How to resize images using terminal on Mac OSX?

I need a simple and free way to resize images and do batch jobs, if necessary. Free image manipulation software has been trickier to use than it should be.

As pointed out by LifeHacker, the following command will do this very easily:

sips -Z 640 *.jpg

To quote their explanation:

"sips is the command being used and -Z tells it to maintain the image's aspect ratio. "640" is the maximum height and width to be used and "*.jpg" instructs your computer to downsize every image ending in .jpg. It's really simple and shrinks your images very quickly. Be sure to make a copy first if you want to preserve their larger size as well."

Source: http://lifehacker.com/5962420/batch-resize-images-quickly-in-the-os-x-terminal

imagemagick helps:

$ convert foo.jpg -resize 50% bar.jpg

There are a lot more things it can do, including the conversion between formats, applying effects, crop, colorize and much, much more.

Here is script that uses sips to recursively resize all the images in a given folder (and its sub-folders), and places the resized images in a resized folder on the same tree level as the image: https://gist.github.com/lopespm/893f323a04fcc59466d7

#!/bin/bash
# This script resizes all the images it finds in a folder (and its subfolders) and resizes them
# The resized image is placed in the /resized folder which will reside in the same directory as the image
#
# Usage: > ./batch_resize.sh

initial_folder="/your/images/folder" # You can use "." to target the folder in which you are running the script for example
resized_folder_name="resized"

all_images=$(find -E $initial_folder -iregex ".*\.(jpg|gif|png|jpeg)")

while read -r image_full_path; do
    filename=$(basename "$image_full_path");
    source_folder=$(dirname "$image_full_path");
    destination_folder=$source_folder"/"$resized_folder_name"/";
    destination_full_path=$destination_folder$filename;

    if [ ! -z "$image_full_path" -a "$image_full_path" != " " ] &&
        # Do not resize images inside a folder that was already resized
        [ "$(basename "$source_folder")" != "$resized_folder_name" ]; then 

        mkdir "$destination_folder";
        sips -Z 700 "$image_full_path" --out "$destination_full_path";

    fi

done <<< "$all_images"

Previous answers are correct, you can use mogrify too. For example, if you want to reduce the size of many images in a directory by 60% then you can use the command below:

of course always make a backup of your images in to another directory before playing with this command.

mogrify -resize 60% *

magic trick for itunesconnect :)

    mkdir ./iPhone5-5-Portrait
    sips -z 2208 1242 *.jpg -s formatOptions 70 --out ./iPhone5-5-Portrait
    sips -z 2208 1242 *.png --out ./iPhone5-5-Portrait

Additionally of @grepit reply

The correct syntax is:

magick mogrify -resize 60% *

And you need to install ImageMagick, the easiest way is using homebrew:

brew install imagemagick