What's in the /etc directory?

bashrc & csh.cshrc

These files control the default settings for any users logging in to the computer who use the bash shell or the tcsh shell. Do not mess with them without backing them up; damage to them will render the shells inoperable and make it impossible for anyone to log in at any level.

fstab

This controls the locations to which Linux assigns your hardware. For example, the fstab file in ariel assigns the CD-ROM drive to /mnt/cdrom. When hardware conflicts occur, this is a good file to check. Again, make certain to back up the file before making changes to it.

motd

This is the message file that is displayed to users upon logging in. For a good example of the use of the motd file, Ariel's was:

April 28 -- There were a whole lot of collisions showing up on my hub this
early morning, and I don't know why.  I shut Ariel down until I could
troubleshoot; apologies if this caused any inconvenience.

April 26 -- Well, the power went off this weekend, and because of the way
Ariel's keyboard is plugged in, she didn't come back up.  And I wasn't
around to bring her back up either. (Sorry about that.)  Let me know 
if you lost data due to the crash.

Mar. 30 -- tcsh works again.  If you would rather have that as your
default shell, go ahead and change it now.

This is a good place to put announcements that users should see and pay attention to; examples of this would be planned downtime, explanations of unexpected downtime, or announcemnets of new software.

passwd

This is where all user accounts are stored. NEVER edit this file without first making a backup. For more information, see the passwd page.

shadow

This is a root-accessible-only file that will appear in the /etc directory after password shadowing has been enabled. It contains all the encrypted passwords for the user accounts on the system. There is no reason to modify this file unless it is to set a user's password to * to lock her out of her account.