Linux Dummies 9th

Chapter 6: Getting to Know the Linux Filesystem
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✓ The third character is either an x or a dash. The x stands for execute permission.
If x is set, the triplet allows the entity it stands for to access the
files contained in this directory or to run the particular program in this file.
In all cases, if the dash sits in place of r, w, or x, the triplet doesn’t allow the
entity the read, write, or execute permission.
The following sections describe owners and groups in more detail.
Beware of owners
You may have noticed by now that we talk a great deal about owners (users)
and groups in Linux. Every file and directory has both of these components:
a user from the /etc/passwd file that’s assigned as its owner and a group
from the /etc/group that’s assigned as the group.
Although an everyday user probably doesn’t need to change file ownerships
often, the root user does so regularly. If you add the file comments,
for example, to /home/tom while you’re logged on as the superuser (another
term for the Administrator, who is the person who owns the root account),
the root user owns that file. The user tom can’t do anything with it unless the
file’s owner (root) sets the last triplet’s permissions to allow the other folks
(those who aren’t the file’s owner or in the specified group) to read and write
to the file. But this method is a pretty sloppy way of doing things because the
whole idea of permissions is to reduce access, not to give everyone access.
Instead, remember to change the file’s owner to the user tom. You do this
with the chown (change owner) command. For example, by typing chown
mike comments, rich changes the ownership over to mike. Then mike can
work with this file and even change its permissions to something he prefers:
rich@testbox:~$ ls -al comments
-rw-r--r-- 1 rich rich 5 2009-05-16 14:25 comments
rich@testbox:~$ sudo chown mike comments
[sudo] password for rich:
rich@testbox:~$ ls -al comments
-rw-r--r-- 1 mike rich 5 2009-05-16 14:25 comments
rich@testbox:~$
Hanging out in groups
Groups are more interesting to work with than owners. You use groups to
allow the root user to assign multiple users the ability to share certain filesystem
areas. For example, in many versions of Linux, all users are added to
a group named users (openSuSE does this, for example). Then rather than a
long-format file listing such as the one shown earlier in this chapter, you may
see the following: