Here is an explanation of the UNIX file permission system.

Let's say I have a file called

/homes/bell/goats/kinky

that I want to make damn sure nobody else can read.

First cd to my goats directory by typing (go ahead, do it)

cd ~bell/goats

(note:  ~bell is UNIX shorthand for user bell's home directory)

Now type

ls -l

-rw-r-----  1 bell            8 Sep 12 13:09 kinkier
-rw-------  1 bell            9 Sep 12 13:08 kinkiest
-rw-r--r--  1 bell           61 Sep 12 13:07 kinky
 |  |  |
 |  |  |___ OH MY GOD!  This little  r  right here indicates
 |  |       that the whole world (OTHERS) can READ my file!
 |  |
 |  |_____ This little r indicates that my GROUP can read
 |         the file.  (Since my group is "general," this also
 |         means that just about everybody with an account on
 |         math can read my file.)
 |
 |________ This little r indicates that I, the USER, can read
           my own file and the little w means that I can
           write, i.e., edit, my file.

Go ahead and see for yourself.  If you are still in my goats
directory, you can type

more kinky

and you will see my file.  (If you are not in my goats directory,
you will have to type   more ~bell/goats/kinky )

Notice that if you type  more kinkiest, you will get PERMISSION
DENIED because the absence of those little r's means that others
and group members do not have permission to read my file.  (Only
I and my Group can read  kinkier.)

To fix this embarrassing situation, I would cd to my goats
directory and type

chmod og-r kinky

This "changes the mode" of my kinky file, taking away r=Read
permission from o=Others and g=Group members.  (To give them
back, I would type   chmod og+r kinky )

(Donu Arapura notes that if you create a file on your
machine at home and transfer it to the math dept SUNs
via ftp (Fetch on a Mac), it will be world readable by
default.  If the file is sensitive, you'll want to 
make sure to log  in and change permissions immediately.)

You now know enough to save yourself from leaking out sensitive
information (like promotion documents and qualifier exams).

However, there are some more facts about directory permissions
and permission defaults that you might want to know about.

If you use subdirectories to sort your files, you can make the
entire contents of the directory unreadable by making the
directory itself unreadable.  For example, if you have a
directory called "confidential," with filemode

drwx------ 

it doesn't make a difference what the permissions are on files
in that directory, only you will be able to see them.

cd to my goat directory again and type

ls -l -a

drwxr-xr-x  2 bell          512 Sep 12 15:44 ./    <-----+
-rw-r-----  1 bell            8 Sep 12 13:09 kinkier     |
-rw-------  1 bell            9 Sep 12 13:08 kinkiest    |
-rw-r--r--  1 bell          110 Sep 12 15:57 kinky       |
                                                         |
                                                         |
             This line describes the permissions on ./ (which is UNIX
             shorthand for the "present directory," i.e. goats).

drwxr-xr-x  2 bell          512 Sep 12 15:44 goats
|      | |
|      | |_____ This little  x  means that others can cd to goats
|      |
|      |_______ This little  r  means that others can ls goats
| 
|______________ This d means that  goats  is a directory

(x stands for EXECUTE.)

To remove these permissions for Others and Group members, I would
cd to the directory that contains  goats  and type

chmod og-rx goats

If you are constantly worried about security, you will want to change
your "umask" line in your  .cshrc  file.  Here's what mine looks like:

umask   077     # No access to anyone -- the most paranoid choice.

This makes all files I create or copy automatically have the

-rw-------

permission (r and w for u=user only), and all directories I create
have the

drwx------

permission.  (But I can change them with chmod afterwards.)

Here are the relevant lines in your  .cshrc   file to fix.

#umask  022     # Group and others may read and dir search (but not write).
#umask  027     # Group may read and directory search, but not others.
#umask  077     # No access to anyone -- the most paranoid choice.
|
|______ delete this little  #  to make your  umask 077 and make sure
        the  # is in front of the other  umask lines.

The change takes effect the next time you log in.

Type

man chmod

for more details.  (See also the man pages for umask, ls, and groups.)

To see an explanation of the required file permissions
for home pages on the web, you can type

more ~bell/pub/home_page

(or copy the file like this:

cp ~bell/pub/home_page your_file_name

Neil Carlson pointed out something I hadn't known about.  It seems
kind of dumb to have groups if EVERYBODY belongs to the same
"general" group by default.  I discovered how to change
the group ownership of my files to the smaller group "scv"
of which I am a member.  (Type "finger your_login_id" to
find out if you belong to any smaller groups.)

For example, if I want to change the group ownership of a
file called "myfile" to "scv," I would type

chgrp scv myfile

If I want all the new files I create to have scv group
ownership from now on, I might even put the line

newgrp scv

in my .cshrc file.

Type  "man chgrp"  and  "man newgrp" for more details.

-Steve Bell