ACK and AG

Alternatives for grep -> ack and ag (much faster)
http://blog.newrelic.com/2015/01/28/grep-ack-ag/

It works on Linux and MAC. Use brew to install on MAC:
brew install ack
brew install ag 

replace a string in multiple files

grep -rl 'from' ./ | xargs sed -i 's/from/to/g'