Make all app icons with imagemagick, iOS and Android

prepare_icons.sh

#!/bin/sh
base=$1
convert "$base" -resize '29x29'     -unsharp 1x4 "Icon-Small.png"
convert "$base" -resize '40x40'     -unsharp 1x4 "Icon-Small-40.png"
convert "$base" -resize '50x50'     -unsharp 1x4 "Icon-Small-50.png"
convert "$base" -resize '57x57'     -unsharp 1x4 "Icon.png"
convert "$base" -resize '58x58'     -unsharp 1x4 "Icon-Small@2x.png"
convert "$base" -resize '60x60'     -unsharp 1x4 "Icon-60.png"
convert "$base" -resize '72x72'     -unsharp 1x4 "Icon-72.png"
convert "$base" -resize '76x76'     -unsharp 1x4 "Icon-76.png"
convert "$base" -resize '80x80'     -unsharp 1x4 "Icon-Small-40@2x.png"
convert "$base" -resize '100x100'   -unsharp 1x4 "Icon-Small-50@2x.png"
convert "$base" -resize '114x114'   -unsharp 1x4 "Icon@2x.png"
convert "$base" -resize '120x120'   -unsharp 1x4 "Icon-60@2x.png"
convert "$base" -resize '144x144'   -unsharp 1x4 "Icon-72@2x.png"
convert "$base" -resize '152x152'   -unsharp 1x4 "Icon-76@2x.png"
convert "$base" -resize '180x180'   -unsharp 1x4 "Icon-60@3x.png"
convert "$base" -resize '512x512'   -unsharp 1x4 "iTunesArtwork"
convert "$base" -resize '1024x1024' -unsharp 1x4 "iTunesArtwork@2x"

convert "$base" -resize '36x36'     -unsharp 1x4 "Icon-ldpi.png"
convert "$base" -resize '48x48'     -unsharp 1x4 "Icon-mdpi.png"
convert "$base" -resize '72x72'     -unsharp 1x4 "Icon-hdpi.png"
convert "$base" -resize '96x96'     -unsharp 1x4 "Icon-xhdpi.png"
convert "$base" -resize '144x144'   -unsharp 1x4 "Icon-xxhdpi.png"
convert "$base" -resize '192x192'   -unsharp 1x4 "Icon-xxxhdpi.png"

You need imagemagick installed.
Put this file into /usr/bin,
Give execution mod: chmod +x

Usage:
Type in the terminal inside your app directory

prepare_icons.sh icon1024.png

icon1024.png - source image file, 1024x1024 or bigger

unsharp is not blurring, it's sharpening.
It sharpens the image after resizing it.

Nice, thank you, @Lerg!

I've extended the script to do a simple input argument check and force resolutions (with !). The reason is that I made a script for launch images as well and here the aspect ratio was kept by imagemagick, resulting in some resolutions not being exactly as desired.

#!/bin/sh
base=$1

if [ -z $base ]
  then
    echo No argument given
else
  ##
  ## iOS files
  convert "$base" -resize 29x29!     "Icon-Small.png"
  convert "$base" -resize 40x40!     "Icon-Small-40.png"
  convert "$base" -resize 50x50!     "Icon-Small-50.png"
  convert "$base" -resize 57x57!     "Icon.png"
  convert "$base" -resize 58x58!     "Icon-Small@2x.png"
  convert "$base" -resize 60x60!     "Icon-60.png"
  convert "$base" -resize 72x72!     "Icon-72.png"
  convert "$base" -resize 76x76!     "Icon-76.png"
  convert "$base" -resize 80x80!     "Icon-Small-40@2x.png"
  convert "$base" -resize 100x100!   "Icon-Small-50@2x.png"
  convert "$base" -resize 114x114!   "Icon@2x.png"
  convert "$base" -resize 120x120!   "Icon-60@2x.png"
  convert "$base" -resize 144x144!   "Icon-72@2x.png"
  convert "$base" -resize 152x152!   "Icon-76@2x.png"
  convert "$base" -resize 180x180!   "Icon-60@3x.png"
  convert "$base" -resize 512x512!   "iTunesArtwork"
  convert "$base" -resize 1024x1024! "iTunesArtwork@2x"
  ##
  ## Android files
  convert "$base" -resize 36x36!    "Icon-ldpi.png"
  convert "$base" -resize 48x48!    "Icon-mdpi.png"
  convert "$base" -resize 72x72!    "Icon-hdpi.png"
  convert "$base" -resize 96x96!    "Icon-xhdpi.png"
  convert "$base" -resize 144x144!  "Icon-xxhdpi.png"
  convert "$base" -resize 192x192!  "Icon-xxxhdpi.png"
fi

Added a couple of new sizes in there (83.5 and icon-small@3x)

