Search and Replace Inline many files

One line way if you have perl to search and replace instances of a certain text with something else. Don't forget to escape special characters w/ a backslash (\).

perl -pi -e 's/find/replace/g' *.txt

Or, to change matching files in a hierarchy:

find . -name '*.txt' |xargs perl -pi -e 's/find/replace/g'