Shell variables and positional parameters.

The Bourne-Again Shell has some reserved variables, named:

    1, 2, 3, 4, 5, 6, 7, 8 and 9

These are used for positional parameters (arguments) in shell scripts.
That means: if you invoke a script like this

./script a b c

then the variables 'a' 'b' and 'c' are available to the script, and
the script assigns their values to positional parameters 1, 2 and 3.
That is, inside the script, whenever the script uses $2, it uses
the value of the second argument 'b'.

That is to say, outside the script the values of a,b,c are $a, $b, $c,
whereas inside the script the values are $1, $2, $3.


You can quite happily create a directory that has a numerical name, 
like this:

    mkdir 2

but numerical names won't work for plain variables; 
you can't use them yourself, because it would confuse the shell 
(poor shell!). 

So trying to give a variable named 'a' the value 'zap' 
like this:

    a=zap

will work, but trying to give a variable named 2 the value zap 
like this:

    2=zap

won't work (try it). You get the response:

    bash: 2=zap: command not found

The value of a reserved shell variable may be accessed in the same
way as you access the value of any other variable, that is,
by prefixing it with the $ symbol, like this: 

    echo $2



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