Linux 版 (精华区)
发信人: netiscpu (说不如做), 信区: Linux
标 题: [B] Red Hat Linux Unleashed (6)
发信站: 紫 丁 香 (Sat Jul 25 03:09:01 1998), 转信
Getting Started with Linux
_________________________________________________________________
o Starting (and Stopping!) Your Linux System
# What's This About Logging In?
# Why You Shouldn't Use the root Login
# Your First Login
o Passwords
o Creating a New Login
o Logging Out
o Trying Out Your New Login
# Linux Error Messages
# Search Paths
o Virtual Terminals
o Commands and Programs
o Summary
_________________________________________________________________
6
Getting 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 Linux
* Logging in and out
* Creating a new user with adduser
* Changing your password
* Using virtual terminals
* Displaying system users with who
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.
darkstar login:
______________________________________________________________
NOTE: A Linux system must always be shut down properly. Improper
shutdown, such as simply turning off your system, can cause serious
damage to your Linux system! When you are finished using your Linux
system, you must shut it down properly, as described in the next
section. If you start to boot Linux, and then change your mind, you
should let the system start up fully and then follow the shutdown
procedure.
______________________________________________________________
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.
______________________________________________________________
NOTE: If you supplied a name to your system when installing Linux,
the system name is used at the prompt. In the login shown earlier,
the system has been called darkstar. The system name enables you to
identify your machine to others when using networks or modem
connections. Don't worry if you didn't name your system yet,
because you can change the system's name at any time.
______________________________________________________________
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.
______________________________________________________________
NOTE: Usually, login names reflect a person's real name. Although
you can't have two identically named logins on your system, you can
easily create logins for users with the same (real) name by having
one or two characters different. So, for example, the login names
suej and suek are treated by Linux as completely separate logins.
Conversely, there is no reason that one human being (for instance,
yourself) can't have two, three, or a dozen login names. In fact,
because you will be the system administrator of your Linux system,
you will have one or more administrative logins, and one or more
regular user logins.
______________________________________________________________
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.
______________________________________________________________
NOTE: The login prompt is actually produced by a program called
login whose only task is to accept your user ID and password,
verify it, and then display a message preventing your access or
letting you through to the next program that starts your user
session.
______________________________________________________________
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.
______________________________________________________________
NOTE: The root login does not restrict you in any way. With one
simple command, issued either on purpose or by accident, you can
destroy your entire Linux installation. For this reason, use the
root login only when necessary. Avoid experimenting with commands
when you do log in as root.
______________________________________________________________
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!
______________________________________________________________
NOTE: System administrator is another term you will see often. A
system administrator is the actual person who sets up and maintains
the Linux system. The amount of work involved in system
administration varies from system to system. A full-time system
administrator may be required in an office for powerful machines
that have many users, peripheral units such as printers and tape
drives, and are connected to a network. Your Linux system will not
require that level of dedication!
______________________________________________________________
______________________________________________________________
NOTE: System administration, because it deals with sensitive
matters such as creating or deleting log-ins, requires superuser
privileges. These privileges are provided by the root login. So,
the sys-tem administrator is an actual person wielding superuser
powers gained by logging in as root.
______________________________________________________________
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
darkstar login:
type
root
and press the Enter key.
Password:
Type in the root password that you set during installation and press
the Enter key.
______________________________________________________________
NOTE: Linux is case-sensitive (as are all UNIX versions). A capital
R is, to Linux, a completely different letter from a lowercase r.
When you type Linux commands, you must use the proper case or Linux
will not understand them. The majority of Linux commands are typed
in lowercase. This includes the login root; if you type Root or
rOoT, Linux will reject the login.
There is a curious exception, though. If you type the login IN ALL
CAPITALS, the system will accept it—but from then on,
everything on your screen will be in capital letters! This is left
over from the days when some terminals only had uppercase letters.
Although these terminals are now all gone or in museums, the login
program retains this historical curiosity.
______________________________________________________________
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
Last login: Sun Dec 11 17:26:18 on tty1
Linux 1.2.13.
You have mail.
