X Terminal Colors (and dark and light backgrounds) (Shallow Thoughts)

Akkana's Musings on Open Source, Science, and Nature.

Tue, 18 Jan 2011

X Terminal Colors (and dark and light backgrounds)

[Displaying colors in an xterm] At work, I'm testing some web programming on a server where we use a shared account -- everybody logs in as the same user. That wouldn't be a problem, except nearly all Linuxes are set up to use colors in programs like ls and vim that are only readable against a dark background. I prefer a light background (not white) for my terminal windows.

How, then, can I set things up so that both dark- and light-backgrounded people can use the account? I could set up a script that would set up a different set of aliases and configuration files, like when I changed my vim colors. Better, I could fix all of them at once by changing my terminal's idea of colors -- so when the remote machine thinks it's feeding me a light color, I see a dark one.

I use xterm, which has an easy way of setting colors: it has a list of 16 colors defined in X resources. So I can change them in ~/.Xdefaults.

That's all very well. But first I needed a way of seeing the existing colors, so I knew what needed changing, and of testing my changes.

Script to show all terminal colors

I thought I remembered once seeing a program to display terminal colors, but now that I needed one, I couldn't find it. Surely it should be trivial to write. Just find the escape sequences and write a script to substitute 0 through 15, right?

Except finding the escape sequences turned out to be harder than I expected. Sure, I found them -- lots of them, pages that conflicted with each other, most giving sequences that didn't do anything visible in my xterm.

Eventually I used script to capture output from a vim session to see what it used. It used <ESC>[38;5;Nm to set color N, and <ESC>[m to reset to the default color. This more or less agreed Wikipedia's ANSI escape code page, which says <ESC>[38;5; does "Set xterm-256 text coloor" with a note "Dubious - discuss". The discussion says this isn't very standard. That page also mentions the simpler sequence <ESC>[0;Nm to set the first 8 colors.

Okay, so why not write a script that shows both? Like this:

#! /usr/bin/env python

# Display the colors available in a terminal.

print "16-color mode:"
for color in range(0, 16) :
    for i in range(0, 3) :
        print "\033[0;%sm%02s\033[m" % (str(color + 30), str(color)),
    print

# Programs like ls and vim use the first 16 colors of the 256-color palette.
print "256-color mode:"
for color in range(0, 256) :
    for i in range(0, 3) :
        print "\033[38;5;%sm%03s\033[m" % (str(color), str(color)),
    print

Voilà! That shows the 8 colors I needed to see what vim and ls were doing, plus a lovely rainbow of other possible colors in case I ever want to do any serious ASCII graphics in my terminal.

Changing the X resources

The next step was to change the X resources. I started by looking for where the current resources were set, and found them in /etc/X11/app-defaults/XTerm-color:

$ grep color /etc/X11/app-defaults/XTerm-color
irrelevant stuff snipped
*VT100*color0: black
*VT100*color1: red3
*VT100*color2: green3
*VT100*color3: yellow3
*VT100*color4: blue2
*VT100*color5: magenta3
*VT100*color6: cyan3
*VT100*color7: gray90
*VT100*color8: gray50
*VT100*color9: red
*VT100*color10: green
*VT100*color11: yellow
*VT100*color12: rgb:5c/5c/ff
*VT100*color13: magenta
*VT100*color14: cyan
*VT100*color15: white
! Disclaimer: there are no standard colors used in terminal emulation.
! The choice for color4 and color12 is a tradeoff between contrast, depending
! on whether they are used for text or backgrounds.  Note that either color4 or
! color12 would be used for text, while only color4 would be used for a
! Originally color4/color12 were set to the names blue3/blue
!*VT100*color4: blue3
!*VT100*color12: blue
!*VT100*color4: DodgerBlue1
!*VT100*color12: SteelBlue1

So all I needed to do was take the ones that don't show up well -- yellow, green and so forth -- and change them to colors that work better, choosing from the color names in /etc/X11/rgb.txt or my own RGB values. So I added lines like this to my ~/.Xdefaults:

!! color2 was green3
*VT100*color2: green4
!! color8 was gray50
*VT100*color8: gray30
!! color10 was green
*VT100*color10: rgb:00/aa/00
!! color11 was yellow
*VT100*color11: dark orange
!! color14 was cyan
*VT100*color14: dark cyan
... and so on.

Now I can share accounts, and I no longer have to curse at those default ls and vim settings!

Update: Tip from Mikachu: ctlseqs.txt is an excellent reference on terminal control sequences.


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[ 09:56 Jan 18, 2011    More linux | permalink to this entry ]