How do I run a rake task from Capistrano?

A little bit more explicit, in your \config\deploy.rb, add outside any task or namespace:

namespace :rake do  
  desc "Run a task on a remote server."  
  # run like: cap staging rake:invoke task=a_certain_task  
  task :invoke do  
    run("cd #{deploy_to}/current; /usr/bin/env rake #{ENV['task']} RAILS_ENV=#{rails_env}")  
  end  
end

Then, from /rails_root/, you can run:

cap staging rake:invoke task=rebuild_table_abc

run("cd #{deploy_to}/current && /usr/bin/env rake `<task_name>` RAILS_ENV=production")

Found it with Google -- http://ananelson.com/said/on/2007/12/30/remote-rake-tasks-with-capistrano/

The RAILS_ENV=production was a gotcha -- I didn't think of it at first and couldn't figure out why the task wasn't doing anything.

Previous answers didn't help me and i found this: From http://kenglish.co/run-rake-tasks-on-the-server-with-capistrano-3-and-rbenv/

namespace :deploy do
  # ....
  # @example
  #   bundle exec cap uat deploy:invoke task=users:update_defaults
  desc 'Invoke rake task on the server'
  task :invoke do
    fail 'no task provided' unless ENV['task']

    on roles(:app) do
      within release_path do
        with rails_env: fetch(:rails_env) do
          execute :rake, ENV['task']
        end
      end
    end
  end

end

to run your task use

bundle exec cap uat deploy:invoke task=users:update_defaults

Maybe it will be useful for someone

You can use this:

namespace :rails_staging_task do
  desc "Create custom role"
  task :create_custom_role do
    on roles(:app), in: :sequence, wait: 5 do
      within "#{deploy_to}/current" do
        with rails_env: :staging do
          rake "create_role:my_custom_role"
        end
      end
    end
  end

  # other task here
end

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