Vim: Difference between t_Co=256 and term=xterm-256color in conjunction with TMUX

terminal, tmux, vim, xterm

Solution

When you don't use `tmux` or `screen`, you only need to configure your terminal emulators to advertise themselves as "capable of displaying 256 colors" by setting their `TERM` to `xterm-256color` or any comparable value that works with your terminals and platforms. How you do it will depend on the terminal emulator and is outside of the scope of your question and this answer.

You don't need to do anything in Vim as it's perfectly capable to do the right thing by itself.

When you use `tmux` or `screen`, those programs set their own default value for `$TERM`, usually `screen`, and Vim does what it has to do with the info it is given.

If you want a more uniform (and colorful) behavior, you must configure them to use a "better" value for `$TERM`:

tmux

Add this line to `~/.tmux.conf`:

set -g default-terminal "screen-256color"

screen

Add this line to `~/.screenrc`:

term "screen-256color"

Now, both multiplexers will tell Vim they support 256 colors and Vim will do what you expect it to do.

edit

My answer assumes that you are able to edit those configuration files but, since you are able to edit your `~/.vimrc`, I don't think that I'm that far off the mark.

edit 2

The value of the `term` option (retrieved with `&term`) is the name of the terminal as picked up by Vim upon startup. That name is what you are supposed to setup in your terminal emulator itself.

The value of the `t_Co` option (`&t_Co`) is what Vim considers to be the maximum number of colors that can be displayed by the host terminal. It is defined according to the entry corresponding to `$TERM` in `terminfo`:

 term            | t_Co
-----------------+------ 
 xterm           | 8
 xterm-256color  | 256
 screen          | 8
 screen-256color | 256

When Vim starts up, it gets the value of the `TERM` environment variable, queries the `terminfo` database with that value and stores a number of informations on its environment in several `t_…` variables among which… the number of colors available in `t_Co`. Given a "legal" terminal type (one that Vim can lookup), Vim always assumes the correct number of colors.

Setting `t_Co` to `256` while leaving `term` to its Vim-defined value — or, more generally, setting `t_Co` and/or `term` to values that don't match with the host terminal — makes no sense and will likely create troubles when Vim sends a signal that is not understood by the terminal or vice-versa.

While it is entirely possible to do so, messing with `t_Co` and `term` in Vim is both totally useless and possibly harmful.

Again, just setup your terminal emulators and terminal multiplexers correctly. That's really all you need.

If you end up in a terminal multiplexer or a terminal emulator where you can't define a correct `TERM`, then and only then you can force Vim to assume 256 colors. To that end, changing the value of `t_Co` is the only thing that makes sense:

if &term == "screen"
  set t_Co=256
endif

So… if you can configure each individual part:

- terminal emulator: `xterm-256color`

- tmux/screen: `screen-256color`

- vim: nothing

and you are done.

If you can't control every part, use a simple conditional in your `~/.vimrc` to set `t_Co` according to `&term` but don't change the value of `term`.

But if you can edit a `~/.vimrc` there's no reason you can't edit a `~/.screenrc` or `~/.tmux.conf` or `~/.bashrc` or whatever.

Problem

Original source