I have a hash in format 'cert1' => {}, 'cert2 => {},.. I would like to create a array with hash keys in format [ '/etc/cert/cert1', '/etc/cert/cert2',...]. What function to use to get there?

you can do in one line with

$_watch_files = $certificates.keys().map |$x| { ... }

also, when chaining function calls like that and there are no arguments to pass in a call, the parentheses can be skipped - thus making it:

$_watch_files = $certificates.keys.map |$x| { ... }

And you don't even necessarily need the

keys()

function. Here it is with 1 function call:

$certificates = { 'cert1' => {}, 'cert2' => {} }
$_watch_files = $certificates.map |$k, $_v| { "/etc/letsencrypt/${k}/cert.pem" }

Though the clarity of

keys()

is nice.