Most VHDL simulators use the Tool Command Language (Tcl) as their scripting language. When you type a command in the console of the simulator, you are using Tcl. Furthermore, you can create scripts with Tcl that run in the simulator and interact with your VHDL code.
In this article, we will create a self-checking testbench that used Tcl instead of VHDL to verify that a VHDL module behaves correctly.