Check if VimPlug is installed in .vimrc

Question

I'm trying to have a .vimrc that is portable between machines. Some computers I use have VimPlugin installed, while others don't. On the ones with VimPlug, I'd like to do some setup using call plug#begin and call plug#end. Currently though, I get an error on the computers without VimPlug, and on those, I'd just like to move on to the rest of the .vimrc.

Optimally, I'd want something like if has('Vim-Plug') and just put the relevant code inside of that. It looks like some embed the installation of the manager right within their rc in an if statement, similarly to what I'd like to do:

if empty(glob('~/.vim/autoload/plug.vim'))
  silent !curl -fLo ~/.vim/autoload/plug.vim --create-dirs
    \ https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim
endif

But this doesn't exactly work since vim still interprets the contents of the if statement, even if it doesn't execute it. I get an error: E117: Unknown function: plug#begin even though that code is inside the if statement.

While researching this, I found tons of posts about checking if a particular plugin is installed, but none to check if the manager was installed. Is this not something I should be doing? Does anyone know a way to accomplish this? I'd rather not have to install Vim-Plug on every computer I use the rc on in order to keep it portable.

Answer 1

If you know a file is precisely installed in a specific place, you can use filereadable().

if filereadable($HOME.'/.vim/autoload/plug.vim')
   call plug#Begin(.....

However, plugins should not be tested to an exact path but to any path in the 'rtp'. This way if some day you migrate to nvim, or to Windows, it'll still work. The right function to do so is globpath().

if ! empty(globpath(&rtp, 'autoload/plug.vim'))
   call plug#Begin(.....

Answer 2

You are almost there! The problem here is, that you need to re-source your config because the change in ~/.vim/autoload/plug.vim won't be recognised by vim since it's loading the files in the runtime only once during the startup. So all you need to do is adding source ~/.config/vim/init.vim (if that's the path to your vim config file) to your if-branch:

if empty(glob('~/.vim/autoload/plug.vim')) 
    silent !curl -fLo ~/.vim/autoload/plug.vim --create-dirs \
        https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim source
    ~/.config/vim/init.vim " this needs to be added
endif

Edit

If you just want to check if your plugin manager exists, to load your config, then I'd do it as follows:

~/.config/vim/plugins.vim

call plug#begin("<path>")
Plug "bla bla"
" ...
call plag#end()

and then I'd add the following to your ~/.config/vim/init.vim a check, which sources this file, if you have the plugin manager installed:

if !empty(glob('~/.vim/autoload/plug.vim'))
    source ~/.config/vim/plugins.vim " start sourcing your plugins if the package-manager exists
endif