In Linux and other UNIX operating systems, every file is associated with a user who is the owner. Every file is also associated with a group which has an interest in the file and certain rights, or permissions: read, write, and execute.
| Command | Result |
|---|---|
| chown | Used to change user ownership of a file or directory |
| chgrp | Used to change group ownership |
| chmod | Used to change the permissions on the file |
Files have three kinds of permissions: read (r), write (w), execute (x). These are generally represented as in the following order rwx. These permissions affect three groups of owners: user (u), group (g), and others (o). As a result, you have the following three groups of three permissions:
| rwx: | rwx: | rwx |
|---|---|---|
| u: | g: | o |
There are a number of different ways to use the chmod command. For instance, to give the owner execute permission:
$ ls -l test1
-rw-rw-r-- 1 joy caldera 1601 Mar 9 15:04 test1
$ chmod u+x test1
$ ls -l test1
-rwxrw-r-- 1 joy caldera 1601 Mar 9 15:04 test1
This kind of syntax can be difficult to type and remember, so one often uses a shorthand which lets you set all the permissions in one step. This is done with a simple algorithm, and a single digit suffices to specify all three permission bits for each entity. This digit is the sum of:
- 4 if read permission is desired.
- 2 if write permission is desired.
- 1 if execute permission is desired.
Thus 7 means read+write+execute, 6 means read+write, and 5 means read+execute.
When you apply this to the chmod command you have to give three digits for each degree of freedom, such as in
$ chmod 755 test1
$ ls -l test1
-rwxr-xr-x 1 joy caldera 1601 Mar 9 15:04 test1
The group ownership is changed by using the chgrp command
# ll /home/mina/myfile.txt
-rw-rw-r--. 1 mina caldera 679 Feb 19 16:51 /home/mina/myfile.txt
# chgrp root /home/mina/myfile.txt
# ll /home/mina/myfile.txt
-rw-rw-r--. 1 mina root 679 Feb 19 16:51 /home/mina/myfile.txt