If I would want to change the permission in a whole directory tree (many many subdirs everywhere, and file), let's say I want directories to have 755 and regular files to have 644. IIRC I can use umask to mask away some bits and then do a chmod.

My questions are: can I make umask differ between directories and file. And should I do a chmod 777 after making the umask.

Or am I out riding my bike in the wrong park?

In such a case, what I usually do is:
find dir/ -type d -exec chmod XXX {} \;
find dir/ ! -type d -exec chmod XXX {} \;

Yves.

umask sets the default permissions that files and directories have when they're created. If my user's umask is 0027 any files or directories I create when logged in as me will be inaccessible to 'others' and not writeable to 'group' and will give full access to 'user' (the extra zero on the end if for setting special permission bits like setuid, setgid etc.).

So to get the effect of the umask subtract 7 from the umask number. For the group permissions in this example 7-2=5 which is read and execute permissions. It will actually create directories with read and execute permission but files with just read permission - its smart enough to know that making files executable by default is unecessary and could lead to security problems.