How do I yank to system clipboard in Kakoune?

In case all the links go away this is the complete solution:

<a-|> xsel --input --clipboard <ret>

Explanation:

• <a-|> pipes the current selection to what follows.
• xsel is a program for manipulating X's clipboard.
• --input tells xsel to read from stdin.
• --clipboard tells xsel which 'selection' store to use. There are three of them and you want 'clipboard' in order to paste elsewhere.

This will work on Linux. Not sure about MacOS, I'd imagine piping to pbcopy would Just Work.

EDIT: To make live easier define a key binding in User mode like so (in your .kakrc):

map global user y '<a-|>xsel -i -b<ret>'

Now you can select the text and press ,y to yank to the system clipboard. The leading comma tells Kakoune to look for the key binding in User mode.

Just so you are aware, there is also a great plugin to help with this issue. Kakboard works everywhere, including Mac, Xorg and even Wayland environments. You can also set your own custom command in more exotic environments by setting the kakboard_(copy|paste)_cmd options.

To add to the accepted answer, according to the documentation if your version of Kakoune is post July 2020 then you can add the following hook to your kakrc:

hook global RegisterModified '"' %{ nop %sh{
  printf %s "$kak_main_reg_dquote" | xsel --input --clipboard
}}