Using 256 colour terminals with applications like Vim and tmux is pretty much a no-brainer; it’s been well-supported on most GUI terminal emulators for ages, including Windows emulators like PuTTY, and gives you a much wider spectrum of colour with which to work and to apply useful features like syntax highlighting and contextual colours for shell prompts.

The problem is that not all terminals are created equal. Occasionally, you’ll find yourself needing to use a console, perhaps with the linux type, when your X server doesn’t start or you’re using a KVM. Following the principles of graceful degradation, it’s a good idea to arrange your terminal configuration and any other applications that normally take advantage of the wider 256 colour spectrum to start up normally with no errors, and to use sensible values instead as supported by the terminal.

Bash

You can check the number of colours supported by your $TERM setting using the tput command:

$ tput colors
256

On systems using termcap, the syntax is slightly different:

$ tput Co
256

This provides you with an accessible way to set up graceful degradation for any colours that you use in your .bashrc file or other startup scripts. By way of example, in my .bashrc file I include the following stanza that adjusts the colour of my prompt depending on whether I have root privileges:

if ((EUID == 0)); then
    PS1=${color_root}${PS1}${color_undo}
else
    PS1=${color_user}${PS1}${color_undo}
fi

The variables contain the terminal escape codes used to start painting text in appropriate colours for the terminal type. I set these earlier on in the file by checking the output of tput colors:

colors=$(tput colors)

# Terminal supports 256 colours
if ((colors >= 256)); then
    color_root='\[\e[38;5;9m\]'
    color_user='\[\e[38;5;10m\]'
    color_undo='\[\e[0m\]'

# Terminal supports only eight colours
elif ((colors >= 8)); then
    color_root='\[\e[1;31m\]'
    color_user='\[\e[1;32m\]'
    color_undo='\[\e[0m\]'

# Terminal may not support colour at all
else
    color_root=
    color_user=
    color_undo=
fi

The above conditional restricts the colour space to conform to what the terminal explicitly specifies is supported. If no colours at all are supported, the variables are empty, and the prompt is simply printed with no colour at all.

Vim

A familiar structure from a lot of Vim colorscheme definition files is:

if has("gui_running") || &t_Co >= 256
    ...
endif

This has the effect of checking that either gVim is running, or that the terminal is capable of printing 256 colours. You can therefore use this and similar structures to delineate different sections of the file to apply based on the number of colours supported by the terminal.

if has("gui_running") || &t_Co >= 256
    ...
elseif &t_Co >= 88
    ...
elseif &t_Co >= 8
    ...
endif

It’s not really a good idea to define this sort of stuff in your .vimrc file; colour information should go into your colour scheme file.

Tmux

Things are a little tricker in tmux. Conditionals based on shell output are supported in recent versions. If you have a particular line of your configuration that will only work on terminals that support a certain number of colours, you can prefix that line with an #if-shell call:

if-shell 'test $(tput colors) -ge 256' 'set-option -g default-terminal "screen-256color"'

The above will only set the default terminal to screen-256color for new sessions if the test call returns a result greater than or equal to 256. Note that you need to wrap both the shell test and the configuration to run in quotes for this to work.

You can preface every applicable line with this test, or if you’d prefer to only run one test for efficiency reasons, you could arrange to load a separate local configuration file only if a single test passes:

if-shell 'test "$(tput colors)" -ge 256' 'source-file ~/.tmux.256.conf'

Unfortunately, there’s a race condition in tmux that means that the first shell can be established before the default-terminal configuration item is actually applied, meaning that the default $TERM of screen is used for the first window, but subsequent windows are correctly created with the screen-256color setting for $TERM. One possible workaround for this is to preserve the old terminal’s name in an environment variable:

containing_term=$TERM
if-shell 'test "$(tput colors)" -ge 256' 'set-option -g default-terminal "screen-256color"'

You can then check this variable’s value in your .bashrc file to see if it’s a terminal known to support 256 colours, and if it is, force the screen-256color terminal type for the shell:

case $containing_term in
    *256color) 
        TERM=screen-256color
        unset containing_term
        ;;
esac

This problem may well be fixed in future versions of tmux, but it still seems to be an issue even in the developing 1.7 version.

All of the above goes a long way to making your personal set of dotfiles more robust, so that when you need to bust them out on some older machine or less capable terminal, they’re more likely to work properly.