Special shell variables

$1 $9 ----- these variables are the positional parameters.

$0 ----- the name of the command currently being executed.

$# ----- the number of positional arguments given to this
invocation of the shell.

$? ----- the exit status of the last command executed is
given as a decimal string. When a command
completes successfully, it returns the exit status
of 0 (zero), otherwise it returns a non-zero exit
status.

$$ ----- the process number of this shell - useful for
including in filenames, to make them unique.

$! ----- the process id of the last command run in
the background.

$- ----- the current options supplied to this invocation
of the shell.

$* ----- a string containing all the arguments to the
shell, starting at $1.

$@ ----- same as above, except when quoted.