Why piping find and grep returns nothing?

Question

I'm trying to have grep search inside specified files that are returned by find:

find . -type d -name 'mydir*' -exec find '{}' -name '*.java' \; | grep 'MyClass'

This doesn't work.

Meanwhile, this works.

grep 'MyClass' $(find . -type d -name 'mydir*' -exec find '{}' -name '*.java' \;)

What's the problem with my first example?

Answer 1

In first case you grep the list of filenames returned by find, whereas in the second case the filename list is an argument to grep, so grep will search inside the files.

Pipe is a standard input, not a named file. That's why grep behaves differently.
From man grep:

grep  searches  the  named input FILEs (or standard input if no files are named, 
or if a single hyphen-minus (-) is given as file name) for lines containing a match
to the given PATTERN.

Answer 2

Because in the first case grep tries fo find 'MyClass' in filenames and in the second one it tries to find 'MyClass' in file contents.

The first one is equal to

find . -type d -name 'mydir*' -exec find '{}' -name '*MyClass*.java' \;

while the second one is equal to

find . -type d -name 'mydir*' -exec sh -c \
"find '{}' -name '*.java' -exec grep 'MyClass' {} \;" \;

Answer 3

I think your find command should be:

find . -type f -path 'mydir*.java'| xargs grep -H 'MyClass'

Which finds files in mydir with .java extension and containing MyClass. -H instructs grep to output filename in front of result.