How can I easily bulk generate icons on macOS?

Around 1 minute

Source: Originally mentioned by strogonoff on Hacker News

You can use sips together with iconutil to generate a complete .icns file for your app from a single 1024 by 1024 PNG without any third party software: 

mkdir MyIcon.iconset
cp Icon1024.png MyIcon.iconset/icon_512x512@2x.png
sips -z 16 16     Icon1024.png --out MyIcon.iconset/icon_16x16.png
sips -z 32 32     Icon1024.png --out MyIcon.iconset/icon_16x16@2x.png
sips -z 32 32     Icon1024.png --out MyIcon.iconset/icon_32x32.png
sips -z 64 64     Icon1024.png --out MyIcon.iconset/icon_32x32@2x.png
sips -z 128 128   Icon1024.png --out MyIcon.iconset/icon_128x128.png
sips -z 256 256   Icon1024.png --out MyIcon.iconset/icon_128x128@2x.png
sips -z 256 256   Icon1024.png --out MyIcon.iconset/icon_256x256.png
sips -z 512 512   Icon1024.png --out MyIcon.iconset/icon_256x256@2x.png
sips -z 512 512   Icon1024.png --out MyIcon.iconset/icon_512x512.png
iconutil -c icns MyIcon.iconset
mkdir MyIcon.iconset
cp Icon1024.png MyIcon.iconset/icon_512x512@2x.png
sips -z 16 16     Icon1024.png --out MyIcon.iconset/icon_16x16.png
sips -z 32 32     Icon1024.png --out MyIcon.iconset/icon_16x16@2x.png
sips -z 32 32     Icon1024.png --out MyIcon.iconset/icon_32x32.png
sips -z 64 64     Icon1024.png --out MyIcon.iconset/icon_32x32@2x.png
sips -z 128 128   Icon1024.png --out MyIcon.iconset/icon_128x128.png
sips -z 256 256   Icon1024.png --out MyIcon.iconset/icon_128x128@2x.png
sips -z 256 256   Icon1024.png --out MyIcon.iconset/icon_256x256.png
sips -z 512 512   Icon1024.png --out MyIcon.iconset/icon_256x256@2x.png
sips -z 512 512   Icon1024.png --out MyIcon.iconset/icon_512x512.png
iconutil -c icns MyIcon.iconset

As a bonus, generate .ico with ffmpeg:

ffmpeg -i MyIcon.iconset/icon_256x256.png icon.ico
ffmpeg -i MyIcon.iconset/icon_256x256.png icon.ico