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This chapter is from the book

Working as Root

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 rm command like this:

matthew@seymour:~$ sudo rm -rf /

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 you are configuring the file system, for example, or to repair or maintain 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 the super-user (root) without logging in as root can help avoid serious missteps when configuring your system. In Ubuntu, you can use sudo to allow you to execute single commands as root and then quickly return 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 sudo like this:

matthew@seymour:~$ sudo 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 nano editor with line wrapping disabled (thanks to the -w).

Creating Users

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

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

matthew@seymour:~$ sudo useradd ryan

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

matthew@seymour:~$ sudo passwd ryan


Changing password for user ryan.
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's default new user settings by using the command and its -D option, like this:

matthew@seymour:~$ useradd -D
GROUP=100
HOME=/home
INACTIVE=-1
EXPIRE=
SHELL=/bin/sh
SKEL=/etc/skel
CREATE_MAIL_SPOOL=no

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 useradd command has many 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 other aspects of a user's account. See man useradd for more info.

Deleting Users

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

matthew@seymour:~$ sudo userdel -r andrew

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

Shutting Down the System

Use the shutdown command to shut down your system. The shutdown command 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 -h or halt option, followed by the word now or the numeral zero (0), like this:

matthew@seymour:~$ sudo shutdown -h now

or

matthew@seymour:~$ sudo shutdown -h 0

To incorporate a timed shutdown and a pertinent message to all active users, use shutdown's time and message options, like so:

matthew@seymour:~$ sudo shutdown -h 18:30 "System is going down for maintenance this
evening at 6:30 p.m. Please make sure you have saved your work and logged out by
then or you may lose data."

This example shuts down your system and provides a warning to all active users 15 minutes before the shutdown (or reboot). Shutting down a running server can be considered drastic, especially if there are active users or exchanges of important data occurring (such as a backup in progress). One good approach is to warn users ahead of time. This can be done by editing the system Message of the Day (MOTD) motd file, which displays a message to users when they login using the command line interface, as is common on multi-user systems.

It used to be that to create a custom MOTD you only had to use a text editor and change the contents of /etc/motd. However, this has changed in Ubuntu as the developers have added a way to automatically and regularly update some useful information contained in MOTD using cron. To modify how the MOTD is updated, you should install update-motd and read the man page.

You can also make downtimes part of a regular schedule, perhaps to coincide with security audits, software updates, or hardware maintenance.

You should shut down Ubuntu for only a few very specific reasons:

  • You are not using the computer, no other users are logged in or expected to need or use the system, such as your personal desktop or laptop computer, and you want to conserve electrical power.
  • You need to perform system maintenance that requires any or all system services to be stopped.
  • You want to replace integral hardware.

Rebooting the System

You should also use the shutdown command to reboot your system. The fastest way to cleanly reboot Linux is to use the -r option, and the word now or the numeral zero (0):

matthew@seymour:~$ sudo shutdown -r now

or

matthew@seymour:~$ sudo shutdown -r 0

Both rebooting and shutting down can have dire consequences if performed at the wrong time (such as during backups or critical file transfers, which arouses the ire of your system's users). However, Linux-based operating systems are designed to properly stop active system services in an orderly fashion. Other commands you can use to shut down and reboot Linux are the halt and reboot commands, but the shutdown command is more flexible.

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