Unit 6: File Permission Management

File permissions determine who can do what to a file. Typically, you do not need to fiddle with the file permission, but when you need to, it is usually for an important reason and it is critical to do it right.

The What of File Permissions

Let's look at what you can do to a file first. Unix file permissions allow control of three actions: r (read), w (write) and x (execute). These permission settings allow the following different actions to be done for regular files and directories.

These three actions can be controlled independently.

The permissions on a file can be expressed in two ways:

• using symbolic notation. For instance, rwx, r-x, -wx, where a - means that the corresponding permission is not given (in the order of r, w, x).

• using a numerical notation. This notation uses a digit between 0 and 7, which is computed as a sum of the individual digit representing the permissions: r is represented with 4, w is represented with 2, and x is represented with 1. For instance, r-x has a numerical representation of 5, and -wx has a numerical representation of 3.

The Who of File Permissions

Unix divides the users into three classes: u is the user who owns the file; g refers to the users in the same group as the user; and o are all the other users.

The permissions can be controlled separately for these classes of users. The permission notation simply concatenates the file permissions of each class of users together, in the order of u, g, and o.

For instance, the permission of 644, or rw-r--r--, on a file means that:

• the owner can read and write
• the group users can only read
• all the other users can only read

Checking file permission

You can view the permission of a file by using the ls -l command (l for long format):

$ ls -l test.txt
-rw-r--r--@ 1 ooiwt  staff  64 Jul 28 09:52 test.txt

Ignoring the first - and the last @, you can see that the permission of test.txt is 644.

The chmod command

You can use chmod command to change the permissions of a file or a directory.

For instance,

$ chmod 666 test.txt
$ ls -l test.txt
-rw-rw-rw-@ 1 ooiwt  staff  64 Jul 28 09:52 test.txt

An alternative way is to just specify the changes. To remove the write permission from others, you can write:

$ chmod o-w test.txt
$ ls -l test.txt
-rw-rw-r--@ 1 ooiwt  staff  64 Jul 28 09:52 test.txt

Common Scenarios for chmod

Here are some scenarios where you might need to use the chmod command:

• If you use the SoC Unix server to do your homework, you should prevent the directory that stores your homework from being accessible by other users. Make sure that your homework directory as the permission of 700.

• If you download a file from the Internet and you do not have the permission to read it, you should do a u+r to give yourself the read permission.

• A program should have execution permission to run. If you have a script or an executable file that you can't run, give yourself the execution permission u+x.