Linux file permissions

Question

I own a particular file on a Linux system. I would like to give 2 groups (accounting, shipping) read access and only read access, and 3 users(Mike, Raj and Wally) write access and only write access.

How can I accomplish this?

Answer 1

I suggest using POSIX ACLs (Access Control Lists). Your filesystem must support them / have the acl option enabled, but if it does, then your problem simplifies to:

setfacl -m u:mike:w myfile
setfacl -m u:raj:w myfile
setfacl -m u:wally:w myfile
setfacl -m g:accounting:r myfile
setfacl -m g:shipping:r myfile

And you can do that for arbitrary sets of permissions.

Answer 2

group read access, specify the second bit as 4, but what is that 3 users ?

chmod 240 file

write to user, read to group.

Answer 3

If you have 2 groups A,B and 3 users x,y,z

first give the correct ownership:

for the group:

chgrp A my_file

for the user

chown x my_file

then change permissions:

chmod g+r my_file # for the group

chmod u+w my_file # for the file