Tcl Tool Command Language
A scripting language by John Ousterhout, Berkeley 1988. Built on a single idea: everything is a string, and every line is a command followed by arguments. Pronounced "tickle".
The syntax is minimal in a way that resembles Lisp — even if and while are ordinary commands taking code blocks as string arguments, which makes the language trivial to extend. It was originally meant as an embeddable configuration language for other programs, and that's where it truly stuck: Cisco IOS, the EDA tools from Cadence and Synopsys, test suites and a lot of lab equipment. The Tk toolkit (1991) was the first practical way to build a graphical interface across Unix, Windows and Mac from one codebase, and survives as Python's tkinter. The expect tool for automating interactive programs is Tcl too.