what is the meaning of \ | in a map vim command?

Question

Here is a map command.

nnoremap  :w\\|!R %

1.what is the meaning of \\ here?
2. does | mean pipe ?

Answer 1

The | character separates two Ex commands, see :help :|. It's like ; in programming languages like C and Java. It has nothing to do with pipes; Vim hasn't that concept (which is typically found in shells).

It is escaped here so that the entire command sequence belongs to the mapping; i.e. it maps to :w|!R %<CR>. Without escaping, Vim would execute the following instead:

:nnoremap <F5> :w
:!R %<CR>

Note that you can also write <Bar> (cp. :help key-notation) instead of \|, and the former is more frequently used.