Here's an
+example:
 
 ```yaml
 name: pull-request
@@ -128,7 +131,22 @@ jobs:
 
     steps:
       - name: Checking if fast forwarding is possible
+        id: fast-forward
         uses: sequoia-pgp/fast-forward@main
         with:
           comment: false
+      - name: Display comment
+        env:
+          COMMENT: ${{ steps.fast-forward.outputs.comment }}
+        run: echo "The comment is... $COMMENT"
 ```
+
+This would display something like:
+
+```text
+The comment is... {
+  "body": "..."
+}
+```
+
+Additional fields may be added to the JSON document in the future.

action.yml

@@ -35,10 +35,21 @@ inputs:
       may also execute some additional RPCs, and so should normally
       be disabled.
     default: 0
+outputs:
+  comment:
+    description: >
+      The comment.
+
+      A comment explaining whether fast forwarding is possible, and,
+      if `merge` is true, whether it was successful.
+
+      The comment is a JSON document with a single field, `body`.
+    value: ${{ steps.fast-forward.outputs.comment }}
 runs:
   using: "composite"
   steps:
-    - run: |
+    - id: fast-forward
+      run: |
         export GITHUB_TOKEN=${{ inputs.github_token }}
         export DEBUG=${{ inputs.debug }}
         export COMMENT=${{ inputs.comment }}

src/fast-forward.sh

@@ -305,6 +305,15 @@ LOG=$(mktemp)
 COMMENT_CONTENT=$(mktemp)
 jq -n --rawfile log "$LOG" '{ "body": $log }' >"$COMMENT_CONTENT"
 
+# Set the comment output variable.
+{
+    # https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#multiline-strings
+    echo "comment<<EOF_$COMMENT_CONTENT"
+    cat "$COMMENT_CONTENT"
+    echo "EOF_$COMMENT_CONTENT"
+} | tee -a "$GITHUB_OUTPUT"
+
+# Post the comment.
 if test $COMMENT -gt 0
 then
     COMMENTS_URL="$(github_pull_request .comments_url)"