$BASH_VERSION .. Set when shell is bash. Contains version number.
$PS4 ........... Prefix for 'set -x' debug output lines.
$SHLVL ......... Shell Level. Incremented by 1 for every new shell.
$- $SHELLOPTS .. Active shell options (also see 'set -o').
$$ ............. PID of the shell process.
$SECONDS ....... Seconds since shell invocation.