#!/bin/sh
base=$1

if [ -z $base ]
  then
    echo No argument given
else
  ##
  ## iOS files
  convert "$base" -resize 29x29!     "Icon-Small.png"
  convert "$base" -resize 40x40!     "Icon-Small-40.png"
  convert "$base" -resize 50x50!     "Icon-Small-50.png"
  convert "$base" -resize 57x57!     "Icon.png"
  convert "$base" -resize 58x58!     "Icon-Small@2x.png"
  convert "$base" -resize 87x87!     "Icon-Small@3x.png"
  convert "$base" -resize 60x60!     "Icon-60.png"
  convert "$base" -resize 72x72!     "Icon-72.png"
  convert "$base" -resize 76x76!     "Icon-76.png"
  convert "$base" -resize 80x80!     "Icon-Small-40@2x.png"
  convert "$base" -resize 100x100!   "Icon-Small-50@2x.png"
  convert "$base" -resize 114x114!   "Icon@2x.png"
  convert "$base" -resize 120x120!   "Icon-60@2x.png"
  convert "$base" -resize 144x144!   "Icon-72@2x.png"
  convert "$base" -resize 152x152!   "Icon-76@2x.png"
  convert "$base" -resize 83.5x83.5! "Icon-83.5@2x.png"
  convert "$base" -resize 180x180!   "Icon-60@3x.png"
  convert "$base" -resize 512x512!   "iTunesArtwork"
  convert "$base" -resize 1024x1024! "iTunesArtwork@2x"
  ##
  ## Android files
  convert "$base" -resize 36x36!    "Icon-ldpi.png"
  convert "$base" -resize 48x48!    "Icon-mdpi.png"
  convert "$base" -resize 72x72!    "Icon-hdpi.png"
  convert "$base" -resize 96x96!    "Icon-xhdpi.png"
  convert "$base" -resize 144x144!  "Icon-xxhdpi.png"
  convert "$base" -resize 192x192!  "Icon-xxxhdpi.png"
fi

For iOS 10 message extension icons.

#!/bin/sh
base=$1

if [ -z $base ]
  then
    echo No argument given
else
  ##
  ## iOS files
  convert "$base" -resize 148x110!   "Messages-74@2x.png"
  convert "$base" -resize 134x100!   "Messages-67@2x.png"
  convert "$base" -resize 120x90!    "Messages-60@2x.png"
  convert "$base" -resize 180x135!   "Messages-60@3x.png"
  convert "$base" -resize 54x40!     "Messages-27@2x.png"
  convert "$base" -resize 81x60!     "Messages-27@3x.png"
  convert "$base" -resize 64x48!     "Messages-32@2x.png"
  convert "$base" -resize 96x72!     "Messages-32@3x.png"
  convert "$base" -resize 1024x768!  "Messages-AppStore.png"
fi

For other people coming here, some of the above follow-up scripts input the POINTS values for sizes rather than PIXEL and thus need to be doubled.

#!/bin/sh

sizes="20x1 20x2 20x3 29x1 29x2 29x3 40x1 40x2 40x3 60x2 60x3 76x1 76x2 83.5x2 1024x1"
source=${1:-logo.svg}
dst=${2:-.}
for s in $sizes;do
  size=${s%x*}
  scale=${s##*x}
  resize=$(bc <<< ${size}*${scale} )
  convert "$source" -resize ${resize}x${resize}     -unsharp 1x4 $dst/"Icon-App-${size}x${size}@${scale}x.png"
done

Added some new sizes, to be used with Zabbix Monitor.

#!/usr/bin/env bash

#
__imgSource=${1:-icon.png}
__imgDest=${__imgSource%.*}
__imgRepository=${2:-Icons}

# 
[ -f ${__imgSource} ] ||  exit 0
[ -d ${__imgRepository} ] || mkdir -p ${__imgRepository}

#
__imgSize=(
   "24x1" "48x1" "64x1" "96x1" "128x1"
   "20x1" "20x2" "20x3"
   "29x1" "29x2" "29x3"
   "40x1" "40x2" "40x3"
   "60x2" "60x3"
   "76x1" "76x2"
   "83.5x2" "1024x1"
)

#
for eSize in ${__imgSize[*]}; do
   size=${eSize%x*}
   scale=${eSize##*x}
   resize=$(bc <<< ${size}*${scale} )
   echo "resizing ${__imgSource} to ${size}x${size}@${scale}"
   convert ${__imgSource} \
      -resize ${resize}x${resize} \
      -unsharp '1.5x1+0.7+0.02' \
      ${__imgRepository}/Icon-${__imgDest}-${size}x${size}@${scale}x.png
done

FWIW, I made a fork with the apple watch icon sizes here: https://gist.github.com/mgrider/3c25a49accf46f8953e7d4dc49fca2c1