Errrrmmmm, but Linux has its basis in Unix has a number of text processing utilities that can pipe their output into each other. Sed, awk, tr, wc, split, join. It also has the ability to pipe stdin, stdout and stderr into files.

A bit of bash scripting with these utilities can achieve much that the majority of ETL tools are used for.

It's probably a heretical thought but the ability to pipe together transforms in a script would probably be something that could be more easily unit tested than a GUI based ETL tool.