UNIX for Dummies Questions & Answers

I'm hoping this is a pretty simple question. I have a problem were memebers of a group can't delete or overwrite a file. The box is setup several users all part of the same group. Now default umask is 002. The problem I run into is there are processes that add to, del, and over write files. Now if I start this processes and own the files everything works great, but if someone else starts that processes and I own the files it 's hit and miss. Sometimes is works and sometimes there are permissions errors. Any ideas on what could cause this? and yes the processes does have to be ran by other users and its just not practical to change the file owner everytime some else starts the processes nor is it good to set default permissions to rwxrwxrw.

Thanks

Deleting a file requires write permission on the directory that contains it. But overwriting a file needs write permission on the file itself. See Unix File Permissions

Here is an example of what I mean
user1 and user2 both part of group1

rwxrwxrwx user1 group1 file1
rwxrwxrwx user1 group1 file2
rwxrwxrwx user1 group1 prog1

user2 starts prog 1 "nohup prog1 &"

What happens is prog1 accesss file1 after is does that permissions change to
rw r r user2 group1 file1
then prog1 ends and user1 comes in and restarts it not looking at permissions.
When prog1 goes to access file1, gets a access denied error.

In both user1 and user2 profiles umask is set to 002. Why is file1 getting changed to rw r r then?

prog1 must be trying to create the file with group write disabled. umask can turn off a bit that the program wants to turn on. But umask cannot ever turn on a bit if the program wants it off.