I want to send a string to a Slack channel, and have this test code:

@Library('my-shared-library') _
import groovy.json.JsonOutput

final String diffs = """
diff --git a/company-core/src/main/java/com/company/common/metadata/SchemaVersionV2.java b/company-core/src/main/java/com/company/common/metadata/SchemaVersionV2.java
index e3c9c4c2741..211b08629ca 100644
--- a/company-core/src/main/java/com/company/common/metadata/SchemaVersionV2.java
+++ b/company-core/src/main/java/com/company/common/metadata/SchemaVersionV2.java
@@ -22,6 +22,12 @@ public enum SchemaVersionV2 implements CompanySchema {
     JIRA_2295(2295,0,0, new String[] {"alter_JIRA_2295_1.sql"}, "create ai_expert_kb_sync table"),
     JIRA_3693(3693, 1, 0, new String[]{"alter_JIRA_3693_1.sql"}, "Adding user-level tracking of a subscriptions' type (following vs connected)"),
     JIRA_3689(3689,0,0, new String[] {"alter_JIRA_3689_1.sql"}, "create supplier message tables"),
+    JIRA_4773(4773,1,0, new String[]{"alter_JIRA_4773_1.sql"}, "Add index on partner_social_feed_post_approval table"),
+    JIRA_2339(DB_LOCATION.ZIFTDM,2339,0,0, new String[] {"alter_JIRA2339_1.sql"}, "Add lead source id"),
+    JIRA_3702(3702, 1, 0, new String[]{"alter_JIRA_3702_1.sql"}, "Adding z123_client ids to subscriptions"),
+    JIRA_4397(4397,1,0, new String[]{"alter_JIRA_4397.sql"}, "Add name, description, effective dates and thumbnail url to supplier_user_activity"),
+    JIRA_2317(2317,1,0, new String[]{"alter_JIRA_2317_1.sql"}, "Add column neverExpire to client table"),
+    JIRA_4374_1(4374, 1, 0, new String[]{"alter_JIRA4374_1.sql"}, "Adding partner filter to supplier event activity table"),
     ;
 
     public static boolean isSuitableForSchemaUpdate(int major, int release) {
"""

def notifySlack(String text, String channel) {
    def slackURL = ''
    def payload = JsonOutput.toJson([text      : text,
                                     channel   : channel,
                                     username  : "jenkins",
                                     icon_emoji: ":jenkins:"])
    try {
        sh(script: "curl -X POST --data-urlencode 'payload=${payload}' ${slackURL}", returnStatus: true)
    }
    catch(Exception ex) {
        println "Slack app may be down, error(if any): "+ex.toString()
    }
}

node('build-node') {
    stage("Notify Slack") {
        final String testMsg = "<channel!> Schema changed:\n```${diffs}```"
        notifySlack(testMsg, "#slack-channel")
    }
}

When I run it, I get

/home/jenkins/workspace/Utilities/Playground/test-slack@tmp/durable-96cd2c08/script.sh.copy: 1: Syntax error: "(" unexpected

In my real code, the diffs string is dynamically generated. Why am I getting the error and what's the workaround?

If I change the diffs string to a simple Hello world! it works.

I printed out the payload in the notifySlack() function and I get

{"text":"<!channel> Schema changed:\n```diff --git a/company-core/src/main/java/com/company/common/metadata/SchemaVersionV2.java b/company-core/src/main/java/com/company/common/metadata/SchemaVersionV2.java\nindex e3c9c4c2741..211b08629ca 100644\n--- a/company-core/src/main/java/com/company/common/metadata/SchemaVersionV2.java\n+++ b/company-core/src/main/java/com/company/common/metadata/SchemaVersionV2.java\n@@ -22,6 +22,12 @@ public enum SchemaVersionV2 implements CompanySchema {\n     JIRA_2295(2295,0,0, new String[] {\"alter_JIRA_2295_1.sql\"}, \"create ai_expert_kb_sync table\"),\n     JIRA_3693(3693, 1, 0, new String[]{\"alter_JIRA_3693_1.sql\"}, \"Adding user-level tracking of a subscriptions' type (following vs connected)\"),\n     JIRA_3689(3689,0,0, new String[] {\"alter_JIRA_3689_1.sql\"}, \"create supplier message tables\"),\n+    JIRA_4773(4773,1,0, new String[]{\"alter_JIRA_4773_1.sql\"}, \"Add index on partner_social_feed_post_approval table\"),\n+    JIRA_2339(DB_LOCATION.ZIFTDM,2339,0,0, new String[] {\"alter_JIRA2339_1.sql\"}, \"Add lead source id\"),\n+    JIRA_3702(3702, 1, 0, new String[]{\"alter_JIRA_3702_1.sql\"}, \"Adding z123_client ids to subscriptions\"),\n+    JIRA_4397(4397,1,0, new String[]{\"alter_JIRA_4397.sql\"}, \"Add name, description, effective dates and thumbnail url to supplier_user_activity\"),\n+    JIRA_2317(2317,1,0, new String[]{\"alter_JIRA_2317_1.sql\"}, \"Add column neverExpire to client table\"),\n+    JIRA_4374_1(4374, 1, 0, new String[]{\"alter_JIRA4374_1.sql\"}, \"Adding partner filter to supplier event activity table\"),\n     ;\n\n     public static boolean isSuitableForSchemaUpdate(int major, int release) {```","channel":"#ecs-feature","username":"jenkins","icon_emoji":":jenkins:"}