Andrew Hudson - Fedora™ Unleashed, 2008 edition

The emacseditor uses an extensive set of keystroke and named commands, but you can work with it by using a basic command subset. Many of these basic commands require you to hold down the Ctrl key, or to first press a meta key (generally mapped to the Alt key). The basic commands are listed in Table 4.2.

TABLE 4.2 Emacs Editing Commands

One of the best reasons to learn how to use emacsis that you can use nearly all the same keystrokes to edit commands on the bashshell command line. Another reason is that like vi, emacsis universally available on nearly every UNIX and Linux system, including Apple's Mac OS X.

The root, or super-user account, is a special account and user on UNIX and Linux systems. Super-user permissions are required in part because of the restrictive file permissions assigned to important system configuration files. You must have root permission to edit these files or to access or modify certain devices (such as hard drives). When logged in as root, you have total control over your system, which can be dangerous.

When you work in root, you can destroy a running system with a simple invocation of the rmcommand like this:

# rm -fr /

This command line not only deletes files and directories, but also could wipe out file systems on other partitions and even remote computers. This alone is reason enough to take precautions when using root access.

The only time you should run Linux as the super-user is when booting to runlevel 1, or system maintenance mode, to configure the file system, for example, or to repair or main tain the system. Logging in and using Linux as the root operator isn't a good idea because it defeats the entire concept of file permissions.

Knowing how to run commands as root without logging in as root can help avoid serious missteps when configuring your system. Linux comes with a command named su that enables you to run one or more commands as root and then quickly returns you to normal user status. For example, if you would like to edit your system's file system table (a simple text file that describes local or remote storage devices, their type, and location), you can use the su command like this:

$ su -c "nano -w /etc/fstab"

Password:

After you press Enter, you are prompted for a password that gives you access to root. This extra step can also help you "think before you leap" into the command. Enter the root password, and you are then editing /etc/fstab, using the nanoeditor with line wrapping disabled.

Before editing any important system or software service configuration file, make a backup copy. Then make sure to launch your text editor with line wrapping disabled. If you edit a configuration file without disabling line wrapping, you could insert spurious carriage returns and line feeds into its contents, causing the configured service to fail when restarting. By convention, nearly all configuration files are formatted for 80-character text width, but this is not always the case. By default, the viand emacseditors don't use line wrap.

You can use sudoto assign specific users or groups permission to perform specific tasks (similar to BSD UNIX and its "wheel" group of users). The sudocommand works by first examining the file named sudoersunder the /etcdirectory; you modify this file with the visudocommand. See the section "Granting Root Privileges on Occasion — The sudoCommand" in Chapter 10, "Managing Users," for details on how to configure and use sudo.

When a Linux system administrator creates a user, an entry in /etc/passwdfor the user is created. The system also creates a directory, labeled with the user's username, in the /homedirectory. For example, if you create a user named bernice,the user's home directory is /home/bernice.

In this chapter, you learn how to manage users from the command line. See Chapter 10 for more information on user administration with Fedora using graphical administration utilities, such as the system-config-usersclient.

Use the useraddcommand, along with a user's name, to quickly create a user:

# useradd andrew

After creating the user, you must also create the user's initial password with the passwd command:

# passwd andrew

Changing password for user andrew.

New password:

Retype new password:

passwd: all authentication tokens updated successfully.

Enter the new password twice. If you do not create an initial password for a new user, the user cannot log in.

You can view useradd'sdefault new user settings by using the command and its -Doption, like this:

# useradd -D

GROUP=100

HOME=/home

INACTIVE=-1

EXPIRE=

SHELL=/bin/bash

SKEL=/etc/skel

CREATE_MAIL_SPOOL=yes

These options display the default group ID, home directory, account and password policy (active forever with no password expiration), the default shell, and the directory containing defaults for the shell.

The useraddcommand has many different command-line options. The command can be used to set policies and dates for the new user's password, assign a login shell, assign group membership, and manage other aspects of a user's account.

Use the userdelcommand to delete users from your system. This command removes a user's entry in the system's /etc/passwdfile. You should also use the command's -roption to remove all the user's files and directories (such as the user's mail spool file under /var/spool/mail):

# userdel -r andrew

If you do not use the -roption, you have to manually delete the user's directory under /home, along with the user's /var/spool/mailqueue.

Use the shutdowncommand to shut down your system. The shutdowncommand has a number of different command-line options (such as shutting down at a predetermined time), but the fastest way to cleanly shut down Linux is to use the -hor halt option, followed by the word nowor the numeral zero ( 0), like this:

# shutdown -h now