Flack is a script for the fish shell. It sends every command you type to a Slack channel, which is helpful for lecture situations when students may not be able to see exactly what you type. It also sets your prompt to flack>, to avoid accidentally broadcasting unwanted commands.
Create a Slack webhook, if you haven't already. Then, using fish and in the Flack directory, run
set flack_webhook_url "https://hooks.slack.com/services/..."
source flack.fish
That's it. To stop, exit your shell.