say_hello.rb

# Server is defined in stage file deploy/staging.rb
#     server 'staging.example.com', user: 'staginguser', roles: %i(app db web)

# Expect `say_hello` task to output:
#     echo 'hello from "staginguser"'
# What `say_hello` task actually outputs:
#     echo 'hello from nil'
task :say_hello do
  on roles(:app) do
    execute "echo 'hello from #{fetch(:user).inspect}'"
  end
end

# By inspecting the output of `Capistrano::Configuration.env`, we can see
# that the server user is in the servers collection under configuration, but 
# this variable is not exposed in the variables available through `fetch`.
namespace :dump do
  task :env do
    pp Capistrano::Configuration.env
  end
  task :servers do
    pp Capistrano::Configuration.env.servers
  end
end