Use fly with a temporary flyrc - useful for situations where you absolutely positively want your session to be temporary

temp-fly.rb

#!/usr/bin/env ruby

# -*- mode: ruby -*-
# # vi: set ft=ruby :

require 'tmpdir'

class Input
  attr_reader :get_method

  def initialize(prompt)
    require 'readline'
    @get_method = ->() { Readline.readline(prompt, true) }
  rescue LoadError
    @get_method = -> () { print(prompt); gets }
  end

  def get
    get_method.call.squeeze(" ").strip
  end
end

class Repl
  def initialize(prompt, env_vars, executable)
    @env_vars = env_vars
    @executable = executable
    @input = Input.new(prompt)
  end

  def run
    while buf = @input.get
      system(@env_vars, @executable, buf)
    end
  end
end

WELCOME_MESSAGE = <<WELCOME
  Welcome to temp-fly. This script starts fly with a temporary configuration, you may login like so:
  `login -t target --concourse-url https://concourse.example.com --team-name team-name`
WELCOME

Dir.mktmpdir do |home|
  puts WELCOME_MESSAGE
  Repl.new("temp-fly> ", { "HOME" => home }, 'fly').run
end