08/12/2012
Ownership and Permissions
Earlier in this chapter, when you tried to cd to root's login directory, you received the following message:
[newuser@localhost newuser]$ cd /root
bash: /root: Permission denied
[newuser@localhost newuser]$
That was one demonstration of Linux's security features. Linux, like UNIX, is a multi-user system, and file permissions are one way the system protects against any type of damage.
One way to gain entry when you are denied permission is to su to root, as you learned earlier. That is because whoever knows the root password has complete access.
[newuser@localhost newuser]$ su
Password: your root password
[root@localhost newuser] # cd /root
[root@localhost /root] #
But switching to superuser is not always convenient, or wise, since it is easy to make mistakes and alter important configuration files.
All files and directories are "owned" by the person who created them. You created the file sneakers.txt (see the section called Using Redirection in your login directory, so sneakers.txt "belongs" to you.
That means you can specify who is allowed to read the file, write to the file or, if it is an application instead of a text file, who can execute the file.
Reading, writing, and executing are the three main settings in permissions.
Since users are placed into a group when their accounts are created, then you can also specify whether certain groups can read, write to, or execute a file.
Take a closer look at sneakers.txt with the ls command using the -l (long) option (see Figure 10-12).
[newuser@localhost newuser]$ ls -l sneakers.txt
-rw-rw-r-- 1 newuser newuser 150 Mar 19 08:08 sneakers.txt
There is a lot of detail provided here. You can see who can read (r) and write to (w) the file, as well as who created the file (newuser) and to which group the owner belongs (newuser).
Tip Your default group
Remember that, by default, your group is the same as your login name.
Figure 10-12. Permissions for sneakers.txt
Other information to the right of the group includes file size, date and time of file creation, and file name.
-rw-rw-r--
The first column (shown above) shows current permissions; it has ten slots. The first slot represents the type of file. The remaining nine slots are actually three sets of permissions for three different categories of users.
Those three sets are: the owner of the file, the group in which the file belongs, and "others," meaning users and groups other than the owner of the file (newuser), and those in newuser's group (which is also newuser).
- (rw-) (rw-) (r--) 1 newuser newuser
| | | |
type owner group others
The first item, which specifies the file type, can show one of the following:
d — a directory
-(dash) — a regular file (rather than directory or link)
l — a symbolic link to another program or file elsewhere on the system
Beyond the first item, in the following three sets, you will see one of the following:
r — file can be read
w — file can be written to
x — file can be executed (if it is a program)
When you see a dash in owner, group, or others, it means that particular permission has not been granted.
Look again at first column of sneakers.txt and identify its permissions. (See Figure 10-13)
[newuser@localhost newuser]$ ls -l sneakers.txt
-rw-rw-r-- 1 newuser newuser 150 Mar 19 08:08 sneakers.txt
[newuser@localhost newuser]$
Figure 10-13. A Closer View of Permissions
The file's owner (in this case, newuser) has permission to read and write to the file. It is not a program, so newuser does not have permission to execute it. The group, newuser, has permission to read and write to sneakers.txt, as well. Similar to the program notation for owner newuser, there is no execute permission for group newuser.
In the last set, you can see that those who are not either the user newuser or in the group called newuser can read the file, but can not write to it or execute it.
Change the permissions on sneakers.txt with the chmod command.
The original file looks like this, with its initial permissions settings:
-rw-rw-r-- 1 newuser newuser 150 Mar 19 08:08 sneakers.txt
If you are the owner of the file or are logged into the root account you can change any permissions for the owner, group, and others.
Right now, the owner and group can read and write to the file. Anyone outside of the group can only read the file (r--).
Caution Permissions Are Necessary
Remember that file permissions are a security feature. Whenever you allow everyone to read, write to, and execute files, you are increasing the risk of files being tampered with, altered, or deleted. As a rule, then, you should only grant read and write permissions to those who truly need them.
In the following example, you want to allow everyone to write to the file, so they can read it, write notes in it, and save it. That means you will have to change the "others" section of the file permissions.
Since you are the owner of the file, you do not have to su to root to do it. Take a look at the file first. At the shell prompt, type:
ls -l sneakers.txt
The previous command displays this file information:
-rw-rw-r-- 1 newuser newuser 150 Mar 19 08:08 sneakers.txt
Now, type the following:
chmod o+w sneakers.txt
To check the results, list the file's details again. Now, the file looks like this:
-rw-rw-rw- 1 newuser newuser 150 Mar 19 08:08 sneakers.txt
Now, everyone can read and write to the file (Figure 10-14).
Figure 10-14. Changing Permissions for sneakers.txt
The o+w command tells the system you want to give others write permission to the file sneakers.txt.
To remove read and write permissions from sneakers.txt use the chmod command to take away both the read and write permissions like so:
chmod go-rw sneakers.txt
and the result will look like this:
-rw------- 1 newuser newuser 150 Mar 19 08:08 sneakers.txt
By typing go-rw, you are telling the system to remove read and write permissions for the group and for others from the file sneakers.txt.
You might think of these settings as a kind of shorthand when you want to change permissions with chmod, because all you really have to do is remember a few symbols and letters with the chmod command.
Here a list of what the shorthand represents:
Identities
u — the user who owns the file (that is, the owner)
g — the group to which the user belongs
o — others (not the owner or the owner's group)
a — everyone or all (u, g, and o)
Permissions
r — read access
w — write access
x — execute access
Actions
+ — adds the permission
- — removes the permission
=—makes it the only permission
Note An Additional Permission
Another permission symbol is t, for the sticky bit. If a sticky bit is assigned to a file, a user who wants to remove or rename that file must own the file, own the directory, have write permission, or be root (see the section called File Properties in Chapter 11).
Want to test your permissions skills? Remove all permissions from sneakers.txt — for everyone.
chmod a-rwx sneakers.txt
Now, see if you can read the file:
[newuser@localhost newuser]$ cat sneakers.txt
cat: sneakers.txt: Permission denied
[newuser@localhost newuser]$
It worked. But since the file belongs to you, you can always change its permissions back (see Figure 10-15).
[newuser@localhost newuser]$ chmod u+rw sneakers.txt
[newuser@localhost newuser]$ cat sneakers.txt
buy some sneakers
then go to the coffee shop
then buy some coffee
bring the coffee home
take off shoes
put on sneakers
make some coffee
relax!
[newuser@localhost newuser]$
Figure 10-15. Removing and Restoring Permissions
Here are some common examples of settings that can be used with chmod:
g+w — adds write access for the group
o-rwx — removes all permissions for others
u+x — allows the file owner to execute the file
a+rw — allows everyone to read and write to the file
ug+r — allows the owner and group to read the file
g=rx — lets the group only read and execute (not write)
By adding the -R option, you can change permissions for entire directory trees.
Because you can not really "execute" a directory as you would an application, when you add or remove execute permission for a directory, you are really allowing (or denying) permission to search through that directory.
To allow everyone read and write access to every file in the tigger directory in your login directory, type:
chmod -R a+rw tigger
But… if you do not allow others to have execute permission to tigger, it does not matter who has read or write access, because no one will be able to get into the directory — unless they know the exact filename they want.
For example, type:
chmod a-x tigger
to remove everyone's execute permissions.
Here is what happens now when you try to cd to into tigger:
[newuser@localhost newuser]$ cd tigger
bash: tigger: Permission denied
[newuser@localhost newuser]$
Restore your own and your group's access.
chmod ug+x tigger
Now, if you check your work with ls -dl you will see that only others will be denied access to the tigger directory.
