Building Web Assets Using Shell Scripts

With the various pre-processed languages used for web (e.g. Coffeescript, LESS .etc), we often reach for tools like Gulp, Grunt and Brunch. However, what if we could just strip it back to basics and use a shell script? This could potentially save you the entire Node.js stack if you only plan to use SCSS (via LibSass).

Firstly, we’re going to need to run both our development server (for the respective framework) and one or more other commands to watch and pre-process files.

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# Convert SIGINT and SIGTERM to an EXIT signal to avoid duplicate traps
trap 'exit' INT TERM

# Kill all processes in the current process group
trap 'kill 0' EXIT

# Run the web server in the background (e.g. Hugo)
hugo server &

# Run various commands to watch and compile files
# TODO: add appropriate commands here

# Wait for all background processes to complete
wait

This solution will allow us to run as many processes as required and see their output all combined in one window. As soon as you hit Ctrl+C, all background processes will be stopped automatically.

Now we’re going to need a tool which can watch for changes in files and compile them when they change. For this, we’ll use fswatch which is written in C, extremely fast and easy to install on OS X with Homebrew.

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brew install fswatch

For the sake of this demonstration, I’m going to use sassc to compile Twitter’s Bootstrap. So let’s install this first:

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brew install sassc

Using fswatch is extremely easy, for example:

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fswatch -0 vendor/bootstrap-sass/assets/stylesheets | while IFS= read -r -d "" path
do
  echo "Re-building Bootstrap CSS (due to change in ${path})"
  sassc --style compressed \
    vendor/bootstrap-sass/assets/stylesheets/_bootstrap.scss \
    public/css/bootstrap.css
done &

So let’s break this down. We start by watching the stylsheets directory for any changes and output null separated filenames when they change.

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fswatch -0 vendor/bootstrap-sass/assets/stylesheets

Next up, we read entry by entry separated by null and save the result into the shell script variable $path:

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while IFS= read -r -d "" path

We can make this even a little neater using an alias and little bash function:

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# Helpers
shopt -s expand_aliases
alias on_change="while IFS= read -r -d ''"
monitor()
{
  [[ $1 == "-i" ]] && { echo -n -e "\0"; shift; }
  fswatch -0 "$@"
}

I have implemented a little extra feature above. As well as running fswatch, you may optionally specify -i to the monitor function and this will trigger a refresh as soon as you start your script to ensure everything is up to date before you start developing.

Now, we can write watchers as follows:

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monitor -i vendor/bootstrap-sass/assets/stylesheets | on_change path
do
  echo "Re-building Bootstrap CSS (due to change in ${path})"
  sassc --style compressed \
    vendor/bootstrap-sass/assets/stylesheets/_bootstrap.scss \
    public/css/bootstrap.css
done &

Of course, if you’re using a tool that contains a watch option, you may simply add that command followed by an & to run that in the background too.

Each one of these commands will create a sub-process to the shell script and be terminated when you hit Ctrl-C.

Have fun bashing away :)

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