$0 :name of shell or shell script.
$1, $2, $3, ... :positional parameters.
$# :number of positional parameters.
$? :most recent foreground pipeline exit status.
$- :current options set for the shell.
$$ :pid of the current shell (not subshell).
$! :is the PID of the most recent background command.
$DESKTOP_SESSION current display manager
$EDITOR preferred text editor.
$LANG current language.
$PATH list of directories to search for executable files (i.e. ready-to-run programs)
$PWD current directory
$SHELL current shell
$USER current username
$HOSTNAME current hostname
Variable
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