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I've noticed that after I use :vs <file> or :sp <file> I have no buffer variables defined for this buffer. So I get E121: Undefined variable: b:Var after :echo b:Var there. But in the first opened buffer I have no errors. What am I doing wrong?

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    I realize that I may have answered a bit too fast: when you do :vs <file> do you mean that you do that on a file you had not open before (in this case my answer is still valid) or do you mean that you opened a buffer with this file in it before, set a buffer variable for this buffer and then use the command with a file which was already in a buffer (in this case I can not reproduce your problem and my answer is not interesting for you)? – statox Dec 12 '17 at 14:03
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The important thing to understand is that buffer variables are not inherited from the buffer from where you load/create the new buffer.

Buffer variables can be created manually, but the typical scenario is that a buffer related autocommand will create the variable. In other words, most of the time it's done automatically in ftplugins or in project management plugins.

Corollary: never define buffer local variable in plugin/ files, or in your .vimrc -- unless it's done from a function triggered from a buffer related autocommand. This is the same with buffer local mappings, commands, abbreviations...

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The doc (:h b:var) says:

A variable name that is preceded with "b:" is local to the current buffer. Thus you can have several "b:foo" variables, one for each buffer.

When you create a variable prefixed with b: it is local to a buffer so if you create a new buffer the variable will not exist.

If you need a variable accessible from every buffer you can use g: or s: if you are in a script. See :h internal-variables for a description of the different possible scopes:

There are several name spaces for variables.  Which one is to be used is
specified by what is prepended:

                (nothing) In a function: local to a function; otherwise: global
buffer-variable    b:     Local to the current buffer.
window-variable    w:     Local to the current window.
tabpage-variable   t:     Local to the current tab page.
global-variable    g:     Global.
local-variable     l:     Local to a function.
script-variable    s:     Local to a |:source|'ed Vim script.
function-argument  a:     Function argument (only inside a function).
vim-variable       v:     Global, predefined by Vim.

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