Terminal colour tolerance

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.


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

$ tput colors

On systems using termcap, the syntax is slightly different:

$ tput Co

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

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

# Terminal supports only eight colours
elif ((colors >= 8)); then

# Terminal may not support colour at all

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.


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

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

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

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.


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:

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
        unset containing_term

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.

Gracefully degrading .vimrc

If you work on many machines with varying operating systems (Windows, GNU/Linux, MacOS X, BSD) and in various upgrade states, particularly if some of the machines are a lot older or have more minimal or custom installations of Vim, you might not be using your .vimrc file on all of them because it includes features that aren’t available on some other machines, meaning that Vim spits a lot of errors at you on startup.

This might have prompted you to perhaps keep a simpler .vimrc file, a “lesser” .vimrc, that you put onto your remote machines, and you keep the “real” .vimrc on your own machine to include lines that use all of the features only available to you on that machine. If you like to version your configuration files, maintaining and testing both of the .vimrc files on all the machines gets old fast; it would be much better to have a single .vimrc file that simply ignored configuration it didn’t understand. There are several ways to approach this.

Check features

Perhaps the best way to manage this is to group all of your configuration items by Vim feature, and to check for their presence in your .vimrc file before attempting to set any of the relevant options. You can do this with the has() function. As an example, here’s a stanza from my .vimrc:

if has("spell")
    set spelllang=en_nz
    nnoremap <leader>s :set spell!<CR>

I set the spellang option and perform the remapping only if the +spell feature is actually available.

If an option is dependent on a feature having been compiled into Vim, you can usually tell by calling :help on it and looking for a phrase like “not available when compiled without the +xyz feature.” You can also view a list all the features available with :help feature-list, and see which features are compiled into a given vim binary with the --version parameter:

$ vim --version
VIM - Vi IMproved 7.3 (2010 Aug 15, compiled Feb 11 2012 03:54:05) Included patches: 1-429
Modified by pkg-vim-maintainers@lists.alioth.debian.org
Compiled by jamessan@debian.org
Huge version with GTK2 GUI.  Features included (+) or not (-):
+arabic +autocmd +balloon_eval +browse ++builtin_terms +byte_offset +cindent
+clientserver +clipboard +cmdline_compl +cmdline_hist +cmdline_info +comments
+conceal +cryptv +cscope +cursorbind +cursorshape +dialog_con_gui +diff ...

There are certain special features, like +unix, that you can use to check whether the Vim instance running is on a platform suitable for an option or not. I find this is handy for choosing the correct syntax for specifying fonts on Windows:

if has("unix")
    set guifont=Inconsolata\ 14
    set guifont=Consolas:h12

Check options

You can check whether an option itself exists rather than a feature with exists():

if exists("&foldenable")
    set foldenable

Check version number

Another way of filtering out options for older versions of Vim is by version number, which you can perform by checking the value of v:version. For example, to only set folding options if you’re working with at least Vim 7, you could do:

if v:version >= 700
    set foldenable

In this particular case, though, it’s a little clearer and more robust to check the condition with if has("folding"), because the version number being recent enough does not necessarily mean the feature exists. However, one good application of using version numbers is fixing bugs or unexpected behaviour in older instances of Vim, to make it behave consistently with newer versions, or even vice-versa if necessary.

Silent calls

If you can’t find a way to filter by feature or version number, a simple way to suppress any error messages from a configuration line is to preface it with silent!. I find this is a nice compact way to call plugin-dependent functions like pathogen#infect(), or custom colorschemes that you can’t be sure actually exist on the machine:

silent! call pathogen#infect()
silent! colorscheme zellner


If you’re not dealing with versions of Vim older than 7.0, another possibility is the try/catch/endtry block. This is handy for setting a default or fallback option if a call fails, such as selecting colorschemes:

    colorscheme zenburn
    colorscheme torte

This is my least-favoured method of making .vimrc files degrade gracefully, as it breaks completely on older instances of Vim.

Thanks to Reddit user bouncingsoul for suggesting the second method which I initially missed.