Command-line Basics: Resizing Images with ImageMagick

Introduction

If you’ve ever done programmatic image manipulation, you have probably encountered the ImageMagick library or its major fork, GraphicsMagick. Essentially, ImageMagick is the most commonly-used program for resizing, converting, or otherwise manipulating images on the command line. It has libraries for integration into almost all popular programming languages , and you can use it directly with its included commands, mogrify and convert.

Prerequisites

Before you begin this guide, you should have a regular, non-root user with sudo privileges configured on your server. You can learn how to configure a regular user account by following our Initial server setup guide for Ubuntu 22.04.

Getting started

The ImageMagick library is very popular, but doesn’t usually come installed by default. However, it is widely available in package managers for all platforms. On Ubuntu 22.04, you can install it with apt.

First, update your package sources:

sudo apt update

Then, install imagemagick:

sudo apt install imagemagick

If you don’t already have a sample image handy to work with, you can download the header image from this tutorial using curl, and save it as image.jpg:

curl https://images.prismic.io/digitalocean/0b619d51-a723-4748-997f-39ed5697a540_intro-to-cloud.jpg?auto=compress,format --output image.jpg

Resize to specific dimensions and keep aspect ratio

To resize an image to specific dimensions, use the convert command with an input file, the -resize parameter, your preferred dimensions, and an output filename:

convert original.png -resize 100x100 new.png

This won’t actually resize the image to the exact dimensions specified. What it will do is resize the image to fit within those dimensions.

You can also append ^ to the dimensions to tell ImageMagick that you’d like to resize the image to fill the dimensions, potentially overlapping on one side.

One of the two dimensions (either width or height) will be scaled exactly, while the other will be scaled proportionately and may overlap:

convert original.png -resize 100x100^ new.png

Resize to specific dimensions and keep the aspect ratio

Sometimes you’ll need to not only resize an image, but also crop it so there’s nothing overlapping. A good use case for this is user avatars. You should avoid stretching or squashing a user’s profile picture, so cropping it to a square is an acceptable solution:

convert original.png -resize 100x100^ -gravity center -extent 100x100 new.png

If needed, you can change the file extension of your output file, for example from .png to .jpg, and ImageMagick will convert your image to that format. You can use ImageMagick this way even without resizing or otherwise modifying an image. Refer to the ImageMagick documentation for a more complete list of options.

Resizing files in place

Thus far, we’ve been converting a file and saving it to a brand new file. While being a safer option, as it’s non-destructive to your original file, this is not always an ideal workflow.

If you need to edit a file in place, you can swap the convert command with mogrify. The mogrify command accepts an input file that will be modified in place.

Here’s the “mogrifyed” versions of the previous commands:

mogrify -resize 100x100 original.png

Resizing multiple files

mogrify can also operate on an entire directory of files at once, unlike convert. In general, both ImageMagick commands use similar syntax, but convert only works with a single specified input and output.

To edit an entire directory of images and resize them all in the same way, you can pass mogrify a * wildcard:

mogrify -resize 100x100! ./some/path/*.png
mogrify -resize 100x100^ -gravity center -extent 100x100 *.png

Conclusion

Resizing images from the command-line is really just the tip of the iceberg. ImageMagick supports many additional options that allow you to optimize images, resample their colors, and convert them to other formats.

Next, you may want to learn how to Serve Modern Image Formats Using imgproxy with Docker.