Test 24 bit colors in terminals

24-bit-color.sh

#!/bin/bash
#
#   This file echoes a bunch of 24-bit color codes
#   to the terminal to demonstrate its functionality.
#   The foreground escape sequence is ^[38;2;<r>;<g>;<b>m
#   The background escape sequence is ^[48;2;<r>;<g>;<b>m
#   <r> <g> <b> range from 0 to 255 inclusive.
#   The escape sequence ^[0m returns output to default

setBackgroundColor()
{
    echo -en "\x1b[48;2;$1;$2;$3""m"
}

resetOutput()
{
    echo -en "\x1b[0m\n"
}

# Gives a color $1/255 % along HSV
# Who knows what happens when $1 is outside 0-255
# Echoes "$red $green $blue" where
# $red $green and $blue are integers
# ranging between 0 and 255 inclusive
rainbowColor()
{ 
    let h=$1/43
    let f=$1-43*$h
    let t=$f*255/43
    let q=255-t

    if [ $h -eq 0 ]
    then
        echo "255 $t 0"
    elif [ $h -eq 1 ]
    then
        echo "$q 255 0"
    elif [ $h -eq 2 ]
    then
        echo "0 255 $t"
    elif [ $h -eq 3 ]
    then
        echo "0 $q 255"
    elif [ $h -eq 4 ]
    then
        echo "$t 0 255"
    elif [ $h -eq 5 ]
    then
        echo "255 0 $q"
    else
        # execution should never reach here
        echo "0 0 0"
    fi
}

for i in `seq 0 127`; do
    setBackgroundColor $i 0 0
    echo -en " "
done
resetOutput
for i in `seq 255 128`; do
    setBackgroundColor $i 0 0
    echo -en " "
done
resetOutput

for i in `seq 0 127`; do
    setBackgroundColor 0 $i 0
    echo -n " "
done
resetOutput
for i in `seq 255 128`; do
    setBackgroundColor 0 $i 0
    echo -n " "
done
resetOutput

for i in `seq 0 127`; do
    setBackgroundColor 0 0 $i
    echo -n " "
done
resetOutput
for i in `seq 255 128`; do
    setBackgroundColor 0 0 $i
    echo -n " "
done
resetOutput

for i in `seq 0 127`; do
    setBackgroundColor `rainbowColor $i`
    echo -n " "
done
resetOutput
for i in `seq 255 128`; do
    setBackgroundColor `rainbowColor $i`
    echo -n " "
done
resetOutput

Eh, if the terminal is narrower than 129 columns (e.g. xterm with 9x18 font on 1024x768 laptop) the output is… weird.

I see the same output as Windows Terminal on libvte terminals on Debian sid:

ii  libvte-2.91-0:amd64 0.68.0-1+b1  amd64        Terminal emulator widget for GTK+ 3.0 - runtime files

Tested with mate-terminal and kmscon. I could try others but generally if you've seen one VTE, you've seen them all. No idea when the regression happened.

This is written for Bash, so why bother with seq at all? Do this instead:

for i in {255..128}; do
    setBackgroundColor $i 0 0
    echo -en " "
done

With that correction, the script works correctly on bash 5.1.16, alacritty 0.11.0, and NixOS 22.11.

Antonio Gurgel dixit:

This is written for Bash, so [why bother with `seq`](http://mywiki.wooledge.org/BadUtils?highlight=%28seq%29#seq) at all? Do this instead: ``` for i in {255..128}; do

Because that makes it even harder for people to port to serious shells. Do this instead: i=256 while (( --i >= 128 )); do bye, //mirabilos -- „Cool, /usr/share/doc/mksh/examples/uhr.gz ist ja ein Grund, mksh auf jedem System zu installieren.“ -- XTaran auf der OpenRheinRuhr, ganz begeistert (EN: “[…]uhr.gz is a reason to install mksh on every system.”)