UNIX FILE PERMISSIONS


Unix can control file access in a number of ways. There are three levels of 
permissions for three classes of users. To view the permissions on a file use 
the ls command with the -l option. Example:

	C:indyunix:~/public_html/pfind>ls -l
	total 40
	-rw-r--r--   1 dbewley  staff        139 Jun 18 14:14 home.html
	-rwxr-xr-x   1 dbewley  staff       9145 Aug 14 07:06 pfind
	drwxr-xr--   2 dbewley  staff        512 Aug 15 07:11 tmp

The permissions are listed in the first column as a string nine characters 
precluded by a file type character which we won't cover. Usually the first 
character will be a dash or a letter d. The dash indicates that this entry 
is a regular file. A letter d tells us that this entry is a directory. So 
what does all that other junk mean?

Permissions should be thought of in groups of three, for the three classes of 
user. 

	user    group   others
	r w x   r w x   r w x

The three levels of permissions are: 

	r  read the file or directory
	w  write to the file or directory
	x  execute the file or list the directory

Each of these permissions can be set individually for the three classes of 
users:

	u  user, the owner of the file or directory
	g  group, members of the group to which the file belongs
	o  other users, everyone else

If a permission is not available its position is filled with a dash.

Examples:

	ls -l hform.html
	-rwx------   1 dbewley  staff      11816 May  9 09:19 slideshow.pl

The owner, dbewley has full permissions - read, write, and execute for this 
file. The group, staff, and everyone else have no permissions.  

NOTE: Since perl scripts are not compiled the must be read by the perl 
interpretor each time they are ran. Therefore perl scripts unlike compiled 
programs must have execute AND read permissions.

	ls -l pfind.pl
	-rwxr-x---   1 dbewley  staff       2863 Oct 10 1995  pfind.pl

This time the owner has full access while the group staff can read and execute 
the file. All others have no permissions for this file.

	ls -l schedule.html
	-rw-r--r--   1 dbewley  staff       2439 Feb  8 1996  schedule.html

This is the normal permission of an HTML file. Everyone can read it, but 
only the user can modify or delete it. There is no need have execute 
permission.


CHANGING PERMISSIONS

The chmod command is used to change permissions on files. The chmod command 
recognizes the three classes of user as u, g, and o and the three levels of 
permissions as r, w, and x. It grants and revokes permissions with a + or -. 
It also will accept a for all three classes of users at once.

Use the chmod command to change permissions:
	chmod <permissions> <file>

Examples:
	
	ls -l pfind.pl
	-rw-------   1 dbewley  staff       2863 Oct 10 1995  pfind.pl
	chmod u+x pfind.pl
	ls -l pfind.pl
	-rwx------   1 dbewley  staff       2863 Oct 10 1995  pfind.pl

This added execute permission for the owner of pfind.pl. To add these 
permissions for the group staff and others use go+rx. Remember, users must 
have at least read and execute permissions to run perl scripts.
	
	ls -l pfind.pl
	-rwx------   1 dbewley  staff       2863 Oct 10 1995  pfind.pl
	chmod go+rx pfind.pl
	ls -l pfind.pl
	-rwxr-xr-x   1 dbewley  staff       2863 Oct 10 1995  pfind.pl

Now, any user can read and execute pfind.pl. Let's say a serious bug was 
found in pfind.pl and we don't want it to be executed by anyone. To revoke 
execute permission for all classes of user use a-x.

	ls -l pfind.pl
	-rwxr-xr-x   1 dbewley  staff       2863 Oct 10 1995  pfind.pl
	chmod a-x pfind.pl
	ls -l pfind.pl
	-rw-r--r--   1 dbewley  staff       2863 Oct 10 1995  pfind.pl

Now, all users can read pfind.pl, but no one can execute it.