The meaning of special dollar sign shell variables (blog.sudoask.com)

    $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.