gistfile1.txt

#!/bin/sh

string="abc\nxyz"

case $string in
  (*[[:blank:]]*) echo "string contains at least one character classified as blank";;
  (*[[:space:]]*) echo "string contains at least one character classified as whitespace (but not blank)";;
  (*) echo no character classified as whitespace;;
esac

if [[ "$string" =~ [[:blank:]] ]]; then echo "Contains whitespace"; else echo "Doesn't contain whitespace"; fi

# the above outputs:
# no character classified as whitespace
# Doesn't contain whitespace