Direct Link: [stackoverflow](https://stackoverflow.com/questions/5163144/what-are-the-special-dollar-sign-shell-variables)
$1, $2, $3, ... are the positional parameters.
"$@" is an array-like construct of all positional parameters, {$1, $2, $3 ...}.
"$*" is the IFS expansion of all positional parameters, $1 $2 $3 ....
$# is the number of positional parameters.
$- 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 is the (input) field separator.
$? is the most recent foreground pipeline exit status.
$! is the PID of the most recent background command.
$0 is the name of the shell or shell script.
Most of the above can be found under Special Parameters in the Bash Reference Manual.
There are all the environment variables set by the shell.