Bulk Image Conversion to WebP with a Simple Zsh Function

In today’s web development landscape, image optimisation is crucial for performance. WebP format offers superior compression compared to traditional formats like JPEG and PNG, often reducing file sizes by 25-50% while maintaining similar quality. After repeatedly converting images manually, I created a simple Zsh function to automate the process.

Prerequisites

First, you’ll need ImageMagick installed on your system:

macOS (with Homebrew):

brew install imagemagick

Ubuntu/Debian:

sudo apt-get install imagemagick

Ubuntu and Debian still ship ImageMagick 6, which has no magick command: use convert instead in the function below.

The Simple Solution

Here’s the function I use daily, simple, effective, and gets the job done:

convert_to_webp() {
  setopt local_options null_glob  # prevent 'no matches found' errors
  for ext in jpg jpeg png; do
    for f in *.$ext; do
      [ -e "$f" ] || continue
      output="${f%.*}.webp"
      magick "$f" -quality 80 -define webp:method=6 "$output"
      echo "Converted: $f -> $output"
    done
  done
}

That’s it. No complicated options, no verbose output, just converts every JPG, JPEG, and PNG in the current directory to WebP with sensible defaults.

Why These Defaults?

• Quality 80: Good balance between file size and visual quality
• Method 6: Best compression (slower but smaller files)
• Supported formats: JPG, JPEG, PNG cover 95% of use cases

Usage

Simply navigate to your image directory and run:

convert_to_webp

Example output:

Converted: photo1.jpg -> photo1.webp
Converted: banner.jpeg -> banner.webp
Converted: screenshot.png -> screenshot.webp

File Size Comparisons

Here are some real-world examples of the savings you can expect:

The savings vary depending on the image content, screenshots and graphics with large solid areas compress exceptionally well, while detailed photos see lower but still significant reductions.

Adding to Your Dotfiles

For my dotfiles setup, I added the function to my zsh/functions file, then it’s automatically available in any new shell session.