Debugging with JShell

There is some undocumented /debug command (Jshell in Java 11). After /help /debug command you can see:

jshell> /help /debug
  Display debugging information for the jshell tool implementation.
  0: Debugging off
  r: Tool level debugging on
  g: General debugging on
  f: File manager debugging on
  c: Completion analysis debugging on
  d: Dependency debugging on
  e: Event debugging on

Notice, if you write /help only there is no information about /debug command.

Out of Jshell there is jdb tool for debug.

Just to clarify further, from the JEP: The Java Shell (Read-Eval-Print Loop) itself, the Non-Goals states :

Out of scope are graphical interfaces and debugger support. The JShell API is intended to allow JShell functionality in IDEs and other tools, but the jshell tool is not intended to be an IDE.