|
Red Hat Linux: Getting Started With LinuxGetting Started with Linux
Congratulations! Now that you have successfully installed Linux, you can start
using it. In this chapter we will look at the steps you need to take to begin
working with Linux, including the following:
Starting (and Stopping!) Your Linux System Depending on the setup you chose during Linux installation and configuration,
either Linux starts automatically when you power on your computer, or it
requires you to type something (such as Linux) to specify that you want to boot
Linux. As your Linux system starts up, you see quite a few Linux initialization
messages scroll through your screen. When Linux has completed its startup, you
should see the following prompt: Welcome to Linux 1.2.13.
Because you know how to start Linux, it's even more important to know how to
shut it down properly. Like many UNIX systems, if Linux is not powered down
properly, damage to files can result. The easiest way to ensure a proper
shutdown is to press the Ctrl, Alt, and Delete keys simultaneously. (This is the
famous Ctrl-Alt-Delete "three-finger salute" used in DOS.) Pressing Ctrl-Alt-Delete causes a number of advisory messages and Linux
shutdown messages to be displayed. You must wait until the Linux shutdown
procedure has finished, at which point your monitor shows the initial "power-on"
screen, before turning your computer off. What's This About Logging In? Linux waits for a login. A login is simply the name that you supply to Linux
to identify yourself to the operating system. Linux keeps track of which names
are permitted to log in or access the system, and only allows valid users to
have access.
Every login name on the system is unique. Normally, a password is assigned to
each login, too. This secret password is like the identification number you use
with your bank card to prove that you really are who you say you are. Also, the
things you can do with your login—the login's privileges—are controlled by
Linux; different logins have different levels of privileges.
At the login prompt, try typing your name, your dog's name, or any other
random name that occurs to you. None of these are valid logins (at least not
yet). The system asks you for a password; it won't matter what you type, so just
press Enter or type a random string of characters. Because the logins are not
valid on the system, Linux won't let you in. It displays the message Login
incorrect to tell you that either the name or the password you entered is not
valid. The only valid login on your Linux system after installation is the most
powerful and dangerous login Linux offers: root. In the section "Creating a New
Login," later in this chapter, we will create a safe login for you to use. This
login can have your name, your dog's name, or whatever else you choose.
Why You Shouldn't Use the root Login You will have to use the root login from time to time. Some things simply
cannot be done on the Linux system without logging in as root. You should not,
however, use the root login as your regular login. This is especially true if
you are exploring the system, poking around, and trying out new commands that
may not do what you thought they would! Linux, as you already know, is a multiuser, multitasking operating system.
Multiuser means that several people can be using Linux at the same time (of
course, you have to add some additional terminals to your system, or it will get
very crowded around the keyboard). Multitasking means that Linux can do more
than one thing at a time. For example, you can spell-check a document while
downloading information from some remote system. (Multiuser implies
multitasking, because all users must be able to do their own work at the same
time.) Linux, therefore, is very good at juggling all these tasks, keeping them
from interfering with each other, and providing safeguards so that you cannot
damage the system or another user's work.
When you log in as root, you become the system. The root login is also
sometimes called the superuser login, and with good reason. To use an analogy,
instead of being a passenger on an airplane, you suddenly have all the
privileges of the flight crew, the mechanics, and the cabin crew. "Hmm, what
does this do?" becomes an extremely dangerous phrase when logged in as root. One of the oldest stories in UNIX lore tells of new users who log in as root
and, in ten keystrokes, destroy their system completely and irrevocably. But if
you're careful to follow the steps given here, and stop and take a moment to
think about the commands you are giving, none of the "How many new users does it
take to nuke a system?" jokes will apply to you!
Your First Login After all the cautions about using the root login, we're going to have you
log in as root. Because root is the only authorized login on a newly installed
Linux system, this is unavoidable. Also, we will be performing a couple of
important procedures that require root privileges. However, after this first
login, we will create a user ID that can prevent accidental damage to the
operating system. At the login prompt type and press the Enter key. Type in the root password that you set during installation and press the
Enter key.
