Special shell variables : Unix/Linux

Special shell variables  There are some variables which are set internally by the shell and which are available to the user:

Name		Description
$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 impletes 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.

Tips ::
$* and $@ when unquoted are identical and expand into the arguments.
"$*" is a single word, comprising all the arguments to the shell, joined together with spaces. For example '1 2' 3 becomes "1 2 3".
"$@" is identical to the arguments received by the shell, the resulting list of words completely match what was given to the shell. For example '1 2' 3 becomes "1 2" "3"..