This package leet you send messages in to a Slack Channel and can be activated as a service for when server with Ubuntu was turned off or on we received a notification on the slack channel
sudo ./build.sh
sudo dpkg -i slack-webhooks_0.8-amd64.deb
slack-webhook -r "my_room" -m "test me"
sudo bash /etc/slack-webhooks/install-asservice.sh
sudo systemctl restart slack-notify-start-stop.service
sudo dpkg --remove slack-webhooks
This example uses the emoji :ubuntu: to display the emoji as a user icon. The :ubuntu: emoji does not exist in grab the image from this folder custom emojis
If you are not familiar with how to do this here is a slack how-to article https://get.slack.help/hc/en-us/articles/206870177-Create-custom-emoji