I'm trying to answer the following question about file permissions in Unix. Consider a file with the following permissions:
rwx---r--
I am not the owner of this file, but I am a member of the group of this file.
My question is: do I have read access to this file?
I thought the answer was YES, since the world has read access to this file. But, a sysadmin is telling me that the answer is NO, since the group access bits are all off.
Can someone provide an authoritative answer?
I would try to answer this for myself, but at the moment, I don't have access to a Unix box with sufficient priveleges.
If your kernel is working correctly, you should be unable to read the file. This behavior is mandated by Posix and all posix compliant systems should behave this way.
Which way this should work was debated during the early 80's. The overriding element was that rwx---r-- and rwxr--r-- really should behave differently. The currently accepted behavior allows a user to revoke permission for one group while allowing all other groups access. If you want rwxr--r-- then all you need is a chmod to get it.
The way Unix handles the permission is by reading the permission from the left to the right.
So
If the permission looks like rwx---r--
And you are not the owner but are a group member you cannot read this file.
##############Demo on HP-UX 11i, Logged on as rene
Home dir /home/rene
$ id
uid=102(rene) gid=20(users)
$ ls -la
total 14
drwxr-xr-x 2 rene users 1024 Jul 16 21:17 .
drwxr-xr-x 4 root root 96 Jul 9 23:13 ..
-rw-r--r-- 1 rene users 832 Jul 9 23:13 .cshrc
-rw-r--r-- 1 rene users 347 Jul 9 23:13 .exrc
-rw-r--r-- 1 rene users 334 Jul 9 23:13 .login
-rw-r--r-- 1 rene users 439 Jul 9 23:13 .profile
-rw------- 1 rene users 146 Jul 16 21:22 .sh_history
-rwx---r-- 1 root users 32 Jul 16 21:19 permtest
$ cat permtest
cat: Cannot open permtest: Permission denied
$
# Permissions are read from the left to the right and it first tells that the group users have NO permissions bits set.
-- I hope this helps