Slack — alert when an incoming webhook is broken

#!/usr/bin/env bash
# /etc/cron.d/slack-self-test
# */5 * * * * root /opt/slack-self-test.sh

WEBHOOK=${SLACK_WEBHOOK}
ALT=${HEARTBEAT_URL}                 # different channel/transport

CODE=$(curl -s -o /dev/null -w "%{http_code}" -X POST "$WEBHOOK" \
  -H 'Content-Type: application/json' \
  --data '{"text":"silent self-test","username":"healthcheck"}')

# 200 = ok. 4xx = revoked/misconfigured. 5xx / curl error = Slack down.
if [ "$CODE" != "200" ]; then
  curl -fsS "$ALT" --data "slack_code=$CODE,webhook_failing=true"
  exit 2
fi
echo "OK (Slack returned $CODE)"