Unix Lesson 6: Users

Just like on Facebook, every Unix system has you log in to a user account. Unix is a multi-user system, and to use it effectively, you must employ more than one user account. Luckily, users and groups are not that complicated.

Usually, you will log in through your own personal user account. To do things as different users, you will use different commands to access them. To find out what user you are at any time, use the whoami command:

$ whoami
erikh

Which just prints what user you are by name. There are also groups, which are collections of users. These users and groups are used as permissions on files, as well as programs which run under those users. Programs that run as the user have the same rights as you, executing commands. That's how it all works.

You can see the user/group relationship by looking at files with ls -l (lower-case L), for a "long" listing. This shows the permissions:

 $ ls -l
 -rwxr-x---   1 erikh erikh    1288 Mar 18 13:32 a_file.txt

These detail the types of permissions, the "owner" of the file, the "group owner" of the file, and the name of the file, respectively. The user who "owns" this file is "erikh". The group is the "users" group, and the filename is a_file.txt. The permissions are a little complicated, let's break that down.

-rwxr-x---

The first - is for special permissions and file types. You'll notice directories get a d here, but other things exist as well.

The second section, which is the next 3 characters, is the "owner" permissions. The r means "readable", the w means "writable", and the x means "executable". These permissions are repeated for the 3-character sections of "group" and "other", where "group" is the permissions for all users in the named group, and "other" is for everyone who does not fit into the other two categories. A dash (-) means there is no permission.

There are several commands which modify the ownership and permissions of a file:

• chown: Change ownership
• chgrp: Change group
• chmod: Change "modes" (modes are the different permissions described above)

Octal modes

In chmod there is a special syntax for changing permissions, it is important to know. These modes are expressed in "octal", which means the numbers are in "base 8" notation. Contrast with our normal decimal system which is "base 10". This is a mathematical way of saying the numbers will count from 0-7, never higher.

You can evaluate the numbers like so, for each position in the number:

0777

• The first position are the special attributes, and is usually 0.
• The second position is the owner permission.
• The third position is the group permission.
• The final position is the other permission.

Calculating the last three positions is really easy. Simply add the numbers together for the different positions:

• 4: readable
• 2: writable
• 1: executable

So 7 is all permissions, 6 is read and write, 5 is read and execute, 0 is none. Then, put the 3 numbers together in the proper order for chmod.

Different Users

To use Unix effectively, you will need to be different users at different times. The most common switch you will make is to the administrator account, otherwise called "root".

Each account has a unique User ID, or UID for short. To see your UID, you can use the id command like so:

$ id -u
1000

To see other user activity on the system, you can use w or last to see who is online, and a log of who logged in, respectively.

Some other users are present to manage system facilities, you can review them by reading the "passwd" (Unix likes to abbreviate everything) database.

$ cat /etc/passwd

There is also one for groups:

$ cat /etc/group

Note that on modern systems, the actual "password" for the accounts is in a separate file only accessible by "root", called /etc/shadow. If an account does not have a password, it means you can't normally log into it. You have to access it indirectly. Even though you can only access the real passwords as "root", they are encrypted in a special way so that they cannot be read.

You can do this by using the "switch user" or su command. It chooses "root" by default, but you can change it (see the manual pages for how). Another, more modern tool lets you execute one command as root called sudo. This way you can just run one command without switching temporarily. su and sudo both require passwords to use; su requires the target account password (e.g., root's password) and sudo requires the password of the user account that is doing the switching (e.g., your password). They are somewhat different tools; sudo is much more complicated to configure, but easier to use on the surface.

To change your password, use the passwd command. To modify group memberships, look at gpasswd.

Assignment

Unpack the octal permissions. Show read, write, and execute permissions for all modes.

• 0750
• 0644
• 0400
• 0550
• 0777
• 0666

What is root's UID?