Question

awk supports this:

awk '{print $(NF-1);}'

but not for user-defined variables:

awk '{a=123; b="a"; print $($b);}'

by the way, shell supports this:

a=123;
b="a";
eval echo \${$b};

How can I achieve my purpose in awk?

Answer 1

Not at the moment. However, if you provide a wrapper, it is (somewhat hacky and dirty) possible.
The idea is to use @ operator, introduced in the recent versions of gawk.

This @ operator is normally used to call a function by name.
So if you had

function foo(s){print "Called foo "s}
function bar(s){print "Called bar "s}
{
    var = "";
    if(today_i_feel_like_calling_foo){
        var = "foo";
    }else{
        var = "bar";
    }
    @var( "arg" ); # This calls function foo(), or function bar() with "arg"
}

Now, this is usefull on it’s own.
Assuming we know var names beforehand, we can write a wrapper to indirectly modify and obtain vars

function get(varname, this, call){call="get_"varname;return @call();}
function set(varname, arg, this, call){call="set_"varname; @call(arg);}

So now, for each var name you want to prrvide access by name, you declare these two functions

function get_my_var(){return my_var;}
function set_my_var(arg){my_var = arg;}

And prahaps, somewhere in your BEGIN{} block,

BEGIN{ my_var = ""; }

To declare it for global access.
Then you can use

get("my_var");
set("my_var", "whatever");

This may appear useless at first, however there are perfectly good use cases, such as
keeping a linked list of vars, by holding the var’s name in another var’s array, and such.
It works for arrays too, and to be honest, I use this for nesting and linking Arrays within
Arrays, so I can walk through multiple Arrays like using pointers.

You can also write configure scripts that refer to var names inside awk this way,
in effect having a interpreter-inside-a-interpreter type of things, too…

Not the best way to do things, however, it gets the job done, and I do not have to worry about
null pointer exceptions, or GC and such 🙂