Execute curl command with variable interpolation

Question

I'm trying to adapt this known-working snippet for installing vim-plug:

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
  autocmd VimEnter * PlugInstall | source $MYVIMRC
endif

Seeing as I already have set runtimepath=$XDG_CONFIG_HOME/vim, I tried:

" BROKEN
let vimplug=$XDG_CONFIG_HOME."/vim/.vim/autoload/plug.vim" 
if empty(glob(vimplug))
    silent execute '!curl -fLo '.vimplug.' --create-dirs https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim'
    autocmd VimEnter * PlugInstall | source $MYVIMRC 
endif

Which errors on exit with:

> You don't have permission to create /vim

which suggests to me that it's evaluating $XDG_CONFIG_HOME to an empty string, but it is set, so this is where I'm stuck.

I added the execute because I understood that to be the correct way to evaluate variables before executing a command; what have I missed?

Answer 1

At least on neovim, something like this should work. It uses stdpath() in order to handle XDG env variables and filereadable() which is more readable that empty(glob()):

" Bootstrap Plug 
let autoload_plug_path = stdpath('data') . '/site/autoload/plug.vim'
if !filereadable(autoload_plug_path)
  silent execute '!curl -fLo ' . autoload_plug_path . ' --create-dirs
      \ "https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim"'
  autocmd VimEnter * PlugInstall --sync | source $MYVIMRC
endif
unlet autoload_plug_path