Question

I would like to open NERDTree at vim startup with a specific directory root depending on an environment variable.

Set environment variables will correctly be expanded, like $HOME. The documentation states undefined variables will expand to an empty string.

So this one works correctly with NERD_TREE_ROOT set to an existing directory. But will not if it is undefined. Instead $NERD_TREE_ROOT will be used like a string.

autocmd VimEnter * NERDTree $HOME/$NERD_TREE_ROOT

How can I use undefined environment variables correctly as empty string?

EDIT: To clarify a bit. This is what I wanted to avoid:

if empty($NERD_TREE_ROOT)
    autocmd VimEnter * NERDTree $HOME
else
    autocmd VimEnter * NERDTree $HOME/$NERD_TREE_ROOT
endif

If that is not possible it will do though.

Answer 1

What you observe has nothing to do with eval or expressions: echo eval('$HOME/$NERD_TREE_ROOT') results in -2147483648 just like echo 0/0 because both variables when performing a numeric operation turn out to be zeros. The expansion of $HOME is performed by vim due to presence of -complete=dir in :NERDTree command definition. This is rather unexpected and, by the way, is the third type of expansion: :echo expand('$HOME/$NERD_TREE_ROOT') results in $HOME/$NERD_TREE_ROOT while :echo expand('$HOME/$HOME') results in /home/zyx//home/zyx. I do not see any way to fix this, but you can always do

execute 'autocmd VimEnter * NERDTree '.fnameescape($HOME.'/'.$NERD_TREE_ROOT)

. It is the only case when expansion works as described in the doc because it is the only way when there are any expressions.