WYNTK: UNIX System Admininistrator

The owner of a file can extend read and write permission to just the group.

You may not want to give all users permission to read a file. If you only want to give permission to other people in the file’s group, you can use the g+r syntax instead:

juicy.gossip

	The only files that I’ve paid attention to the security on were personnel files when I was a manager. I don’t restrict file permissions much. However, I have been known to label a sensitive memo with a misleading or obscure name. No sense in leaving a file around labelled juicy.gossip.
	--Linda Lamb

    % chmod g+r filename

Instead of giving permission to all users (a), you are giving permission to just the group (g). Similarly, you can give write permission to others in the group with g+w:

    % chmod g+w filename

and both read and write permission with g+rw:

    % chmod g+rw filename

For example, suppose my coworker Mike wants to edit a file of mine called updates. I can let him do this by giving read and write permission to the world with chmod a+r, but it’s safer to give permission only to members of the file’s group, if you can.

If I run ls -lg on updates, I’ll see that it belongs to the group prod, but is read/write only to the owner, lmui (me).

    % ls -lg updates
    -rw------ 1 lmui prod 249 Oct 25 09:24 updates

I can check that Mike belongs to the prod group by using the groups command:

% groups mike
mike : ora prod

So I don’t have to give read and write permission to the entire world, just to the group:

% chmod g+rw updates

Now Mike and other users in the group prod can read and write the file.