Use the sed -i command
sed -i.BAK 's|foo|bar|g' files # -i does in-place replacement perl -pi.bak -e 's|foo|bar|g' files # or perl -pi.bak -e 's|foo|bar|g' `find /pathname -name "filespec"`
Perl is the preferred way, but on some production system you must use the old good sed. For some tips on sed take a look to its faq.
Another very powerful command is xargs. Xargs is a rapid way to process files containing spaces, using a combo with find:
find . -print0 -type f | xargs -0 ls | grep " "
Xargs is normally fastest then relaying on backtick substitution. For instance:
ls $(find . -type f)
is slower then
find . -type f | xargs ls
because this second form create two process which works at the same time, using the unix pipe.