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

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.