How do I pass a list of files to grep

Well, the generic case that works with any command that writes to stdout is to use xargs, which will let you attach any number of command-line arguments to the end of a command:

$ find … | xargs grep 'search'

Or to embed the command in your grep line with backticks or $(), which will run the command and substitute its output:

$ grep 'search' $(find …)

Note that these commands don't work if the file names contain whitespace, or certain other “weird characters” (\'" for xargs, \[*? for $(find …)).

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However, in the specific case of find the ability to execute a program on the given arguments is built-in:

$ find … -exec grep 'search' {} \;

Everything between -exec and ; is the command to execute; {} is replaced with the filename found by find. That will execute a separate grep for each file; since grep can take many filenames and search them all, you can change the ; to + to tell find to pass all the matching filenames to grep at once:

$ find … -exec grep 'search' {} \+

Some versions of grep (e.g. on non-embedded Linux or BSD or Mac OS X) have a -r option to make a recursive search. On OpenBSD, use -R (and there's no --exclude as in the example below). This covers simple combinations of find with grep.

If your implementation doesn't have the -R flag, or if you want fancier file matching criteria, you can use the -exec primary of find to make it execute grep. A few older find implementations don't support -exec … +; on these systems, use a ; instead of the + (this will call grep once per file, so it'll be slower, but otherwise the result will be the same). Note the /dev/null trick to cause grep to show the file name even if it happens to be called on a single file (GNU grep and FreeBSD/NetBSD/OSX grep have a -H option to achieve the same effect).

find . -type f -name '*.o' -prune -o -exec grep 'needle' /dev/null {} +
grep -r --exclude='*.o' 'needle' .