Fedora™ Unleashed, 2008 edition

nonadministrative tasks and you need to do something that only the super user can do? The su command is available for this purpose.

NOTE

A popular misconception is that the su command is short for super user; it just means substitute user. An important but often overlooked distinction is that between su and su -. In the former instance, you become that user but keep your own environmental variables (such as paths). In the latter, you inherit the environment of that user. This is most noticeable when you use su to become the super user, root. Without appending the -, you do not inherit the path variable that includes /bin or /sbin, so you must always enter the full path to those commands when you just su to root.

Because almost all Linux file system security revolves around file permissions, it can be useful to occasionally become a different user with permission to access files belonging to other users or groups or to access special files (such as the communications port /dev/ttyS0 when using a modem, or the sound device /dev/audio when playing a game). You can use the su command to temporarily switch to another user identity, and then switch back.

TIP

It is never a good idea to use an Internet Relay Chat (IRC) client as the root user, and you might not want to run it using your regular user account. Just create a special new user just for IRC and su to that user in a terminal widow to launch your IRC client.

The su command spawns a new shell, changing both the UID and GID of the existing user and automatically changes the environmental variables associated with that user. This behavior is known as inheriting the environment. See Chapter 4, 'Command-Line Quick Start,' for more information on environmental variables.

The syntax for the su command is this:

$ su option username arguments

The man page for su gives more details, but some highlights of the su command are as follows:

-c, --command COMMAND
pass a single COMMAND to the shell with -c
-m, --preserve-environment
do not reset environment variables
-l a full login simulation for the substituted user, the same as specifying the dash alone

You can invoke the su command in different ways that yield diverse results. By using su alone, you can become root, but you keep your regular user environment. You can verify this by using the printenv command before and after the change. Note that the working directory (you can execute pwd as a command line to print the current working directory) has not changed. By executing the following, you become root and inherit root's environ ment:

$ su -

By executing the following, you become that user and inherit the super user's environment — a pretty handy tool. (Remember: Inheriting the environment comes from using the dash in the command; omit that, and you keep your 'old' environment.) To become another user, specify a different user's name on the command line:

$ su - other_user

When leaving an identity to return to your usual user identity, use the exit command. For example, while logged on as a regular user, use this:

$ su -

The system prompts for a password:

Password:

When the password is entered correctly, the root user's prompt appears:

#

To return to the regular user's identity, just enter the following:

# exit

This takes you to the regular user's prompt:

$

If you need to allow other users access to certain commands with root privileges, it is necessary to give them the root password so that they can use su — that definitely is not a secure solution. The next section describes a more flexible and secure method of allowing normal users to perform selected root tasks.

Granting Root Privileges on Occasion — The sudo Command

It is often necessary to delegate some of the authority that root wields on a system. For a large system, this makes sense because no single individual will always be available to perform super-user functions. The problem is that UNIX permissions come with an all-or- nothing authority. Enter sudo, an application that permits the assignment of one, several, or all of the root-only system commands.

After it is configured, using sudo is simple. An authorized user merely precedes the command that requires super-user authority with the sudo command, as follows:

$ sudo command

After getting the user's password, sudo checks the /etc/sudoers file to see whether that user is authorized to execute that particular command; if so, sudo generates a 'ticket' for a specific length of time that authorizes the use of that command. The user is then prompted for his password (to preserve accountability and provide some measure of security), and then the command is run as if root had issued it. During the life of the ticket, the command can be used again without a password prompt. If an unauthorized user attempts to execute a sudo command, a record of the unauthorized attempt is kept in the system log and a mail message is sent to the super user.

Three man pages are associated with sudo: sudo, sudoers, and visudo. The first covers the command itself, the second the format of the /etc/sudoers file, and the third the use of the special editor for /etc/sudoers. You should use the special editing command because it checks the file for parse errors and locks the file to prevent others from editing it at the same time. The visudo command uses the vi editor, so you might need a quick review of the vi editing commands found in Chapter 4 in the section 'Working with vi.' You begin the editing by executing the visudo command with this:

# visudo

The default /etc/sudoers file looks like this:

# sudoers file.
#
#
# This file MUST be edited with the 'visudo' command as root.
#
# See the sudoers man page for the details on how to write a sudoers file.
#
# Host alias specification