4.3. The /etc directory

Scripts or directories of scripts to run at startup or when changing the run level. See Chapter 9 for further information.

The user database, with fields giving the username, real name, home directory, encrypted password, and other information about each user. The format is documented in the passwd manual page. The encrypted passwords are much more commonly found in the /etc/shadow these days. This means that almost everything about the user except the password is stored in the passwd file. History and convention make a name change undesirable.

Floppy disk parameter table. Describes what different floppy disk formats look like. Used by setfdprm. See the setfdprm manual page for more information.

Lists the filesystems mounted automatically at startup by the mount -a command (in /etc/rc or equivalent startup file). Under Linux, also contains information about swap areas used automatically by swapon -a. See Section 6.8.5 and the mount manual page for more information. Also fstab usually has its own manual page in section 5.

Similar to /etc/passwd, but describes groups instead of users. See the group manual page in section 5 for more information.

Configuration file for init.

Output by getty before the login prompt. Usually contains a short description or welcoming message to the system. The contents are up to the system administrator.

The configuration file for file. Contains the descriptions of various file formats based on which file guesses the type of the file. See the magic and file manual pages for more information.

The message of the day, automatically output after a successful login. Contents are up to the system administrator. Often used for getting information to every user, such as warnings about planned downtimes.

List of currently mounted filesystems. Initially set up by the bootup scripts, and updated automatically by the mount command. Used when a list of mounted filesystems is needed, e.g., by the df command.

Shadow password file on systems with shadow password software installed. Shadow passwords move the encrypted password from /etc/passwd into /etc/shadow; the latter is not readable by anyone except root. This makes it harder to crack passwords. If your distribution gives you a choice (many do) of whether or not to use shadow passwords then you are highly recommended to do so.

Configuration file for the login command. The login.defs file usually has a manual page in section 5.

Like /etc/termcap, but intended for printers. However it uses different syntax. The printcap has a manual page in section 5.

Files executed at login or startup time by the Bourne or C shells. These allow the system administrator to set global defaults for all users. See the manual pages for the respective shells.

Identifies secure terminals, i.e., the terminals from which root is allowed to log in. Typically only the virtual consoles are listed, so that it becomes impossible (or at least harder) to gain superuser privileges by breaking into a system over a modem or a network. Do not allow root logins over a network. Prefer to log in as an unprivileged user and use su or sudo to gain root privileges.

Lists trusted shells. The chsh command allows users to change their login shell only to shells listed in this file. ftpd, the server process that provides FTP services for a machine, will check that the user's shell is listed in /etc/shells and will not let people log in unless the shell is listed there.

The terminal capability database. Describes by what ``escape sequences'' various terminals can be controlled. Programs are written so that instead of directly outputting an escape sequence that only works on a particular brand of terminal, they look up the correct sequence to do whatever it is they want to do in /etc/termcap. As a result most programs work with most kinds of terminals. See the termcap, curs_termcap, and terminfo manual pages for more information.