Declare variables in shell script
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:
$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
$$ 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 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.