    Positional parameters $1,$2,$3… and their corresponding array representation, count and IFS expansion $@, $#, and $*.
    $- current options set for the shell.
    $$ pid of the current shell (not subshell)
    $_ most recent parameter (or the abs path of the command to start the current shell immediately after startup)
    $IFS the (input) field separator
    $? most recent foreground pipeline exit status
    $! PID of the most recent background command
    $0 name of the shell or shell script

