How two users can be the owners of a file?

Question

as the title, I have to create a file, example:

/home/john/file.ini

I created an user and a group, myuser:mygroup and set them to this file.ini

chown myuser:mygroup file.ini

Ok, my problem is

I have another user named mymain this user must read this file, so how can I change the permissions?

IMPORTANT: I ONLY have to allow "mymain" user to read this file, NOT all other users

So myuser (owner of the file) and then mymain. Stop.

Answer 1

In the traditional Unix file permission system that's not possible: a file has only a single owner. You could create a group containing just the two users that should have access and make that the owning group of the file (and give the desired permissions to that group). This approach has some administrative overhead, however (not every user can create a group and place other users in it).

Many (most) modern filesystems support ACLs, however and they aren't too hard to use. If you want to use POSIX ACLs to give another user read (r) and write (w) permissions then you can use this command:

setfacl -m user:mymain:rw file.ini

Two caveats:

• The necessary tools aren't always installed. On Ubuntu/Debian they are found in the acl package (sudo apt-get install acl)
• Even if the filesystem used supports ACLs, it might be that your local filesystem is mounted without support for ACLs. In that case you need to modify the mount parameters to enable it (for ext4, for example, that simply means appending ,acl to the mount options in /etc/fstab).

Answer 2

You should make sure that user mymain is a member of mygroup and give the group permissions, like

chmod g+r <filename>

Some filesystems implement ACL (access control lists), and these may proof to be much more useful to you, but they are beyond the scope of my answer here. You will want to read up on ACLs in an enterprise (ADS) setting_

added

Technically, of course, two users cannot simultaneously be the owner of the same file. Also, if you want to mimic that, you should give full permission to the group, perhaps making a special group for just these two 'owners' and the extent to which this actually mimics owernship will be paltform dependent (old-style UNIX-en tend to be very strict in their rules for allowing chmod/chown)

Answer 3

There are two ways you can achieve this.

1. As noted in @sehe's answer, use the Group facility of Unix permissions.
2. Create a hard link to the file (so the file exists twice) and make mymain the owner of the hard link - ln myfile mymainfile

I explain more about Unix file permissions here:

http://blog.superuser.com/2011/04/22/linux-permissions-demystified/