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Unix Permissions

NOTE: This Tip Sheet assumes that the reader has a copy of a telnet application like EWAN or NCSA Telnet, and is familiar with its use. If you do not have a telnet application, you can get one several ways. If you are a SkyPoint Macintosh user you can download the standard SkyPoint software package, which includes NCSA Telnet. Windows 95 users have a telnet window already installed. Otherwise, all platform users can access www.shareware.com on the Web, do a search for telnet applications, and download one for free.

What Are Permissions?

Permissions dictate who may enter, read from, write to, or run a file in a directory. They also determine who may read, write, or run (execute) an individual file. Each of these functions can be set separately. UNIX categorizes possible users into three types - the owner of the file (user), members of the owner's group (group), and the rest of the world (other). When deciding who you want to have a particular type of access to a file or directory, you will have to make a decision for each of these three categories.

How Do I Know What Permissions Are On A File Or Directory?

To see the permission on every file and subdirectory in your current directory, type ls -l and hit the return key. You will see a list like the one below.

total 14          
drwxrwxrwx 2 aquila skyshell 512 Jul 8 1996 Mail/
-rwxrwxrwx 1 aquila skyshell 0 Jan 16 09:46 config.sys*
lrwxrwxrwx 1 aquila skyshell 35 Feb 6 13:09 ftp -> /var/spool/ftp/pub/members/a/aquila/
-rw-r--r-- 1 aquila skyshell 1756 Apr 2 13:13 rhostinfo

The file, link, or directory name is shown at the far right of each line. The permission for that entry is shown at the far left field of each line. The second field from the left shows the owner, here user "aquila," and the third field from the left shows the owner's group, here "skyshell."

Look at the permissions field carefully. To read the code, divide the field into four parts: the first letter, then three sets of 3 letters. The first letter can be a "-", "l", or "d". The groupings of three can only show "r", "w", "x", and "-". For example, the "rhostinfo" permissions would be divided into:

- | rw- | r-- | r--

The first part, the single letter or dash, signifies what the named entry is. For example, if it had a "d" in it, it would mean that the entry was a directory. In the above example there is only a "-", meaning that this is a regular file.

The leftmost grouping of three shows the owner's permission. In your home directory, you are the owner. Here, the file's owner can read (r) and write (w) this file. Writing includes the ability to delete. The "-" shows that the file can not be executed by the owner, as it is placed in the spot reserved for the "x" permission.

The second set of three shows the group's permission. In the above example, the group is "skyshell." Skyshell is the group that most of SkyPoint's users belong to. The second set of three therefore applies to anyone in the skyshell group, which is an awful lot of people. Here, the read (r) permission is turned on, but the write (w) and execute (x) permissions are off. That means that anyone in that group could read or copy the file, but could not delete, change, or run it.

The third, rightmost set are the set of permissions for the rest of the world (other). Here, they are the same as for the group.

How Do I Change The Permissions? - chmod

Usage: chmod [option/operation/mode] [filename]

Option (permission type): u =user (owner)
g =group
o =other
Operation: + =add permission
- =remove permission
= = assign permission and remove permission from all other fields
Mode: r =read
w =write
x =execute

The easiest way to explain how to use chmod is to give an example and let you see how it works. Let's do a few examples:

Example 1:

-rw-r--r-- 1 aquila skyshell 1756 Apr 2 13:13 rhostinfo

Let's say that we want the owner (u) of this file to be able to execute this file (if it was executable) as well as reading and writing to it, we want the group (g) to be able to read it only, and we don't want the rest of the world (o) to have any access to it at all. Here's the command we would use to make those changes:

chmod u+x,o-r rhostinfo

Notice that we have added (+) the execute (x) permission to the owner (u), left the group (g) untouched by not mentioning it in the command, and removed (-) the read (r) permission from the rest of the world (o). You should also note that the command does not have a space after the comma - this is very important, because if you had a space after the comma the command would not work. The permissions on rhostinfo, if we did an ls -l rhostinfo, would be:

-rwxr----- 1 aquila skyshell 1756 Apr 2 13:13 rhostinfo

Example 2:

-rwxr----- 1 aquila skyshell 2505 May 4 1996 test*

In this example, we will pretend that test is an executable file. We want the owner (u) to be able to read (r), write (w), and execute (x) the file. We also want the group (g) and the rest of the world (o) to be able to read (r) and execute (x) the file but not change or delete it (w). First we note that the permissions for the owner are correct, but we need to change the permissions for the group and the rest of the world. Here are two ways to do this:

chmod u=rwx,g=rx,o=rx test
chmod g+x,o+x test

Notice that in the first method, you would specify exactly what you wanted the permissions to be. When using the assign (=) operation, anything that is not specified as on is automatically turned off. In the second method, you would use the add (+) and remove (-) operators to change only what you specified you wanted changed.

We suggest you take some time to practice the chmod command to become familiar with it. Do not worry about "breaking" something or not being able to undo your changes to a file owned by you. You will be able to fix any error you make when you change a permission on one of your files.

 

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