Only include files that match a given pattern in a recursive diff

Perhaps this is a bit indirect, but it ought to work. You can use find to get a list of files that don't match the pattern, and then "exclude" all those files:

find a b -type f ! -name 'crazy' -printf '%f\n' | diff -r a b -X -

The -X - will make diff read the patterns from stdin and exclude anything that matches. This should work provided your files don't have funny chars like * or ? in their names. The only downside is that your diff won't include the find command, so the listed diff command is not that useful.

(I've only tested it with GNU find and diff).

EDIT:

Since only non-GNU find doesn't have -printf, sed could be used as an alternative:

find a b -type f ! -name '*crazy*' -print | sed -e 's|.*/||' | diff -X - -r a b

That's also assuming that non-GNU diff has -X which I don't know.

I couldn't make it work with -X -, so I used a different approach - let find find the files recursively according to my requirements and let diff compare individual files:

a="/your/dir"b="/your/other/dir"for f in $(find $a -name "*crazy*" | grep -oP "^$a/\K.*"); do  diff -u $a/$f $b/$fdone

Or, in one line:

a="/your/dir" && b="/your/other/dir" && for f in $(find $a -name "*crazy*" | grep -oP "^$a/\K.*"); do diff -u $a/$f $b/$f; done

This grep -oP "^$a/\K.*" is used to extract the path relative to the directory a, i.e. it removes /your/dir/ from /your/dir/and/a/file, to make it and/a/file.