update wallpaper after scaling and cropping the image to the current screen resolution

asetroot.sh

#!/bin/bash
#
# asetroot.sh
#
# Copyright (c) 2014 Takuma Nakajima
#  
# This software is released under the MIT License.
# http://opensource.org/licenses/mit-license.php
# 

set -e

function autocrop() {
    srcimg=$1
    target_x=`echo $2 | awk -F 'x' '{print $1}'`
    target_y=`echo $2 | awk -F 'x' '{print $2}'`
    outimg=$3

    tmpimg=/dev/shm/tmp_mkwallpaper.jpg
    src_x=`identify $srcimg | awk '{print $3}' | awk -F 'x' '{print $1}'`
    src_y=`identify $srcimg | awk '{print $3}' | awk -F 'x' '{print $2}'`

    if [ `expr ${src_x} \* ${target_y} / ${src_y}` -gt $target_x ]; then
	tmp_x=`expr ${src_x} \* ${target_y} / ${src_y}`
	convert -geometry ${tmp_x}x${target_y}! $srcimg $tmpimg
	convert -crop ${target_x}x${target_y}+`perl -e "print abs( $tmp_x - $target_x ) / 2"`+0 $tmpimg $outimg
    else
	tmp_y=`expr ${src_y} \* ${target_x} / ${src_x}`
	convert -geometry ${target_x}x${tmp_y}! $srcimg $tmpimg
	convert -crop ${target_x}x${target_y}+0+`perl -e "print abs( $tmp_y - $target_y ) / 2"` $tmpimg $outimg
    fi

    rm -f $tmpimg
}

wallpaper=$1
if [ $# -ne 1 ]; then
    echo "usage: $0 <image_file>"
    exit 1
elif [ ! -e $wallpaper ]; then
    echo "error: image not found: $wallpaper"
    exit 1
fi
output=/dev/shm/wallpaper.png

current_resolution=`xrandr -q | grep -Eo 'current[^,]+' | sed 's/\(current\)\? //g'`

echo "resizing image to ${current_resolution}..."
autocrop $wallpaper $current_resolution $output

echo "updating wallpaper..."
hsetroot -fill $output