$ find -type f -name '*.conf' -exec sed -Ei 's/foo/bar/' '{}' \;
note that sed -i is non-standard (although both GNU and current BSD systems support it)
Can also be accomplished with
find . -name "*.txt" | xargs perl -pi -e 's/old/new/g'
as shown here - http://www.commandlinefu.com/commands/view/223/a-find-and-replace-within-text-based-files-to-locate-and-rewrite-text-en-mass.
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by David Winterbottom (codeinthehole.com)
