Add Capture An Output Value For Use In A Later Step as a github actions til

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).
 
-_1055 TILs and counting..._
+_1056 TILs and counting..._
 
 ---
 
@@ -25,6 +25,7 @@ _1055 TILs and counting..._
 * [Elixir](#elixir)
 * [Gatsby](#gatsby)
 * [Git](#git)
+* [GitHub Actions](#github-actions)
 * [Go](#go)
 * [HTML](#html)
 * [HTTP](#http)
@@ -298,6 +299,10 @@ _1055 TILs and counting..._
 - [What Is The Current Branch?](git/what-is-the-current-branch.md)
 - [Whitespace Warnings](git/whitespace-warnings.md)
 
+### GitHub Actions
+
+- [Capture An Output Value For Use In A Later Step](github-actions/capture-an-output-value-for-use-in-a-later-step.md)
+
 ### Go
 
 - [Access Go Docs Offline](go/access-go-docs-offline.md)

github-actions/capture-an-output-value-for-use-in-a-later-step.md

@@ -0,0 +1,48 @@
+# Capture An Output Value For Use In A Later Step
+
+GitHub Actions has a workflow command called `set-output`. This can be used to
+capture the output from a shell command in step. That output value can then be
+used in a later step.
+
+A useful example of this is reading the version of a tool from a dot-file to
+tell a later step what version of that tool to install.
+
+Here's the `.tool-versions` file included in my repository:
+
+```
+postgres 13.1
+ruby 3.0.0
+nodejs 15.4.0
+```
+
+Assuming I've already [checked out my
+repo](https://github.com/actions/checkout), I can find and read the `nodejs`
+version from my `.tool-versions` file with a step that uses `set-output`.
+
+```yaml
+  - name: Read Node.js version to install from `.tool-versions`
+    id: nodejs
+    run: >-
+      echo "::set-output name=NODE_VERSION::$(
+        cat .tool-versions |
+        grep nodejs |
+        sed 's/nodejs \(.*\)$/\1/'
+      )"
+```
+
+`echo` runs the command in the string which sets `NODE_VERSION` as an output
+value to what ends up being `15.4.0`.
+
+This output value can be referenced in a later step.
+
+```yaml
+  - name: Install required Node.js version
+    uses: actions/setup-node@v1
+    with:
+      node-version: "${{ steps.nodejs.outputs.NODE_VERSION }}"
+```
+
+`steps` has a reference to the `nodejs` step (note the `id` above) which then
+has `outputs` like the `NODE_VERSION`.
+
+[source](https://docs.github.com/en/actions/reference/workflow-commands-for-github-actions#using-workflow-commands-to-access-toolkit-functions)