Hello, Redis test suite is written in Tcl because for this kind of stuff Tcl is very good. A few reasons:

1. It is very easy to write DSLs in Tcl, you can turn it into a radically different on-purpose language. If you think Ruby is good at DSLs, try Tcl.

2. It has good I/O functions and is event-driven by default. The Redis test for example uses a server-client model using the event driven support of Tcl.

3. It has excellent ways to interact with other Unix commands, see for example the [exec] command: http://www.tcl.tk/man/tcl8.5/TclCmd/exec.htm

4. It has automatic memory management but does not use a garbage collector, reference counting is used instead. This means that the memory footprint is small and there are no GC pauses. Moreover the interpreter is extremely stable.

There are more reasons for using Tcl in certain projects, the above are the reasons I'm using it for Redis testing.