Replace "firstWord" with "newFirstWord", "secondWord" with "newSecondWord", "thirdWord" with "newThirdWord" in all XML files in the current directory tree
find -name "*.xml" -exec sed -s --in-place=.bak -e 's/firstWord/newFirstWord/g;s/secondWord/newSecondWord/g;s/thirdWord/newThirdWord/g' {} \;