Use 'sips' to manipulate images in CLI.

sips (scriptable image processing system) is a prebundled command tool which can used to manipulate images. There are two advantages:

- it is prebundled, you don't need to download and install it.
- it uses CoreImage so it is very fast.

Here are some examples:

1. Get Image size
$sips --getProperty pixelWidth  xxx.jpg
  pixelWidth: 3264

$sips --getProperty pixelHeight xxx.jpg
  pixelHeight: 2448

2. Resize an image
$sips -z 100 100 xxx.jpg
  resize xxx.jpg to 100x100

$sips --resampleWidth 100 xxx.jpg
  resize xxx.jpg 's width to 100, and resize the height to keep its ratio.

$sips --resampleHeight 100 xxx.jpg
  resize xxx.jpg's height to 100, and resize its width to keep the original ratio.

You can check its man page here: https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/sips.1.html

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