If required variables aren't set in cap config, it errors

[mikecmpbll]

It should not cause an exception if the messenger option isn't set, it should just not run the hooks. Messengers may exception though if variables are missing.

[zx1986]

Sorry, just another question, I source the code, and if my guess is right,
here is the way to specify channels?

if ENV['MATTERMOST_URI']
  require 'net/http'
  require 'net/https'
  require 'capistrano/deploy_hooks'
  require 'capistrano/deploy_hooks/messengers/mattermost'

  set :deploy_hooks, {
    channels: '["town-square", "off-topic", "another-channel"]', # channels in string with array format.
    messenger: Capistrano::DeployHooks::Messengers::Mattermost,
    webhook_uri: ENV['MATTERMOST_URI'],
  }
end