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

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.”)