http://qs1969.pair.com?node_id=283508


in reply to substitute a word in the whole file .. how ???

perl -i.bak -pe 's/frog/toad/g' txtfile

This one-liner will save your old file into txtfile.bak, and modify txtfile by changing every occurrence of "frog" into "toad".

See perlrun for details on the command line arguments.