Understanding Linux umask on Redhat / Centos

Hold on Cowboy

This blog post is pretty old. Be careful with the information you find in here. It's likely dead, dying, or wildly inaccurate.

umask is linux’s way of determining default file/folder permissions when new files and folders are created. It’s a little odd in that the umask is subtracted from 777 for folders and 666 for files to get the correct permissions. I don’t claim to understand fully, but it just is.

What is your current umask? umask 0002

By default, normal users will get a umask of 0002 = 775 on a folder and 664 on a file. This equates to

-rwxrwxr—

for a file.

I had a user that had a set umask of 0022, but all other users had a umask of 0002. This puzzled me, I looked in the normal areas that you can override the umask

/etc/profile

and

~/.bashrc

but didn’t see anything obvious. Then looking in

/etc/passwd

I noticed this user had a groupid set to apache’s group (48). This was done for reasons outside this discussion.

So if a userid or groupid < 99 then umask = 0022 (more restrictive).

Resources: http://kbase.redhat.com/faq/docs/DOC-1373