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Colours on Vim and tmux

When using Vim (7.3) through tmux (1.8) I found that the Vim colour scheme looked strange. After much googling this was the solution that worked for me.

Set the following in your .vimrc1:

set term=screen-256color

Set the following in your .tmux.conf2:

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

This was very much trial and error3 but seems to work across both OS X and Debian boxes.

EDIT (5/12/13): Since I was struggling to understand colours in the terminal, I emailed the knowledgable @geraintrjones who sent me an excellent explanation (with some brilliant Serengeti animal analogies):

Terminal emulators tell the world about their capabilities with something called a $TERM variable.

The $TERM variable is a label that says “I am this kind of thing” - for instance, OS X’s Terminal.app has its default $TERM set to xterm256-color. You can see this by typing echo $TERM in the terminal. (you might see something else, and this is where the problem starts…)

This doesn’t tell you much on its own, but there is a big database called terminfo that lists all the $TERM variables, and what features something with, say, xterm256-color, supports.

Its like a label saying ‘Elephant’. When you look up ‘Elephant’ in the animal list, you see it has 4 legs, a trunk, and can support palanquins but requires a mahout.

You can see the terminfo entry for your terminal by typing infocmp $TERM (which will look up that terminal’s $TERM in your system’s terminfo database ).

You will see the number of colours listed like this: colors#[number]. xterm256-color for instance, will show colors#256, xterm-color will show colors#8.

This means that OS X’s Terminal.app is telling the world it can handle that number of colours. Applications like Vim see a label saying “I have all the xterm-color features!” and think, “Okay, I’d better give this old timer 8 colours”. Or they see xterm256-color and think, “Lets give this dude 256 colors”.

What should happen, is that tmux reads the $TERM value of your terminal, sees xterm256-color (or similar) , thinks “Okay, that’s a 256 colour terminal” and sets its $TERM to screen256-color. Vim running in tmux then reads tmux’s $TERM and goes, “Cool, I looked screen256-color up and it has 256 colours - let me crack out the 256 colour palettes, like a colourful boss”.

Terminal tells tmux its an Elephant, tmux tells Vim its an Elephant, Vim gets the mahouts out and preps the palanquin.

Whats probably happening is that your .bash_profile is setting a different $TERM - for instance, xterm-color.

Tmux then goes, “Okay, this is an 8 colour terminal, I’m gonna set my $TERM to screen”, which only has 8 colours. Vim then reads tmux’s $TERM and serves up a paltry 8 colours only.

So, terminal is telling tmux its an Antelope, so tmux tells vim its an Antelope, Vim leaves the mahout at home and you don’t get a palanquin, even though you could handle that sh_t.

So by setting set -g default-terminal "screen-256color" in your .tmux.conf, you are telling tmux to always set its $TERM to screen-256color, and then apps like Vim will look that up in terminfo and see, yup, supports 256 colours.

However, your .bash_profile gets run every new terminal right - so it’s run again , overriding the $TERM.

So, telling vim to use a different $TERM, i.e. set term=screen-256color in your .vimrc makes it use the features terminfo lists for screen-256color ,which includes 256 colours. You can use infocmp screen-256color if you’d like to see this.

So Tmux says, its an Elephant, then your .bash_profile changes the label to Antelope - then you have to have Vim tell itself “ignore that, its really an Elephant”.

Only setting the Vim $TERM means tmux isn’t set up for 256 colour, so Vim tries giving 256 and fails. Only setting the tmux one means tmux is loaded for 256, but due to the .bash_profile changing the $TERM, everything thinks tmux is only prepped for 8.

Really, you shouldn’t be trying to override $TERM in .vimrc - because now, whenever you open Vim it thinks it’s in a terminal that has all the screen-256color features. This is only the case when its being opened in tmux!

Really really, you shouldn’t be overriding tmux either - if you make sure your terminal (in this example, OS X’s Terminal.app) has its $TERM set correctly, tmux will read it, see it can handle 256 colors and automatically set its $TERM to screen-256color.

The best solution would be to gently reassure the terminal that it is in fact an Elephant after all. Which probably means nixing a rouge export TERM=xterm-color from your .bash_profile.

TL;DR echo $TERM in a new terminal. If it doesn’t say xterm256-color, go look at your .bash_profile and wipe out any evil export TERM=.