Copy all files matching pattern from dir and subdirs into a single dir

Try:

find src/ -type f | grep -i so$ | xargs -i cp {} dst

If you're using Bash, you can turn on the globstar shell option to match files and directories recursively:

shopt -s globstar
cp src/**/*.so dst

If you need to find files whose names begin with ., and/or files in and under directories whose names begin with ., set the dotglob option also (e.g., with shopt -s dotglob).  You can set them both in one command:

shopt -s globstar dotglob

I tried the command suggested by Mike:

find src/ -type f | grep -i so$ | xargs -i cp {} dst

but it ended up with dumping all the files into directory dst with their relative paths lost.

To keep the relative paths the command needs to be modified to this:

find src/ -type f | grep -i so$ | xargs -i cp {} dst/{}