drwxr--r--  2 anderson      512 Sep 21 14:00 bin
-rw-------  1 anderson      438 Aug  5 10:03 dream.sig
drwx------  2 anderson      512 Oct 11 16:53 mail
-rw-------  1 anderson      438 Aug  5 10:03 propag.sig
-rwx------  1 anderson       39 Sep 10 16:10 vacation

You will notice that the leftmost column of the listing contains the file permissions. Lets analyze the permissions listing a little more closely:

drwxr--r--

The first position will contain a d if this is a directory. The next three positions are the owner's permissions, the next three the group's permissions, and the last three everyone else's permissions. If there is a dash (-) in a position, it means that a particular permission is denied. For example in the above entry we can see that it is a directory, that the owner has full rights to it, and the group and everyone else has read-only access.

To change your permissions, use the chmod command as follows:

pegasus% chmod who[+/-]permissions filename

where who is u for owner (user), g for group, o for everyone else (other), or a for all three (user, group, and other). These can be combined. permissions is the file permissions (r,w,or x, or a combination of the three) as discussed above, and filename is the file or directory to change permissions for. As an example, suppose we want to grant everyone read access to a file named neat.stuff:

pegasus% chmod o+r neat.stuff

Now suppose we want to deny everyone read and write access to a file named private.stuff:

pegasus% chmod go-rw private.stuff

You can also change your default permissions. Your default permissions is the set of permissions that is automatically assigned to any new file that you create (either directly or by any program that you run). To see what your current default permissions are, use the umask command:

pegasus% umask

When you type the above command you will note that a number is displayed. This number is the octal representation of the rwx set for owner, group, and other users. For example, the default mask for most users is 77 (in reality it should be 077, but the leading zero is often dropped) which means that you have read, write, and execute permissions and all others have no permissions.

Now what do these numbers mean? Each number represents permissions that a particular class of users should not have, and is the sum of the following:

4	read (r) permission
2	write (w) permission
1	execute (x) permission

For example, to deny write and execute permissions, you would add 2 and 1 together to get 3. To deny all permissions: 4 + 2 + 1 = 7, which is the default permission as mentioned in the previous paragraph. A value of 0 means that no permissions have been denied.

Now how to use these numbers? You use the umask command again:

pegasus% umask value

where value is the set of three numbers describing what permissions to deny to owner, group, and other. For example, suppose by default we want to grant the owner all permissions, but allow group and other to have only read permission for any file that is created:

pegasus% umask 033

The first value is 0 since we are not denying the owner any permissions. The second and third numbers are 3 since we are denying group and other write (2) and execute (1) permissions (2 + 1 = 3).

WARNING: Be very careful when you change your default permissions so as not to accidentally grant everyone write access to any file that you create.

AUTOMATICALLY EXECUTING COMMANDS WHEN LOGGING ON

This section is for experienced UNIX users only.

If you are an experienced UNIX user and you wish to set up commands that are automatically executed when you log on, please place the commands in a separate file named .localrc (this file will be called from the .cshrc file). Systems maintenance procedures may require replacing all users' .login and .cshrc files from time to time.