Convert GIMP's `.gpl` file to CSS variables

gpl-to-css.sh

#!/bin/bash
# convert a Gimp colour palette (.gpl) to a list of CSS variables
# example:
#   ./gpl-to-css.sh my-palette.gpl > my-palette.css
# converts
# ----- my-palette.gpl -----
# GIMP Palette
# Name: My Palette
# Columns: 3
# 255 0 0 Red
# 0 255 0 Green
# 0 0 255 Blue
# ---------------------------
# to
# ----- my-palette.css -----
# :root {
#   --colour-Red: #ff0000;
#   --colour-Green: #00ff00;
#   --colour-Blue: #0000ff;
# }
# ---------------------------
# only supports 3-column colours
# you could edit the lines below to enable 4 colours by:
# - add another match to the regex
# - shift the indexes of the column items to add a fourth alpha column
# - add an A colour to the print statement

regex='^Columns: ([0-9]+)$'
if [[ $(grep -E "$regex" $1) =~ $regex ]];
then
  columns=${BASH_REMATCH[1]}
else
  echo "Error: could not find columns in $1" >&2
  exit 1
fi

if [[ $columns -ne 3 ]];
then
  echo "Error: only 3-column palettes are supported" >&2
  exit 1
fi

cat $1 | awk -F ' ' '
BEGIN {
  printf ":root {\n"
} /^([0-9]{1,3})\s+([0-9]{1,3})\s+([0-9]{1,3})\s+(.*)$/ {
  colourname = ""
  for (i=4;i<=NF;i++) {
    colourname = colourname "-" $i
  }
  printf "  --colour%s: #%02x%02x%02x;\n", colourname, $1, $2, $3
} END {
  printf "}\n"
}'