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NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.

4.11. Ownership and Permissions

As a regular user, try to enter root's home directory by entering the command cd /root/. Note the error message:

 -bash: cd: /root/: Permission denied 

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 malicious tampering.

One way to gain entry when you are denied permission is to enter the command su -. This is because whoever knows the root password has complete access.

However, switching to the superuser is not always convenient or recommended, since it is easy to make mistakes and alter important configuration files as the superuser.

All files and directories are "owned" by the person who created them. You created the file foo.txt (refer to Section 4.9.2 Using Redirection) in your login directory, so foo.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, you can also specify whether certain groups can read, write to, or execute a file.

Take a closer look at foo.txt with the ls command using the -l option (refer to Figure 4-11).

A lot of detail is provided here. You can see who can read (r) and write to (w) the file, as well as who created the file (user), and to which group the owner belongs (user). (By default, the name of your group is the same as your login name.)

Figure 4-11. Permissions for foo.txt

Other information to the right of the group includes file size, date and time of file creation, and file name.

The first column 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.

For example:

 -rw-rw-r-- 

Those three sets are the owner of the file, the group in which the file belongs, and "others," meaning other users on the system.

 -    (rw-)   (rw-)   (r--) 1 user user

The first item, which specifies the file type, will probably be 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

Others are possible, but are beyond the scope of this manual. Refer to the Red Hat Enterprise Linux System Administration Guide for more information.

Beyond the first item, in each of the following three sets, you may 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)

  • - (dash) — specific permission has not been assigned

When you see a dash in owner, group, or others, it means that particular permission has not been granted. Look again at the first column of foo.txt and identify its permissions.

ls -l foo.txt 
-rw-rw-r--    1 user user     150 Mar 19 08:08 foo.txt

The file's owner (in this case, user) has permission to read and write to the file. The group, user, has permission to read and write to foo.txt, as well. It is not a program, so neither the owner or the group has permission to execute it.

4.11.1. The chmod Command

Use the chmod command to change permissions. This example shows how to change the permissions on foo.txt with the chmod command.

The original file looks like this, with its initial permissions settings:

 -rw-rw-r--    1 user user     150 Mar 19 08:08 foo.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 Caution
 

Remember that file permissions are a security feature. Whenever you allow anyone else to read, write to, and execute files, you are increasing the risk of files being tampered with, altered, or deleted. As a rule, 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 must change the "others" section of the file permissions.

Take a look at the file first. At the shell prompt, type:

ls -l foo.txt

The previous command displays this file information:

 -rw-rw-r--    1 user user     150 Mar 19 08:08 foo.txt 

Now, type the following:

chmod o+w foo.txt

The o+w command tells the system you want to give others write permission to the file foo.txt. To check the results, list the file's details again. Now, the file looks like this:

 -rw-rw-rw-    1 user user     150 Mar 19 08:08 foo.txt 

Now, everyone can read and write to the file.

To remove read and write permissions from foo.txt use the chmod command to take away both the read and write permissions.

chmod go-rw foo.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 foo.txt.

The result looks like this:

 -rw-------    1 user user    150 Mar 19 08:08 foo.txt 

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 is 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

Want to test your permissions skills? Remove all permissions from foo.txt — for everyone.

chmod a-rwx foo.txt

Now, see if you can read the file with the command cat foo.txt, which should return the following:

 cat: foo.txt: Permission denied

Removing all permissions, including your own, successfully locked the file. But since the file belongs to you, you can always change its permissions back with the following command:

chmod u+rw foo.txt

Use the command cat foo.txt to verify that you, the file owner, can read the file again.

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 — allows only the group to 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) the execute permission for a directory, you are really allowing (or denying) permission to search through that directory.

Examine the dir1/ directory you created in section FIXME by listing all of the files in your home directory.

ls -l /home/>user</

The permissions on this directory are:

drwxrwxr-x   2 mgoldin mgoldin  4096 Jan  6 15:05 dir1

If you do not allow others to have execute permission on the dir1/ directory, it does not matter who has read or write access. No one can access the directory unless they know the exact file name.

For example, type

chmod a-x dir1/

to remove everyone's execute permissions.

Here is what happens when you try to change directories using the cd dir1/ command after removing everyone's execute permissions:

 bash: dir1/: Permission denied        

Next, restore your own and your group's access:

chmod ug+x dir1/

If you check your work with ls -l, you can see that only others are denied access to the /dir1/ directory.

4.11.2. Changing Permissions With Numbers

Another way to change permissions uses numeric representations.

Go back to the original permissions for foo.txt:

 -rw-rw-r--    1 user user     150 Mar 19 08:08 foo.txt 

Each permission setting can be represented by a numerical value:

  • r = 4

  • w = 2

  • x = 1

  • - = 0

When these values are added together, the total is used to set specific permissions. For example, if you want read and write permissions, you would have a value of 6; 4 (read) + 2 (write) = 6.

For foo.txt, here are the numerical permissions settings:

 -  (rw-)   (rw-)  (r--)

The total for the user is six(4+2+0), the total for the group is six(4+2+0), and the total for others is four(4+0+0). The permissions setting is read as 664.

If you want to change foo.txt so those in your group do not have write access, but can still read the file, remove the access by subtracting two (2) from that set of numbers.

The numerical values then become six, four, and four (644).

To implement these new settings, type:

chmod 644 foo.txt

Now verify the changes by listing the file. Type:

ls -l foo.txt

The output should be:

 -rw-r--r--    1 user user     150 Mar 19 08:08 foo.txt 

Now, neither the group nor others have write permission to foo.txt. To return the group's write access for the file, add the value of w (2) to the second set of permissions.

chmod 664 foo.txt
Warning Warning
 

Setting permissions to 666 allows everyone to read and write to a file or directory. Setting permissions to 777 allows everyone read, write, and execute permission. These permissions could allow tampering with sensitive files, so in general, it is not a good idea to use these settings.

Here is a list of some common settings, numerical values and their meanings:

Setting Numerical Meaning
-rw------- (600) Only the owner has read and write permissions.
-rw-r--r-- (644) Only the owner has read and write permissions; the group and others have read only.
-rwx------ (700) Only the owner has read, write, and execute permissions.
-rwxr-xr-x (755) The owner has read, write, and execute permissions; the group and others have only read and execute.
-rwx--x--x (711) The owner has read, write, and execute permissions; the group and others have only execute.
-rw-rw-rw- (666) Everyone can read and write to the file. (Be careful with these permissions.)
-rwxrwxrwx (777) Everyone can read, write, and execute. (Again, this permissions setting can be hazardous.)

Table 4-3. File permissions settings, numerical values, and their meanings

Here are some common settings for directories:

Setting Numerical Meaning
drwx------ (700) Only the user can read, write in this directory.
drwxr-xr-x (755) Everyone can read the directory; users and groups have read and execute permissions.

Table 4-4. Directory permissions settings, numerical values, and their meanings

 
 
  Published under the terms of the GNU General Public License Design by Interspire