What's those underscore commands?

Question

When I tab tab _ in terminal, Bash suggests 206 posibilities. I tried to run one of them _git_rm but nothing happend, what are they?

Here is a screenshot:

Answer 1

These functions whose name begins with an underscore are part of the programmable completion engine. Bash follows zsh's convention here, where the function that generates completions for somecommand is called _somecommand, and if that function requires auxiliary functions, they are called _somecommand_stuff.

These completion functions typically do nothing useful or raise an error if you call them manually: they're intended to be called from the completion engine.

This follows on a fairly widespread practice in various programming languages to use a leading underscore to indicate that a function or variable is in some way internal to a library and not intended for the end-user (or end-programmer).