Fixing Copyright Notices

Recently, I've been working on starlight - a project that I've had in my stable since 2021, aimed at replacing the frontend portion of Zigbee2MQTT with a simpler and more user friendly interface. Originally called zigbee-webui, and originally just a React frontend, starlight has now become a monorepo with a React frontend (soon to be reworked with Vite and rewritten with TypeScript) and a TypeScript Express backend (an API).

If you're someone who writes code in the open like me, you have probably seen the classic GNU GPL licence notice in files;

This program is free software: you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free Software
Foundation; either version 3 of the License, or (at your option) any later
version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with
this program. If not, see https://www.gnu.org/licenses/.

Now, this is massive and unwieldy - but GNU.org suggests that you should have these notices in every file, "including build scripts, configure files, and makefiles". Some time ago I came across the Software Package Data Exchange (SPDX) standard, which summates the entire licence of a file down to a single line right at the top of each file:

SPDX-License-Identifier: GPL-3.0-or-later

I've been using SPDX licence IDs in this project since the 26th of June 2022, but only recently did I stumble upon an incredibly useful article about copyright statements, "How and why to properly write copyright statements in your code". As it turns out, there is actually a strong case to add a copyright disclaimer to each file. Thankfully SPDX has us covered here too:

SPDX-FileCopyrightText: © $YEAR $AUTHOR <$EMAIL>

In the above linked article, you'll read that we want only the creation year of the file in our copyright disclaimer. And we need to update this in every file with an SPDX header. Right. So the problem statement is this; can I, programmatically, find and update all of the files with a copyright notice that states the file's creation date?

We can use some of git's subcommands to find out information about our checked in files. git log can get us the commit where any given file was added, using --diff-filter=A. If you've moved files around, then you also want --follow. From this, we can tailor this even further to get just a YYYY-MM-DD string for any given file - the date the file was added - using --format=%as. Using bash parameter expansion, we can grab just the year, too. Now we have a command to get back the addition year for any file:

#!/bin/bash

file_addition_year () {
  local file
  local date

  file=$1
  date=$(git log --diff-filter=A --follow --format=%as "$file")
  echo "${date:0:4}"
}

# file_addition_year frontend/src/App.js
# 2021

We would probably want some extra handling for this in case we're not in a git repository, to be on the safe side, but since this is just a quick script to fetch some data, I think this is ok.

In order to add a copyright header to all the files, we can make use of sed to prepend the text that we want to all our files. To work through all the files I want to update, I'm using find and a while read loop:

find . \
  -iname '*.js' \
  -o -iname '*.ts' \
| while read -r file; do
    year=$(file_addition_year "$file")
    sed -i \
      "1s;^;// SPDX-FileCopyrightText: © ${year} Amber Cronin <software@amber.vision>\n;" \
      "$file"
  done

And that's how I updated all the copyright notices in starlight. I also make use of a good pre-commit hook from Lucas-C on GitHub to add the licence header to specific files, which I'd recommend using for all projects.