The Unix-like operating systems, such as Linux differ from other computing systems in that they are not only multitasking but also multi-user.
What exactly does this mean? It means that more than one user can be operating the computer at the same time. While your computer only has one keyboard and monitor, it can still be used by more than one user. For example, if your computer is attached to a network, or the Internet, remote users can log in via ssh (secure shell) and operate the computer. In fact, remote users can execute graphical applications and have the output displayed on a remote computer. The X Window system supports this.
The multi-user capability of Unix-like systems is a feature that is deeply ingrained into the design of the operating system. If you remember the environment in which Unix was created, this makes perfect sense. Years ago before computers were "personal," they were large, expensive, and centralized. A typical university computer system consisted of a large mainframe computer located in some building on campus and terminals were located throughout the campus, each connected to the large central computer. The computer would support many users at the same time.
In order to make this practical, a method had to be devised to protect the users from each other. After all, you could not allow the actions of one user to crash the computer, nor could you allow one user to interfere with the files belonging to another user.
This lesson will cover the following commands:
On a Linux system, each file and directory is assigned access rights for the owner of the file, the members of a group of related users, and everybody else. Rights can be assigned to read a file, to write a file, and to execute a file (i.e., run the file as a program).
To see the permission settings for a file, we can use the ls command. As an example, we will look at the bash program which is located in the /bin directory:
[me@linuxbox me]$ ls -l /bin/bash
-rwxr-xr-x 1 root root 316848 Feb 27 2000 /bin/bash
Here we can see:
In the diagram below, we see how the first
portion of the listing is interpreted. It consists
of a character indicating the file type, followed
by three sets of three characters that convey the
reading, writing and execution permission for the
owner, group, and everybody else.
The chmod command is used to change the permissions of a file or directory. To use it, you specify the desired permission settings and the file or files that you wish to modify. There are two ways to specify the permissions. In this lesson we will focus on one of these, called the octal notation method.
It is easy to think of the permission settings as a series of bits (which is how the computer thinks about them). Here's how it works:
rwx rwx rwx = 111 111 111 rw- rw- rw- = 110 110 110 rwx --- --- = 111 000 000 and so on... rwx = 111 in binary = 7 rw- = 110 in binary = 6 r-x = 101 in binary = 5 r-- = 100 in binary = 4
Now, if you represent each of the three sets of permissions (owner, group, and other) as a single digit, you have a pretty convenient way of expressing the possible permissions settings. For example, if we wanted to set some_file to have read and write permission for the owner, but wanted to keep the file private from others, we would:
[me@linuxbox me]$ chmod 600 some_file
Here is a table of numbers that covers all the
common settings. The ones beginning with "7" are
used with programs (since they enable execution)
and the rest are for other kinds of files.
The chmod command can also be used to control the access permissions for directories. Again, we can use the octal notation to set permissions, but the meaning of the r, w, and x attributes is different:
Here are some useful
settings for directories:
Becoming The Superuser For A Short While
It is often necessary to become the superuser to perform important system administration tasks, but as you have been warned, you should not stay logged in as the superuser. In most distributions, there is a program that can give you temporary access to the superuser's privileges. This program is called su (short for substitute user) and can be used in those cases when you need to be the superuser for a small number of tasks. To become the superuser, simply type the su command. You will be prompted for the superuser's password:
[me@linuxbox me]$ su
After executing the su command, you have a new shell session as the superuser. To exit the superuser session, type exit and you will return to your previous session.
In some distributions, most notably Ubuntu, an alternate method is used. Rather than using su, these systems employ the sudo command instead. With sudo, one or more users are granted superuser privileges on an as needed basis. To execute a command as the superuser, the desired command is simply preceeded with the sudo command. After the command is entered, the user is prompted for the user's password rather than the superuser's:
[me@linuxbox me]$ sudo some_command
Changing File Ownership
You can change the owner of a file by using the chown command. Here's an example: Suppose I wanted to change the owner of some_file from "me" to "you". I could:
[me@linuxbox me]$ su
Notice that in order to change the owner of a file, you must be the superuser. To do this, our example employed the su command, then we executed chown, and finally we typed exit to return to our previous session.
chown works the same way on directories as it does on files.
Changing Group Ownership
The group ownership of a file or directory may be changed with chgrp. This command is used like this:
[me@linuxbox me]$ chgrp new_group some_file
In the example above, we changed the group ownership of some_file from its previous group to "new_group". You must be the owner of the file or directory to perform a chgrp.
© 2000-2018, William E. Shotts, Jr. Verbatim copying and distribution of this entire article is permitted in any medium, provided this copyright notice is preserved.
Linux® is a registered trademark of Linus Torvalds.