A ZSH command file to produce the various PNG files required for iOS and macOS app Icons using `sips` (scriptable image processing system) available on any macOS. The resulting PNGs in the "AppIcons" folder can then be dragged into Xcode's `Assets.xcassets` onto a placeholder for an AppIcon.

AppIcons.command

#!/bin/zsh
# Shell Script to create all relevant icons from an high resolution artwork

if [ "x$1" != "x" -a -f "$1" ] ; then
    INPUT=$1
else 
    INPUT="Artwork.png"
fi

if [ ! -f "$INPUT" ]; then
    echo "Input file not specified"
    exit 1
fi
mkdir -p AppIcons

while IFS= read -r line ; do
  FILENAME=${line% *}
  SIZE=${line#* }
  if [ "x$FILENAME" != "x" ] ; then
    COMMAND="sips -s format png \"$INPUT\" --resampleHeightWidth $SIZE $SIZE --out \"AppIcons/$FILENAME\""
    echo "$COMMAND"
    eval "$COMMAND"
  else
    echo ""
  fi
done <<EOF
AppIcon~iPhone-20@2x.png 40
AppIcon~iPhone-20@3x.png 60
AppIcon~iPhone-29@1x.png 29
AppIcon~iPhone-29@2x.png 58
AppIcon~iPhone-29@3x.png 87
AppIcon~iPhone-40@2x.png 80
AppIcon~iPhone-40@3x.png 120
AppIcon~iPhone-57@1x.png 57
AppIcon~iPhone-57@2x.png 114
AppIcon~iPhone-60@2x.png 120
AppIcon~iPhone-60@3x.png 180

AppIcon~iPad-20@1x.png 20
AppIcon~iPad-20@2x.png 40
AppIcon~iPad-29@1x.png 29
AppIcon~iPad-29@2x.png 58
AppIcon~iPad-40@1x.png 40
AppIcon~iPad-40@2x.png 80
AppIcon~iPad-76@1x.png 76
AppIcon~iPad-76@2x.png 152
AppIcon~iPad-83.5@2x.png 167

AppIcon~iOS-Marketing-1024@1x.png 1024

AppIcon~macOS-16@1x.png 16
AppIcon~macOS-16@2x.png 32
AppIcon~macOS-32@1x.png 32
AppIcon~macOS-32@2x.png 64
AppIcon~macOS-128@1x.png 128
AppIcon~macOS-128@2x.png 256
AppIcon~macOS-256@1x.png 256
AppIcon~macOS-256@2x.png 512
AppIcon~macOS-512@1x.png 512
AppIcon~macOS-512@2x.png 1024
EOF

Thanks @KevinQuisquater! I've updated my script accordingly, to ensure everybody gets the latest version.

It seems that zsh does not complain about the unnecessary ; I had added. The missing icons you've identified are probably due to my focus on iOS 13 and above. Probably older iOS releases/devices expect even more icon variants...

Thanks for the useful script. I think it's worth noting that when I used the script some of my icons where slightly of center. Resampling is quick approach, but I think I'm going to get better result rendering each on individually for the original art file.

Hi Roy

The result of the script depends on the quality of the input. Either your input image was not square (width = height) or the text was not aligned.
I tried to reproduce the problem myself and got an acceptable quality (see below). None of the icons seems to be misaligned.

I created these icons based on an artwork image with dimensions 1024x1024 and the text properly centered.

Best regards
Philipp

Ah, I thought the pixels where shifting from he resampling algorithm used in your script. I checked my 1024pt icon that I ran the script on and the text is actually off center. Thanks.