Can't move `viminfo` file - &viminfo reverts upon loading vim

Question

I'm trying to change the location of the viminfo file.

Why does the value of &viminfo revert after it is set in vimrc?

TL;DR:

In my vimrc I have:

set nocompatible
exec "set viminfo+=n".expand($XDG_DATA_HOME)."/vim/viminfo"
echom &viminfo

I've noticed that echo &viminfo from command mode shows the previous version of &viminfo even though the echom inside vimrc shows that &viminfo was correctly (but temporarily) updated.

I'd suspect autocommand, but ag -ul viminfo only shows my vimrc and the viminfo file itself.

Why does the value of &viminfo change after I explicitly set it?

Full description

@ravi@boxy:~$ vim --version | head -1
VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Aug 05 2016 16:48:20)

I start vim with VIMINIT set to honour the XDG Base Directory Specification

@ravi@boxy:~$ echo $VIMINIT
let $MYVIMRC="$XDG_CONFIG_HOME/vim/vimrc"|set nocp|exec "set viminfo+=n".expand($XDG_DATA_HOME)."/vim/viminfo"|echom &viminfo|source $MYVIMRC
@ravi@boxy:~$ 

For belts and braces, I also have in my vimrc:

set nocompatible
" Location of the viminfo file
exec "set viminfo+=n".expand($XDG_DATA_HOME)."/vim/viminfo"
echom &viminfo

Now I remove the existing ~/.viminfo, start vim, then exit:

@ravi@boxy:~$ rm ~/.viminfo 
@ravi@boxy:~$ vim
'100,<50,s10,h,n/home/ravi/.local/share/vim/viminfo
'100,<50,s10,h,n/home/ravi/.local/share/vim/viminfo
Press ENTER or type command to continue
@ravi@boxy:~$ file ~/.viminfo 
/home/ravi/.viminfo: ASCII text
@ravi@boxy:~$

The &viminfo variable gets printed twice because of the belt and braces, but as you can see, the file is still created at the default location.

Answer 1

TL;DR

This seems to be a bug in vim where set nocompatible is not idempotent and doesn't follow the principle of least astonishment.

As a workaround, either:

1. Ensure that you set nocompatible (or the equivalent set nocp) only once, and at the top of your vimrc.

2. Don't set it if it's already set:

   if &compatible | set nocompatible | endif " Avoid side effects if `nocp` already set
   

Explanation and bug illustration

From :help compatible (empahsis mine):

This is a special kind of option, because when it's set or reset, other options are also changed as a side effect. CAREFUL: Setting or resetting this option can have a lot of unexpected effects: Mappings are interpreted in another way, undo behaves differently, etc. If you set this option in your vimrc file, you should probably put it at the very start.

Note that &viminfo is not listed in the side-effects, however the following lines clearly show the side effect upon &viminfo:

set nocompatible
set viminfo+=nWatch-my-viminfo-file-location-be-ignored
echom &viminfo
set nocompatible " do side effects even though nocomptible is already set
echom 'After 2nd "set nocompatible":'
echom &viminfo

Output:

'100,<50,s10,h,nWatch-my-viminfo-file-location-be-ignored
After 2nd "set nocompatible":
'100,<50,s10,h

vim --version | head -1

VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Aug 05 2016 16:48:20)

---

Resolution

I have raised two GitHub issues regarding this:

• Undocumented "set nocompatible" side effect upon &viminfo
• set nocompatible not idempotent - setting produces side effects when already set

See also StackExchange's question: How to tell VIM to store the viminfo file somewhere else?

Answer 2

I know its quite a while now, but I am just mentioning this for future readers.

I was facing a similar issue while trying to change the location for viminfo file in vimrc. At last setting the value of viminfofile option worked for me.

Added the following to my vimrc:

set viminfofile=D:\vim\viminfo

It works for me on windows with vim 8.2

Answer 3

The documentation says to use the 'n' option but what works for me and is not in the documentation is:

set viminfofile=~/.vim/viminfo

(I got this idea from tanish's window answer above. Turns out it works on linux too! I'm using vim 9.0.)