README: example that prints the number of the opened PR

Take commands out of the `action.yml` file, and put it in a dedicated
shell script.

.github/workflows/ci.yml

@@ -0,0 +1,17 @@
+name: CI
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  shellcheck:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+      - name: Install Nix
+        uses: cachix/install-nix-action@v16
+      - name: Shellcheck
+        run: nix-shell --run 'shellcheck $(find . -type f -name "*.sh" -executable)'

.github/workflows/update.yml

@@ -0,0 +1,19 @@
+name: update-flake-lock
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: '0 0 * * 0'
+
+jobs:
+  lockfile:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+      - name: Install Nix
+        uses: cachix/install-nix-action@v16
+        with:
+          extra_nix_config: |
+            access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+      - name: Update flake.lock
+        uses: ./.

README.md

@@ -65,6 +65,35 @@ jobs:
           inputs: input1 input2 input3
 ```
 
+## Example that prints the number of the created PR
+
+```yaml
+name: update-flake-lock
+on:
+  workflow_dispatch: # allows manual triggering
+  schedule:
+    - cron: '0 0 * * 0' # runs weekly on Sunday at 00:00
+
+jobs:
+  lockfile:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+      - name: Install Nix
+        uses: cachix/install-nix-action@v16
+        with:
+          extra_nix_config: |
+            access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+      - name: Update flake.lock
+        id: update
+        uses: DeterminateSystems/update-flake-lock@vX
+        with:
+          inputs: input1 input2 input3
+      - name: Print PR number
+        run: echo Pull request number is ${{ steps.update.outputs.pull-request-number }}.
+```
+
 ## Running GitHub Actions CI
 
 GitHub Actions will not run workflows when a branch is pushed by or a PR is opened by a GitHub Action. There are two ways to have GitHub Actions CI run on a PR submitted by this action.

action.yml

@@ -21,25 +21,22 @@ inputs:
     description: 'A comma or newline separated list of labels to set on the Pull Request to be created'
     required: false
     default: ''
+outputs:
+  pull-request-number:
+    description: 'The number of the opened pull request'
+    value: ${{ steps.create-pr.outputs.pull-request-number }}
 runs:
   using: "composite"
   steps:
-    - run: |
-        if [[ -n '${{ inputs.inputs }}' ]]; then
-          inputs=()
-          for input in ${{ inputs.inputs }}; do
-            inputs+=("--update-input" "$input")
-          done
-          nix flake lock "${inputs[@]}" --commit-lock-file --commit-lockfile-summary "${{ inputs.commit-msg }}"
-        else
-          nix flake update --commit-lock-file --commit-lockfile-summary "${{ inputs.commit-msg }}"
-        fi
+    - run: $GITHUB_ACTION_PATH/update-flake-lock.sh
       shell: bash
       env:
         GIT_AUTHOR_NAME: github-actions[bot]
         GIT_AUTHOR_EMAIL: <github-actions[bot]@users.noreply.github.com>
         GIT_COMMITTER_NAME: github-actions[bot]
         GIT_COMMITTER_EMAIL: <github-actions[bot]@users.noreply.github.com>
+        TARGETS: ${{ inputs.inputs }}
+        COMMIT_MSG: ${{ inputs.commit-msg }}
     - run: |
         content="$(git log --format=%b -n 1)"
         content="${content//'%'/'%25'}"
@@ -49,6 +46,7 @@ runs:
       shell: bash
       id: commit_message
     - name: Create PR
+      id: create-pr
       uses: peter-evans/create-pull-request@v3
       with:
         branch: update_flake_lock_action

update-flake-lock.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if [[ -n "$TARGETS" ]]; then
+    inputs=()
+    for input in $TARGETS; do
+        inputs+=("--update-input" "$input")
+    done
+    nix flake lock "${inputs[@]}" --commit-lock-file --commit-lockfile-summary "$COMMIT_MSG"
+else
+    nix flake update --commit-lock-file --commit-lockfile-summary "$COMMIT_MSG"
+fi