List of commonly used built-in shell variables – not a comprehensive list
$# – The number of command-line arguments used in the last command
$- – Options currently in effect
$? – Exit value of the last command – 0 for success
$$ – Process ID of the shell
$! – Process ID of the last background command
$0 – The first word of the last command – i.e. the command itself
$n – Individual arguments on the command line by position starting with 0 for the command – bash allows n to be greater than 9 if specified as ${n}
$* – All arguments on the command line – $@ does the same thing
“$*”- All arguments on the command line – as a string
BASH – Full path name used to invoke bash