Logging In

Once you have access to your UNIX system, you will need to log in with your username and password. The UNIX operating system was designed for multiple users. Requiring each user to log in ensures that the system remains secure, and that each user’s files remain private.

Selecting a Login Name

Every UNIX system has at least one person, called the system administrator, whose job is to maintain the system. The system administrator is also responsible for adding new users to the system, and for setting up the initial work environment. If you are on a multiuser system, you will have to ask the system administrator to set up a login for you. If you are the only user on the system, you will be the system administrator. During the installation of your UNIX variant, you will be asked to select a login name and password.

In general, your login name can be almost any combination of letters and numbers, although there are a few constraints:

• Your login name must be more than two characters long. If it is longer than eight, only the first eight characters are relevant.

• It must contain only lowercase letters and numbers and must begin with a lowercase letter. No symbols or spaces are allowed.

• It cannot be the same as another login name already in use. Some login names are customarily reserved for certain uses; for example, root is often a login name for the system administrator (sometimes called the superuser).

Choosing a login name is similar to choosing an e-mail address. In fact, your login name will become your e-mail address on the UNIX system. Try to pick a login name that is easy to spell and type, and that other users will associate with you. Names (nate), initials (raf), and combinations of names and initials (susanl, jfarber) are common. For example, a user named Marissa Silverman might choose marissa, msilver, mars, or mls as a login name. Of course, you could also choose something unrelated to your name, such as yoda01. Keep in mind that your login name is how you will be known on the system, so it is important to choose something that won’t become embarrassing or confusing later. In some cases the system administrator may select a login name for you.

Choosing a Password

If you begin by installing a UNIX variant on your own system, it will ask you to choose a password when you select a login name. If your account is on a remote system, your system administrator will probably assign you a temporary password, which you should change the first time you log in. UNIX places some requirements on passwords, typically including the following:

• Passwords must have at least six characters.

• Passwords must contain at least two alphabetic characters (uppercase or lowercase letters), and at least one number or symbol. Note that UNIX is sensitive to case, so WIZARD is a different password than w1zard.

• Your login name with its letters reversed or shifted cannot be used as a password. For example, if your login name is msilver, you cannot choose silverm or revlism as a password.

The passwords 3hrts&3lyonz and R0wkS+@r are both valid, but kilipuppy (no numeric or special characters) and Red1 (too short) are not.

UNIX System Password Security

Your first contact with security on your UNIX system is choosing a password. Simple passwords are easily guessed. A large commercial dictionary contains about 250,000 words, which can be checked as passwords in less than two minutes of computer time. All dictionary words spelled backward takes about another minute. All dictionary words preceded or followed by the digits 0–99 can be checked in just a few more minutes. Similar lists can be used for other guesses.

Here are some guidelines:

• Avoid easily guessed passwords, such as your name or the names of family members or pets.

• Also avoid your address, your car’s license plate, and any other phrase that someone might associate with you.

• Avoid words or names that exist in a dictionary (in any language, not just English).

• Avoid trivial modifications of dictionary words. For example, normal words with replacement of certain letters with numbers: mid5umm3r, sn0wball, and so forth.

Pronounceable nonsense words can make good passwords, such as 38fizwik, 6nogbuf7, or met04ikal. These passwords are very difficult to guess, but because they can be pronounced, they are often easy for you to remember.

Resist the temptation to write your password down. In particular, do not stick it to your screen or leave it on your desk. If you have to write it down, keep it in a safe place. If someone gains access to the UNIX system with your password, they will have access to all of your work-they may even be able to find a way to access restricted parts of the system once they are logged in.

Caution 

If you do forget your password, there is no way to retrieve it. Because it is encrypted, even your system administrator cannot look up your password. If you cannot remember it, the administrator will have to give you a new temporary password.

A Successful Login

When you successfully enter your login name and password, the UNIX system responds with a set of messages, similar to this:

 login: corwin Password: Last login: Tues June 27 09:55:17 on tty1 ****************************************************************** *                       Welcome to amber!                        * *  Red Hat Linux release 9 (Shrike) Kernel 2.4.20–8 on an 1686   * *             Report system problems to action@amber             * *                                                                * * amber will be coming down on Sunday Aug 28, 2006 from          * * 8:00am until 12:00pm (noon) for system maintenance.            * * Please schedule your work accordingly. Thank you.              * ****************************************************************** You have new mail $

At the top is a line that tells you when you last logged in. This is a security feature. If the time of your last login seems wrong, call your system administrator. This discrepancy could be an indication that someone has broken into the system and is using your login name.

This is followed by the message of the day (motd). Because every user has to log in, the login sequence is a natural place for your system administrator to put important messages. This sometimes includes general system information, such as the e-mail address for system problems, and often includes important announcements, such as system changes or shutdowns.

In some cases, you may also see other messages when you log in, like the line “You have mail” shown above. In Chapter 4, you will learn how to configure your account to display custom information, such as a list of other users who are currently logged in.

An Incorrect Login

If you make a mistake in typing either your login name or your password, the UNIX system will respond this way:

 login: corwin Password: Login incorrect login:

The system will prompt you to enter a password even if you type an incorrect login name. This prevents someone from guessing login names and learning which ones are valid by discovering the ones that yield the “Password:” prompt.

If you repeatedly type your login or password incorrectly (three to five times, depending on how your system administrator has set the default), the UNIX system may disconnect you, although you will not get locked out of your account. On some systems, the system administrator will be notified of erroneous login attempts as a security measure.

If you have problems logging in, you might check to make sure that your CAPS LOCK key has not been set. If CAPS LOCK has been turned on, you will inadvertently enter an incorrect login name or password, because in UNIX uppercase and lowercase letters are treated differently

The UNIX System Prompt

After you successfully log in, you will see the UNIX System command prompt at the far left side of the current line. The default prompt on many UNIX systems is the dollar sign:

 $

This $ is the indication that the UNIX system is waiting for you to enter a command.

Note 

In the examples in this book, you will see the $ or other prompt at the beginning of a line as it would be seen on the screen, but you are not supposed to type it.

The default prompt may be different on your system. You may see a percent sign (%), or a string of characters, such as ~>, -bash-2.05b$, or corwin@amber:~%. The command prompt is frequently changed by users. In Chapter 4, you will learn how to customize the prompt for yourself.

Graphical Environments

On some systems, when you first log in you may be sent directly to the X Window environment. This is a graphical environment for UNIX. Chapters 6 and 7 of this book describe how to use and configure the most common versions of the X Window System.

It is also possible to set up X when you are using a remote connection to a UNIX system. To do this, you will need a tool like Cygwin/X or VNC (on a PC), or X11 (in Mac OS X). Chapter 18 has information on configuring these to run on your machine.

Many of the most powerful features of UNIX are best accessed through text commands. If you are using a graphical environment, you will need to open a terminal window to see the command prompt so that you can enter these commands. The name of the terminal program varies according to which environment you are using. One very common program is called xterm; others include konsole and gnome-terminal.