What happens when you log in?

The Unix and Internet Fundamentals HOWTO
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# 4. What happens when you log in?

When you log in, you identify yourself to the computer. On modern Unixes you will usually do this through a graphical display manager. But it's possible to switch virtual consoles with a Ctrl-Shift key sequence and do a textual login, too. In that case you go through the getty instance watching that console tto call the program login.

You identify yourself to the display manager or login with a login name and password. That login name is looked up in a file called /etc/passwd, which is a sequence of lines each describing a user account.

One of these fields is an encrypted version of the account password (sometimes the encrypted fields are actually kept in a second /etc/shadow file with tighter permissions; this makes password cracking harder). What you enter as an account password is encrypted in exactly the same way, and the login program checks to see if they match. The security of this method depends on the fact that, while it's easy to go from your clear password to the encrypted version, the reverse is very hard. Thus, even if someone can see the encrypted version of your password, they can't use your account. (It also means that if you forget your password, there's no way to recover it, only to change it to something else you choose.)

Once you have successfully logged in, you get all the privileges associated with the individual account you are using. You may also be recognized as part of a group. A group is a named collection of users set up by the system administrator. Groups can have privileges independently of their members’ privileges. A user can be a member of multiple groups. (For details about how Unix privileges work, see the section below on permissions.)

(Note that although you will normally refer to users and groups by name, they are actually stored internally as numeric IDs. The password file maps your account name to a user ID; the `/etc/group` file maps group names to numeric group IDs. Commands that deal with accounts and groups do the translation automatically.)

Your account entry also contains your home directory, the place in the Unix file system where your personal files will live. Finally, your account entry also sets your shell, the command interpreter that login will start up to accept your commmands.

What happens after you have successfully logged in depends on how you did it. On a text console, login will launch a shell and you'll be off and running. If you logged in through a display manager, the X server will bring up your graphical desktop and you will be able to run programs from it — either through the menus, or through desktop icons, or through a terminal emulator running a shell.

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