to my surprise, the chown command in one child process changed the directory in other child process. I did `ls` to the working dir and you can see the directories' owner ship keep chaning.

Yes, that's what happens. File (and directory) permissions and ownerships are properties of the filesystem, which is shared by all processes. If you change the permissions or ownerships on a file (or on a directory), they do not just change for your process(es) only, but for everyone (assuming they change at all -- i.e., assuming you have the permissions needed to make the change). If you change them and then shut the computer down and turn it back on the next day, they'll *still* be whatever you changed them to, even though all of the processes from the previous day are gone, because the file permissions and ownerships are stored on disk, the same as the actual contents of the files are stored on disk.