In Unix, how do I do a recursive search and replace on a batch of files in a given directory?

I think this is a simple application of "for loop" in shel script.

The traditional answer is the one already given with find and sed. This replaces "foo" with "bar" in all text files: find . -name '*.txt' -print -exec sed -i.bak 's/foo/bar/g' {} \; (Equivalently, you can use "perl -pi -e s/foo/bar/g" instead of sed.) However, for common scenarios, you may find the command http://manpages.ubuntu.com/manpages/trusty/man1/rpl.1.html is easier to remember. Here is replacement on all files, recursively, in the current directory: rpl -R foo bar . It's not available by default on some Linux or Unix installations but is quick to install ("apt-get install rpl" on Ubuntu, for example). For tougher jobs with more complex renaming or changing of file contents or filenames, the most powerful tool I'm aware of is https://github.com/jlevy/repren, a small Python script I wrote a while back for some thornier renaming and refactoring tasks. The situations where you might prefer it include: Renaming and moving of files and directories as well as search and replace on file contents (my-dir/foo-stuff/foo.txt -> my-dir/bar-stuff/bar.txt). See a "dry run" of changes before you commit to performing the search and replace. Support regular expressions with back substitution, whole words, case insensitive, and case preserving (foo -> bar, Foo -> Bar, FOO -> BAR) modes. Multiple simultaneous replacements, including swaps (foo -> bar and bar -> foo, without an intermediate rename) or sets of non-unique replacements (foo -> bar, f -> x). Check the repren page for details.