Add string substitution to template and custom parameters (#469)

* Add env subst for custom, template and channel in notify script

* Only expand when starts by $

* Remove channel and add debug

.circleci/test-deploy.yml

@@ -76,6 +76,16 @@ jobs:
           step_name: "Basic on hold template"
           template: basic_on_hold_1
           event: always
+      - run:
+          name: Generate and Export Slack Message
+          command: |
+            export SLACK_MESSAGE=$(node src/tests/generate-payload.js | jq -c .)
+            echo "export SLACK_MESSAGE='$SLACK_MESSAGE'" >> $BASH_ENV          
+      - slack/notify:
+          debug: true
+          step_name: "Custom template generated dynamicly"
+          event: pass
+          custom: $SLACK_MESSAGE
       - slack/notify:
           debug: true
           step_name: "Custom template with env var in the message"

src/scripts/notify.sh

@@ -1,5 +1,13 @@
 #!/usr/bin/env bash
 # shellcheck disable=SC2016,SC3043
+if [[ "$SLACK_PARAM_CUSTOM" == \$* ]]; then
+    echo "Doing substitution custom"
+    SLACK_PARAM_CUSTOM="$(eval echo "${SLACK_PARAM_CUSTOM}" | circleci env subst)"
+fi
+if [[ "$SLACK_PARAM_TEMPLATE" == \$* ]]; then
+    echo "Doing substitution template"
+    SLACK_PARAM_TEMPLATE="$(eval echo "${SLACK_PARAM_TEMPLATE}" | circleci env subst)"
+fi
 
 if [ "$SLACK_PARAM_DEBUG" -eq 1 ]; then
     set -x

src/tests/generate-payload.js

@@ -0,0 +1,12 @@
+const payload = {
+    blocks: [
+        {
+            type: "section",
+            text: {
+                type: "plain_text",
+                text: "This message was generated with Javascript",
+            },
+        },
+    ],
+};
+console.log(JSON.stringify(payload)); // Outputs JSON