Doing a bulk search and replace across a set of files is actually surprisingly easy. sed is the key. It has a flag - i that will modify the files passed to it in-place.

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$ sed -e 's/TextToFind/Replacement/' -i file1 file2 file3

Tie this power with either grep -l. (Thanks to Steve for pointing out a mistake in the following, now corrected)

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$ grep -l TextToFind * |xargs sed -e 's/TextToFind/Replacement/' -i

or find

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$ find . -exec sed -e 's/TextToFind/Replacement' -i {} ;

If there are multiple changes you want to make, just put them all into a file and pass it in via the -f flag.

file: replacements.patterns

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s/TextToFind1/Replacement1/
s/TextToFind2/Replacement2/
s/TextToFind3/Replacement3/

and the command, using find to iterate through all files in the current directory and subdirectories.

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find . -exec sed -f replacements.patterns -i {} ;

et voila - hope it helps.