If it's Tuesday, this must be someone else's fortune.
darkstar:~#
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:
* darkstar is the system name.
* The ~ character indicates your location in the file system
(explained in Chapter 8, "The Linux File System").
* The # character usually specifically indicates that you're logged
in as root (although the $ sign is used in some operating systems
which makes it difficult to quickly ascertain whether you are
logged in as root or a regular user). According to UNIX
conventions, regular user prompts are either % or $, depending on
the shell; while # is reserved for root. These symbols are called
shell prompts because they are used by the shell to prompt you for
commands.
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.
______________________________________________________________
NOTE: Don't leave your terminal unattended while you're logged in.
The idly malicious may take the opportunity to make some changes to
your files, or send a nasty mail message off to people you'd hate
to alienate. Always log out or lock your terminal when you leave.
______________________________________________________________
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
Changing password for root
Enter new password:
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.
______________________________________________________________
NOTE: Make sure you type the password slowly and carefully! If any
other user's password is lost or forgotten, it can be reset by the
root login. But, if the root password is lost or forgotten, you
must reinstall Linux.
______________________________________________________________
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.
darkstar:~#
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
You misspelled it. Password not changed.
and changes are not made to the password. You need to start over with
the passwd command.
______________________________________________________________
NOTE: Do not forget your new root password! Chant it to yourself
before going to sleep, if necessary. But don't write it down on a
piece of paper and slip it under the keyboard, either!
______________________________________________________________
______________________________________________________________
NOTE: If you want to leave a program right away and return to the
shell prompt, try Ctrl-C (hold down the Ctrl key and press C; this
is sometimes written as ^C). This usually terminates whatever
program you're in (usually without ill effects), and redisplays the
shell prompt.
______________________________________________________________
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
Looking for first available UID_ 505
Looking for first available GID_ 507
Adding login: fido_done.
Creating home directory: /home/fido_done.
Creating mailbox: /var/spool/mail/fido_done.
Don't forget to set the password
To complete adding the user, you'll need to use the passwd command to
set password.
Darkstar:-# passwd
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.
______________________________________________________________
NOTE: The User ID is used by Linux whenever it is referring to
something you have done. The operating system is designed to use a
number rather than the full login name because it takes up less
room and is easier to manipulate. The User ID is important, and
each login on the system has its own unique number. By convention,
UIDs of 500 or less are special system UIDs; root's UID is 0.
Regular users get UIDs starting at 501.
______________________________________________________________
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
Welcome to Linux 1.2.13
darkstar login:
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:
Use "logout" to leave the shell.
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
Password:
Last login: Sun Dec 11 19:14:22 on tty1
Linux 1.2.13.
Quiet! I hear a hacker....
darkstar:~$
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.
______________________________________________________________
NOTE: Linux can be configured to automatically mail a message to
all new users. This can be a greeting, or can give system
information and etiquette.
______________________________________________________________
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
bash: adduser: command not found
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 (discussed in
Chapter 8). 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
bash: /sbin/adduser: Only root may add users to the system.
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
darkstar login:
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
fido tty2 Dec 14 01:42
fido tty1 Dec 14 01:40
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.
Linux also comes with a very powerful multitasking windowing
environment called X. Installing and running X Window systems is
described in Chapter 21, "Installing X Window on Linux."
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."
Summary
In this chapter, we assigned a password to the root login and created
a new user ID to be used in the next few chapters. We tried out Linux
multitasking, and we learned some useful Linux terminology tips that
will serve us well in the future (we will try to avoid seeing too many
error messages, though). At this point, you can either ensure that you
have logged out of all virtual terminals, or move on to the following
chapters.
In the next two chapters, we will become more familiar with using
Linux. In Chapter 7, "Basic Linux Commands and Utilities," we try out
a variety of Linux commands as we exercise our Linux muscles. In
Chapter 8, we learn more about how Linux stores its programs and data,
and explore some of the interesting terrain on your hard drive.
--
Enjoy Linux!
-----It's FREE!-----
※ 修改:．netiscpu 于 Jul 25 03:43:37 修改本文．[FROM: mtlab.hit.edu.cn]
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