(somewhat lame) manipulation of .rbenv-vars in capistrano 3

rbenv-vars.rake

namespace :rbenv do
  namespace :vars do
    desc "Display .rbenv-vars on all servers"
    task :show do
      on roles(:all) do |host|
        execute "cat #{fetch(:deploy_to)}/.rbenv-vars"
      end
    end

    desc "Add or update single .rbenv-vars setting (cap stage rbenv:vars:set[NAME,value])"
    task :set, :varname, :varvalue do |t, args|
      varname = args[:varname]
      varvalue = args[:varvalue] || ""

      on roles(:all) do |host|
        unless /[A-Za-z_][0-9A-Za-z_]*/ =~ varname
          error "rbenv:vars: invalid environment variable name #{varname}"
          exit 1
        end

        if test("[ -f #{fetch(:deploy_to)}/.rbenv-vars ]")
          execute "sed -ni '/^#{varname}=/ !p' #{fetch(:deploy_to)}/.rbenv-vars"
        end

        execute "echo '#{varname}=#{varvalue}' >> #{fetch(:deploy_to)}/.rbenv-vars"
      end
    end

    desc "Remove single .rbenv-vars setting (cap stage rbenv:vars:unset[NAME])"
    task :unset, :varname do |t, args|
      varname = args[:varname]

      on roles(:all) do |host|
        unless /[A-Za-z_][0-9A-Za-z_]*/ =~ varname
          error "rbenv:vars: invalid environment variable name #{varname}"
          exit 1
        end

        if test("[ -f #{fetch(:deploy_to)}/.rbenv-vars ]")
          execute "sed -ni '/^#{varname}=/ !p' #{fetch(:deploy_to)}/.rbenv-vars"
        end
      end
    end
  end
end