After you have logged in as root, the system starts up a user session for
you. At this point you should see the following on your screen: darkstar login: root Linux tells you when the login for this user was last recorded (although this
information may not appear the very first time you log in), and then provides
you with some version information. Linux also tells you that this login has a
mail message waiting to be read. Finally, if games were installed on your
system, Linux gives you a witty saying or aphorism. It is always good practice to scan the line that starts with Last login, and
check that the time given is correct. This is especially important if your Linux
system is accessed by other users or connected to other systems. If the time
given does not look right, it could be that someone is using the login to break
into your system, or using your username without your permission. We will read the mail message later, after taking care of some important
steps. If you are curious, the same mail message is sent by the install
procedure when the operating system is installed. It concerns registration
matters for Linux. Your "fortune" is chosen randomly from a long list, so don't expect to see
the same one shown in the previous example. If you didn't install the games
package during the Linux installation routine, you won't see a fortune. You can
install the games package at any time. The final line you see on the screen is the system prompt. This tells you
that Linux is waiting for you to type in your commands—it's prompting you for
input. The system prompt also displays the following useful information:
Passwords In Linux (and just about all other UNIX systems) the superuser login name is
root. No matter how humble or huge the system, if you can log in as root, the
system is wide open for you to do whatever you want. Obviously, letting just
anyone log in as root is unacceptable because it exposes the system to too much
potential for serious damage. To prevent unauthorized access, the root login should always have a password,
and that password should be secure. You may have noticed that the Red Hat
installation requested that you set a root password during the installation
process. Some Linux installations do not set the root password; it is set to a
null string, which is a word with no characters. With root and any other login,
Linux does not bother asking for the password if it's the null string. The null string is the least secure password there is, because anyone who
knows a valid username (such as root) can access the system. It is up to you to
change the password. Linux lets you choose what the new password will be, and
accepts it without complaint. Unfortunately, this can lead to a false sense of
security. It was noticed a long time ago that users chose passwords that they could
easily remember: their dog's name, their birthday, their hometown, their
spouse's name, and so on. The problem is that these passwords were also easy to
break, either through guessing or by more sophisticated means. This led some
system administrators to insist on difficult-to-break, randomly picked passwords
(such as S8t6WLk). People could not remember these passwords at all, so they
wrote them down on pieces of paper and stuck them on their desks. Others, who
were trying to break into the system, would find these pieces of paper and gain
use of that login. The best passwords are ones with a combination of uppercase letters,
lowercase letters, and numbers, that are still easy to remember. Fri13th,
22Skidoo, and 2Qt4U are just a few examples. These hard-to-guess passwords are
known as strong passwords, while easy-to-guess ones are called weak. Of course, you should never use these exact passwords, or any other published
sample passwords, because they're so easy to guess. There are many mischievous
minds out there who, on strolling by a Linux system, might try root and Fri13th
for the fun of it. You don't want to be the one with the nightmare of getting
your system broken into. For the best security, passwords should be changed every so often. Many
system administrators recommend once every two or three months as reasonable.
This guards against dictionary-based guessing attacks, and also minimizes damage
in cases in which the password has been broken but nothing has really been done
with it yet.
Of course, the amount of system security you require depends on how much
access there is to your system, and how sensitive is the information found on
it. The root password should always be a good, secure one. If nothing else, it
will discourage you from casually logging on as root, especially if you leave
your user logins with null passwords. If you are using Linux at home for experimenting, much of the security
worries mentioned previously may seem silly. However, it doesn't hurt to use
good security, and the practice can be carried over to larger UNIX systems at
work. We must assign a password for the root login using the Linux command passwd.
The spelling of the command has its history in the development of UNIX, when
long commands, such as password, were avoided due to the number of characters
that had to be typed! To change the root password at the system prompt, type the
command passwd, and you see the following: darkstar:~# passwd At the prompt, type your new, secure password. What you type is not displayed
on the screen. This keeps anyone looking over your shoulder (called "shoulder
surfing") from reading the password you've entered.
Because it's so important that passwords are entered correctly, the system
double-checks the spelling of the password for you by asking you to type it
again: Re-type new password: Again, what you type is not displayed on the screen. If your two password
entries match, you see the following: Password changed. The password is now changed in the system's configuration files. If the two
entries do not match completely (remember, case is important), Linux gives you
the message and changes are not made to the password. You need to start over with the
passwd command.
Creating a New Login Now that you have assigned a password for the root account, the next thing
you should do is create a login with which you can safely explore the Linux
system and try out some of the basic commands covered in the following chapters.
Login names are used by valid system users. You can create a login for
yourself that you will use permanently, or you can create a temporary login for
exploring the system and remove it later. Login names can be any character or
number string you want. Typically, login names bear a resemblance to the user's
real name, so Joe Smith's login name may be joe, jsmith, or joes. Linux has a utility called adduser, which simplifies and automates the task
of adding a new user to the system. (This isn't how they did it in the good old
days. You should be glad. In the past, files had to be manually edited to add
users, a tedious and error-prone process.) To create a user, at the shell prompt type adduser and the username you want
to add. darkstar:-# adduser fido To complete adding the user, you'll need to use the passwd command to set
password. The adduser command included with Red Hat automatically assigns the UID, GID,
and the home directory for the user added. The adduser command included with
other Linux distributions will prompt you for these parameters. An even more user-friendly method of adding users is to use the "user and
group management" portion of the control-panel available under X.
Logging Out Now that you have created a new user, you can use it in the next couple of
chapters to explore Linux. To finish with your session as root, log out of the
system by typing logout: darkstar:~# logout You see the login prompt displayed again. At this point, you can log back in
as root, or as the new user you have just created. Some systems enable you to log out with the Ctrl-D sequence. If the shell you
are using supports Ctrl-D as a logout command, the login prompt reappears.
