What is the & mark meaning ahead a variable in vim?

Question

I read echo help file and find a command: :echo "the value of 'shell' is" &shell

I wonder what is '&shell' here.

I check the help file for "&", "variables", "shell" and havent find any useful information. So what is the "&" meaning here. And what should I do to figure it out by myself?

Answer 1

And what should I do to figure it out by myself?

Do :help &<C-d> and "figure out" which tag is more likely to help you (list is approximative, YMMV):

&            't_&8'       /\&      :&         :&&    :let-&    :set-&
:set-&vi     :set-&vim    cpo-&    expr-&&    g&     s/\&      t_&8

• & is normal mode &, irrelevant,
• 't_&8' has a t_ but &shell has not, irrelevant,
• /\& looks like it is related to search, irrelevant,
• :& and :&& look like they are Ex commands, irrelevant,
• :let-& looks like it has something to do with variables, relevant,
• all the :set- seem related, relevant,
• cpo-& is related to :help 'cpoptions', irrelevant,
• expr-&& has two &s, irrelevant,
• g& appears to be normal-mode-something, irrelevant,
• s/\& looks like it is related to substitutions, irrelevant,
• t_&8 is the same as the other but without quotes, weird and irrelevant.

So that leaves you with :help :let-& and a couple of others:

:help :let-&
:let &{option-name} = {expr1}           *:let-option* *:let-&*
        Set option {option-name} to the result of the
        expression {expr1}.

Hooo…

But you are not supposed to open random help sections and "figure it out by yourself". What you are supposed to do is follow the user manual (:help user-manual), which teaches you things in the proper order. &optionname is explained in the chapter about scripting Vim: :help 41.3.