Imports an image from a twain sane scanner, converts to jpg and saves it to the desktop of the current user, then opens it with Preview.app

scan2preview.sh

#!/bin/bash

targetfile="scan_$( date +"%Y-%m-%d_%H-%M-%S" ).jpg"

# Get image from scanner
scanimage --format=tiff --resolution 300 > /tmp/scan.tiff

# TODO: Search for correct error message (need it to be thrown by scanimage before implementing)
if [ $(grep -q 'cups' /tmp/scan.tiff ) ]; then
  # Calculate size of file without error message
  newsize=$(( $(wc -c /tmp/scan.tiff | sed "s/^[^0-9]*\([0-9]*\)[^0-9].*$/\1/") - 22 ))

  # Remove error message
  tail -c $newsize /tmp/scan.tiff > /tmp/scannew.tiff
  mv /tmp/scannew.tiff /tmp/scan.tiff
fi

# Use convert to create a jpg
convert -quality 90 /tmp/scan.tiff "$HOME/Desktop/$targetfile"

# Remove tiff
rm /tmp/scan.tiff

# Open jpg in default JPG viewer (mostly preview) to be edited
open "$HOME/Desktop/$targetfile"

Instead of creating $homedir manually, why not just use $HOME?

Also, you might want to wrap stuff like $homedir/Desktop/$targetfile in quotes, in case the expanded string contains spaces.

Thanks for your hints. :)