Special Variables

  [YSH Vars]      ARGV                ENV
                  __defaults__        __builtins__        X __sh_function__
                  _this_dir
  [YSH Status]    _error
                  _pipeline_status    _process_sub_status
  [YSH Tracing]   SHX_indent          SHX_punct             SHX_pid_str
  [YSH read]      _reply
  [Oils VM]       OILS_VERSION        LIB_YSH
                  OILS_GC_THRESHOLD   OILS_GC_ON_EXIT
                  OILS_GC_STATS       OILS_GC_STATS_FD
                  OILS_LOCALE_OK
  [libc locale]   ysh-locale
  [Interactive]   OILS_COMP_UI        YSH_HISTFILE
  [Float]         NAN                 INFINITY
  [Module]        __provide__ 
  [Other Env]     HOME                PATH
