SHELL

linux replace string in all files

sed -i 's/old-text/new-text/g' input.txt

Linux command line search and replace string in all files

 String Find and replace (folders/sub folders)
> grep -rl "old string" . | xargs sed -i 's/old string/new string/g'

 URL Find and replace  (folders/sub folders) 
> grep -rl "www.google.com" . | xargs sed -i 's/www.google.com/www.yahoo.com/g'