Resizing images on the Linux command line

The convert command from the ImageMagick suite of tools provides ways to make all sorts of changes to image files. Among these is an option to change the resolution of images. The syntax is simple, and the command runs extremely quickly. It can also convert a image from one format to another (e.g., jpg to png) as well as blur, crop, despeckle, dither, flip and join images and more.

Although the commands and scripts in this post mostly focus on jpg files, the convert command also works with a large variety of other image files, including png, bmp, svg, tiff, gif and such.

Basic resizing

To resize an image using the convert, you would use a command like this:

$ convert -resize 1200x1000 smile.jpg smile-2.jpg

The syntax is "convert -resize resolution currentfile newfile".

The resolution should be expressed as the desired width (in pixels), followed by an "x" and then the desired height. Note that if the numbers aren't numerically related to the current dimensions of the image, the resultant resolution might not be what you expect. Generating a 1200x1000 image from a 2400x2000 is one thing. Asking for it to be saved as a 2000x1200 will result in one that is only 1440x1200.

Using a script

If you intend to convert a number of images or will be resizing images often, it's a good idea to use a script. The first script shown below would create a "smile_2.jpg" file from a "smile.jpg" file using the 1200x800 resolution. Note how it extracts the file extension from the filename so that it can build the new filename.

#!/bin/bash
 
if [ $# -eq 0 ]; then
    echo -n "image file: "
    read img
else
    img=$1
fi
add=2

# get filetype and base name from argument
filetype=`echo $img | awk -F . '{print $2}'`
basename=`echo $img | sed 's/.jpg//'`

# target resolution
resolution=1200x800

# new file name will include "_2"
newfile="${basename}_${add}.$filetype"

# run convert command
convert -resize $resolution $img $newfile

# display the new file
ls -l $newfile

Resizing a group of image files

The next script will create a 1200x800 resolution file from each of the jpg files in the current directory and will display each of the new files after it has been set up.

#!/bin/bash

num=2
resolution=1200x800

for image in `ls *.jpg`; do
  basename=`echo $image | sed "s/.jpg//"`
  convert -resize $resolution $image ${basename}_${num}.jpg
  ls -ltr | tail -1
done

If you run the script, you would see something like this:

$ resize_jpg_files
-rw-r--r--. 1 shs shs  49321 May 25 09:52 camper_2.jpg
-rw-r--r--. 1 shs shs   3872 May 25 09:52 map_2.jpg
-rw-r--r--. 1 shs shs   3872 May 25 09:52 pig_2.jpg
-rw-r--r--. 1 shs shs 130432 May 25 09:52 tree-cutting_2.jpg
-rw-r--r--. 1 shs shs  45082 May 25 09:52 volcano_rings_2.jpg

Resizing by file type

The next script will ask you what type of image files to convert and will then run through the files of the type in the current directory. Like the previous script, it adds a "_2" to the file names to differentiate them from the originals. This can easily be changed, of course, if some other character or string works better for you.

#!/bin/bash

echo -n "file type: "
read filetype
add=2
resolution=800x600

for image in `ls *.$filetype`; do
  echo $image
  basename=`echo $image | sed "s/.$filetype//"`
  convert -resize $resolution $image ${basename}_${add}.$filetype
  ls -ltr | tail -1
done

Using multiple image resolutions

This last script asks for the name of a single image file and then creates new versions of the image using three different resolutions. It also adds the intended resolution to the file name.

#!/bin/bash

# ask for image file name
echo -n "image: "
read image

if [ ! -f $image ]; then
  echo "No such file: $image"
  exit 1
fi

# NOTE: resolution is height x width
for reso in 400x500 600x800 800x1200; do
  basename=`echo $image | sed 's/.jpg//'`
  convert -resize $reso $image ${basename}_${reso}.jpg
  ls -ltr | tail -1
done

The resultant files might look like this:

$ ls -l dog*
-rw-r--r--. 1 shs shs 14501 May 25 12:29 dog_400x500.jpg
-rw-r--r--. 1 shs shs 28658 May 25 12:29 dog_600x800.jpg
-rw-r--r--. 1 shs shs 45082 May 25 12:29 dog_800x1200.jpg
-rw-r--r--. 1 shs shs 58628 May 25 12:25 dog.jpg

Wrap-up

The convert command makes resizing image files extremely easy. To learn more about some of the command's other options, check out this post on converting and manipulating image files on the Linux command line.