The Unix CLI is most definitely a REPL. But it is a system-level REPL. So instead of running functions you run programs. You can't interactively develop a program in the UNIX CLI. Especially not in Python, JS, Ruby, etc. You can try to establish a REPL like flow in the CLI, by using a watcher like entr and doing something like $ cat test-input.txt | python ./my-script.py but the granularity of this approach is too coarse for the programming _language_. It is the ideal granularity for orchestrating shell scripts and CLI programs.