Declare variables in shell script

Shell Variables
A variable in a shell script is a means of referencing a numeric or character value. And unlike formal programming languages, a shell script doesn't require you to declare a type for your 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

              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.