bash - Why double underline (__) for private functions? Why (_) for bash completion functions?

Question

When you look at bash functions shipped with your Linux of choice, you often see that private (private in the sense of a recommendation) functions are written like this:

__private_func() {     : } 

So, you first wonder why not just one underscore (_) for private functions... then you see that bash completion functions usually start with a single underscore, usually calling private functions with a double underscore like in the example before:

_complete_func() {     __private_func } 

What I would like to know is: What is the reasoning about this? And is there some convention about how to prefix private functions?

Answer 1

I looked over the bash man page and the POSIX shell standard, but was not able to locate anything regarding this naming convention. That said, there is the use of the underscore for indicating reserved or internal names in C. To quote the libc manual on reserved names:

In addition to the names documented in this manual, reserved names include all external identifiers (global functions and variables) that begin with an underscore (‘_’) and all identifiers regardless of use that begin with either two underscores or an underscore followed by a capital letter are reserved names

The main logic for this naming convention is:

so that the library and header files can define functions, variables, and macros for internal purposes without risk of conflict with names in user programs

It also would have the benefit of being able to grep between "private" and "public" functions (which I put in quotes because a user can call either form regardless of naming) easily.