Otherwise, you may see a message such as this: If you have used other UNIX systems before, you may be used to using Ctrl-D
to log out. The default shell used by Linux does not support Ctrl-D unless the
keymappings are changed to allow it. Trying Out Your New Login Now we can try out our new login. We can also look at some of the interesting
features and capabilities of Linux. At the login prompt, type the login name you have just created. If you were
conscientious and assigned a nonzero-length password to your new login, enter
the password when prompted. You should now see the following: darkstar login: fido Note that your prompt looks different from the root prompt. The $ prompt
indicates that you are a regular user running under the bash shell (which was
the default choice presented by the adduser program). Also, there is no You have
mail message.
To see an example of the difference between the root login and a regular user
login, type adduser at the shell prompt and press Enter. darkstar:~$ adduser The message you get looks somewhat cryptic. However, it has a typical Linux
error message structure, so it's worth taking a little effort to understand it.
Linux Error Messages First of all, the program that's giving you the message is your shell, bash.
It therefore announces itself with bash:, somewhat like the character in a play
script. Next is the shell's "monologue." Being the "strong and silent" type of
character, bash's monologue is very terse and to the point. It declares the
object that is causing it problems (adduser), and the problem with this object:
the command (adduser) can't be found. If the error message were expanded into real English, it would go something
like this: "Hi, I'm bash. You know that adduser command you gave me? I looked
everywhere for adduser but I couldn't find it, so I couldn't perform whatever
actions adduser would have specified." With time, you will get quite good at
understanding Linux error message grammar. Search Paths Why can root find adduser, but an ordinary user cannot? Linux has many
directories, and each directory can hold many files (one of which can be the
elusive adduser). In theory, Linux could go search through the file system until
it found adduser. But if root accidentally mistyped adduser as aduser, Linux
would have to rummage through every nook and cranny before finally giving up.
This could take 10 or more seconds, and cause needless wear and tear on your
hard drive. Therefore, Linux has search paths for finding commands . Usually, only a
small part of the entire Linux file system is on the search path, which
literally is the path along which Linux searches. Because root makes use of many
system administration programs such as adduser, the directories that hold these
programs are in root's search path. Ordinary users do not have system
administration directories in their search path. However, if you explicitly tell Linux where a file is located, it does not
need to look through its search path. As it happens, adduser is found in the /sbin
directory. Try running /sbin/adduser. darkstar:~$ /sbin/adduser This time, bash could find adduser (because you told it exactly where to
look), but discovered that an ordinary user does not have permission to run
adduser. As you can see, Linux limits the actions of logins to their privilege
level. Virtual Terminals Linux, as mentioned earlier, is a multiuser, multitasking system. This means
that more than one login can access the system at the same time, and that each
login can be doing one or more different things all at the same time. A serious
multiuser system will have several terminals (consisting of a keyboard and a
display screen) wired or networked to the main computer unit. Although you probably don't have any terminals attached to your system, you
still can log in several times under the same or different login names, using
your single keyboard and screen! This magic is performed by using virtual
terminals. Press Alt-F2. When you do so, everything on your screen should disappear, to
be replaced by the following: Welcome to Linux 1.2.13 Log in as your "regular" login (not root). When the shell prompt is
displayed, type who at the prompt and press Enter. You should see the following:
darkstar:~$ who When you run the Linux command who, your screen displays the names of all
logins currently logged into the system, and where they are logged in from.
(Your login name will appear, of course, instead of fido in the preceding
example.) By convention, tty1 is the main console screen. It is the "normal" one that
appears after Linux has started up, so you don't have to do anything special to
get it. If you have switched to any other virtual consoles, you can return to
tty1 by pressing Alt-F1. How many virtual screens are active on your system? Try going through all the
Alt-Fn keys. Alternatively, you can scroll through the virtual screens by using
the Alt-right arrow combination to move up through the screens, or Alt-left
arrow to move down. Quite often you find yourself doing something, perhaps in a long and
complicated program, and realize that you should have done something else first.
Flip to another virtual terminal and do whatever it is. Another handy use of virtual terminals is when, through experimentation or
otherwise, your screen locks up or starts typing in strange symbols. From a
different virtual terminal, you can try to fix things, or restart the system if
necessary. Commands and Programs "Run the who command" and "Run who" are much more common ways of saying "Type
who at the prompt and press Enter." We will use the shorter expressions wherever
their meaning is clear. Sometimes people familiar with Linux drop the word
"run," so that one user might tell another, "I tried who but didn't see anything
unusual." It's understood by the context that when they "tried who," they
actually ran it. Something else you will notice if you are reading carefully is that there
seem to be both Linux programs and Linux commands. A command is what you type at
the shell prompt. For this reason, the combination of the shell prompt and what
you type after it is often called a command line. When you press the Enter key,
Linux takes the command you've entered and tries to perform it. The Linux system
has built-in responses to some commands; for other commands it finds the
appropriately named program on your hard disk and executes that program. In the strictest sense, then, the command is what you type, and the program is what performs your command. However, very simple programs with straightforward results, such as who, are often referred to as commands, although there is actually a who program on your hard disk. More complicated programs, usually interactive such as adduser, or open-ended such as a text editor, are called programs. So you might hear one experienced user tell another, "The adduser program worked fine. I tried the who command 15 minutes later and the new user had logged in already." |
