Cache-busting Assets with sed

Thanks to the ongoing quarantine, I’ve found the time to take on some of the long-standing items on my web dev to-do list. One of these items was to implement cache-busting on CSS and JavaScript assets in coordination with service workers.

Being a developer with an already strong but still growing aversion to frameworks and taskrunners, especially when all I really need is a simple feature, I prefer mining old-school Unix tools and using them to building out a custom deploy script, usually using a post-receive hook with Git. It’s hard to find an old-school tool as old as sed, which first appeared in 1974.

For those who don’t know sed (short for “streaming editor”), its friendly overviewpage opens with a fine summary:

sed is commonly used to filter text, i.e., it takes text input, performs some operation (or set of operations) on it, and outputs the modified text. sed is typically used for extracting part of a file using pattern matching or substituting multiple occurrences of a string within a file.

And that’s what I need: to comb through a static set of HTML files for references like <script
src="assets/js/site.​js"> and insert a hash so the file referenced becomes something likeassets/js/site.0a1cb23.​js. The file itself remains site.​js on the server, so I don’t have to worry about moving files around and changing their names. A quick little location-scoped regex rewrite block in Nginx will ignore the hash:

# This assumes assets off of an /assets directory, obviously
location ~ /assets {
  rewrite (.+)\.([a-f0-9]+)\.(js|css)$ $1.$3 break;
}

To grab a meaningful hash, I need to capture the HEAD commit hash from my site’s repository, usinggit rev-parse. As a side note, it’s essential to include the argument --git-dir=<bare repo directory> when trying to run commands on a specific repo—especially bare repos, which I use for all my basic deploy scripts.

So the relevant part of my deploy script (which is written for zsh and its awesome recursive globbing syntax looks something like this:

cd $TMP_GIT_CLONE
SHORTHASH=`git --git-dir=$TMP_GIT_CLONE/.git rev-parse --short HEAD`;
for file in **/*.html;
sed -E -i "s/=([^:]+)\.(css|js)/=\1.$SHORTHASH.\2/g" $file;
done;

With that run, the files have all of their paths updated. I then need to give a similar treatment to my service worker script, which actually needs a variable declaration rewritten.

For the sake of local development and not hanging on too long to an existing service worker cache, I open my service worker script with these lines JavaScript, which falls back to a mockTenMinuteVersion() function that generates a new time-based hash every ten minutes:

var VERSION;
const version = VERSION ? VERSION : mockTenMinuteVersion();

I prefer using the ES 2015 const declaration, but because const by definition cannot have its value reassigned, simply declaring const VERSION; is enough kick out a nasty little syntax error and prevent the rest of the script from running.

To allow for that, I write the regex on this call to sed to also replace var with const and assign the SHORTHASH value to const VERSION as well. These lines get added to thepost-receive lines listed above:

sed -E -i "s/var VERSION/const VERSION = \"$SHORTHASH\"/" _site/sw.​js;
sed -E -i "s/'([^:]+)\.(css|js)/'\1.$SHORTHASH.\2/g" _site/sw.​js;

And that changes the opening lines of my service worker script to:

const VERSION = "0a1cb23";
const version = VERSION ? VERSION : mockTenMinuteVersion();

Finally, sed needs to do its thing and update the files listed in the manifest of preloaded files togo from this:

const site_preloaded_assets = {
  essential: [
    '/assets/css/screen.​css',
    '/assets/js/site.​js',
    '/',
    site_offline_path
  ],
  supporting: []
};

to this:

const site_preloaded_assets = {
  essential: [
    '/assets/css/screen.0a1cb23.css',
    '/assets/js/site.0a1cb23.​js',
    '/',
    site_offline_path
  ],
  supporting: []
};