Generate all Icons in single shot by providing 1024x1024.png as an input.

GenerateAppIcons.sh

cp 1024x1024.png AppIcon-20.png
sips -Z 20 AppIcon-20.png

cp 1024x1024.png AppIcon-20@2x.png
sips -Z 40 AppIcon-20@2x.png

cp 1024x1024.png AppIcon-20@3x.png
sips -Z 60 AppIcon-20@3x.png

cp 1024x1024.png AppIcon-29.png
sips -Z 29 AppIcon-29.png

cp 1024x1024.png AppIcon-29@2x.png
sips -Z 58 AppIcon-29@2x.png

cp 1024x1024.png AppIcon-29@3x.png
sips -Z 87 AppIcon-29@3x.png

cp 1024x1024.png AppIcon-40.png
sips -Z 40 AppIcon-40.png

cp 1024x1024.png AppIcon-40@2x.png
sips -Z 80 AppIcon-40@2x.png

cp 1024x1024.png AppIcon-40@3x.png
sips -Z 120 AppIcon-40@3x.png

cp 1024x1024.png AppIcon-60@3x.png
sips -Z 180 AppIcon-60@3x.png

cp 1024x1024.png AppIcon-76.png
sips -Z 76 AppIcon-76.png

cp 1024x1024.png AppIcon-76@2x.png
sips -Z 152 AppIcon-76@2x.png

cp 1024x1024.png AppIcon-83.5@2x.png
sips -Z 167 AppIcon-83.5@2x.png

rm 1024x1024.png

mkdir AppIcon.appiconset

mv AppIcon-20.png AppIcon.appiconset/AppIcon-20.png
mv AppIcon-20@2x.png AppIcon.appiconset/AppIcon-20@2x.png
mv AppIcon-20@3x.png AppIcon.appiconset/AppIcon-20@3x.png
mv AppIcon-29.png AppIcon.appiconset/AppIcon-29.png
mv AppIcon-29@2x.png AppIcon.appiconset/AppIcon-29@2x.png
mv AppIcon-29@3x.png AppIcon.appiconset/AppIcon-29@3x.png
mv AppIcon-40.png AppIcon.appiconset/AppIcon-40.png
mv AppIcon-40@2x.png AppIcon.appiconset/AppIcon-40@2x.png
mv AppIcon-40@3x.png AppIcon.appiconset/AppIcon-40@3x.png
mv AppIcon-60@3x.png AppIcon.appiconset/AppIcon-60@3x.png
mv AppIcon-76.png AppIcon.appiconset/AppIcon-76.png
mv AppIcon-76@2x.png AppIcon.appiconset/AppIcon-76@2x.png
mv AppIcon-83.5@2x.png AppIcon.appiconset/AppIcon-83.5@2x.png

echo '{
  "images" : [
    {
      "idiom" : "iphone",
      "size" : "20x20",
	  "filename" : "AppIcon-20@2x.png",
      "scale" : "2x"
    },
    {
      "idiom" : "iphone",
      "size" : "20x20",
	  "filename" : "AppIcon-20@3x.png",
      "scale" : "3x"
    },
    {
      "size" : "29x29",
      "idiom" : "iphone",
      "filename" : "AppIcon-29@2x.png",
      "scale" : "2x"
    },
    {
      "size" : "29x29",
      "idiom" : "iphone",
      "filename" : "AppIcon-29@3x.png",
      "scale" : "3x"
    },
    {
      "size" : "40x40",
      "idiom" : "iphone",
      "filename" : "AppIcon-40@2x.png",
      "scale" : "2x"
    },
    {
      "size" : "40x40",
      "idiom" : "iphone",
      "filename" : "AppIcon-40@3x.png",
      "scale" : "3x"
    },
    {
      "size" : "60x60",
      "idiom" : "iphone",
      "filename" : "AppIcon-40@3x.png",
      "scale" : "2x"
    },
    {
      "size" : "60x60",
      "idiom" : "iphone",
      "filename" : "AppIcon-60@3x.png",
      "scale" : "3x"
    },
    {
      "idiom" : "ipad",
      "size" : "20x20",
	  "filename" : "AppIcon-20.png",
      "scale" : "1x"
    },
    {
      "idiom" : "ipad",
      "size" : "20x20",
	  "filename" : "AppIcon-20@2x.png",
      "scale" : "2x"
    },
    {
      "size" : "29x29",
      "idiom" : "ipad",
      "filename" : "AppIcon-29.png",
      "scale" : "1x"
    },
    {
      "size" : "29x29",
      "idiom" : "ipad",
      "filename" : "AppIcon-29@2x.png",
      "scale" : "2x"
    },
    {
      "size" : "40x40",
      "idiom" : "ipad",
      "filename" : "AppIcon-40.png",
      "scale" : "1x"
    },
    {
      "size" : "40x40",
      "idiom" : "ipad",
      "filename" : "AppIcon-40@2x.png",
      "scale" : "2x"
    },
    {
      "size" : "76x76",
      "idiom" : "ipad",
      "filename" : "AppIcon-76.png",
      "scale" : "1x"
    },
    {
      "size" : "76x76",
      "idiom" : "ipad",
      "filename" : "AppIcon-76@2x.png",
      "scale" : "2x"
    },
    {
      "idiom" : "ipad",
      "size" : "83.5x83.5",
	  "filename" : "AppIcon-83.5@2x.png",
      "scale" : "2x"
    }
  ],
  "info" : {
    "version" : 1,
    "author" : "xcode"
  }
}' > Contents.json

mv Contents.json AppIcon.appiconset/Contents.json