I go with Posix shell scripts, or Korn shell when I need some feature not in Posix.

I like Python too, but have never used it enough to become more than a beginner with it. By the time my project has become complex or demanding enough for shell script to be unsuitable, I'm leaving the interpreted languages and going straight to C. Ideally, this would mean only using C to write any missing scriptable components or performance-critical pieces needed to complete the task in shell, but I don't always stop there and often end up writing the whole thing in C.