Problem Statement

Demonstrate a language's ability for programs to execute code written in the language provided at runtime. Show what kind of program fragments are permitted (e.g. expressions vs. statements), and how to get values in and out (e.g. environments, arguments, return values), if applicable what lexical/static environment the program is evaluated in, and what facilities for restricting (e.g. sandboxes, resource limits) or customizing (e.g. debugging facilities) the execution. You may not invoke a separate evaluator program, or invoke a compiler and then its output, unless the interface of that program, and the syntax and means of executing it, are considered part of your language/library/platform. For a more constrained task giving a specific program fragment to evaluate, see Eval in environment.

Let's start with the solution:

$ a=42
$ b=a
$ eval "echo \$$b"
42