You can change the permissions of your files (or other people's files if you're the root superuser) by using the command "chmod". The syntax is very simple. For instance if George decides to give write permissions to the administrators, he will type:

chmod g+w myfile

g represents the group of the file (administrators).
w represents the write permission.
+ represents the fact that the permission is added.

If George then lists the permissions using ls -l he obtains:

ls -l myfile
-rwxrwx---  1 george administrators 10 2006-03-09 21:31 myfile

As you can see, the administrators now have write access to the file, and permission to change its content.

The "chmod" command takes 4 parameters:

- The type of users to apply the change of permissions for (u for user, g for group, o for others, a combination of them or a for all three of them).
- The type of change to make (+ to add permissions, - to remove permissions, = to define permissions)
- The type of permissions to apply the change with (r for read, w for write, x for execute)
- The file or group of files to apply the change on (filename for a precise file, but wildcard characters for multiple files)

Let's have a look at a few examples:

- chmod o+r myfile adds read permission to the others on myfile;
- chmod ug+rx myfile adds read and execute permissions to both the owner (user) and the group on myfile;
- chmod a-rwx myfile removes all permissions to everybody (all) on myfile;
- chmod a=rx *.txt defines permissions to be read and write to everybody on all files suffixed by .txt.

The chmod command also accepts another syntax which is quite popular among system administrators: the octal system. Rather than using letters such as u, g, o, a, r, w and x.. you can use octal numbers. The main advantage is that once you're used to it, it is faster to use. Also, because it sets permissions rather than adding or removing them, you don't accidentally overlook anything. Here is how the octal numbers work:

Each permission is given a value:

Values add up when you combine permissions. Consequently the total value can go from 0 (no permission at all) to 7 (full permissions):

Finally a value is given for each of the three types of users (User, Group and Other) and these three numbers ranging from 0 to 7 are put together to form the octal number. This is the number you can use with "chmod".

For instance:

chmod 750 myfile

750 means 7 (rwx) for the owner, 5 (r-x) for the group and 0 (---) for others. As a result, the permissions of myfile will be "rwxr-x---". As seen above this command is equivalent to:

chmod u=rwx,g=rx myfile; chmod o-rwx myfile;  

Here are some common uses of the octal numbers:

- chmod 755 myfile : rwxr-xr-x, all rights to the owner, other people only read and execute;
- chmod 644 myfile : rw-r--r--, owner car read and write, other people only read;
- chmod 777 myfile : can be considered bad practice in some cases, full permissions to everybody.