Structure of the GNU/Linux Operating System

Hello TurkHackTeam Family, today I will talk about “Structure of the GNU/Linux Operating System”. It will feel like a lesson and I won’t keep it long.

Titles of Subject

• Root
• GID
• UID
• Group Management
• Creating Group
• Change the Group ID
• Add User to Group
• Delete User from Group
• Setting Password to a Group
• Delete Group
• User Management
• Creating User
• Setting Password to a User
• Changing the User ID
• Delete User
• Getting Information About User
• Shadow Directory
• Difference Between Su and Sudo
• Switch to root user with Su command
• Switch to root user with Sudo Su command
• Switch to another user
• File Ownership and permissions


First, let me talk about the meanings of some terms that I will talk about in my lesson.


Root

They are users that have full authority on Linux/Unix systems. Root users have unlimited power.

GID

GID stands for Group ID. If group IDs are defined as “0”, group has root permissions.

UID

UID stands for User ID. IF user IDs are defined as the same way “0”, the user have root permissions.


First, as you know Linux distributions have a multi user system. Therefore, we don’t only process with only one user, but we can open more than one user and make operation. The most important thing for us is the User ID. If you say what is this User ID; Access rights are granted according to this ID. There can be more than one user with this ID. If “Administrator” can be more than one in Windows, Linux systems have more than one “root” user. Generally, the reason for opening more than one user with the same access rights as the root in Linux systems is if the root user’s password is forgotten, the other backup users who have root user rights can log in the system.

On Linux systems, default users come automatically. We have a code to see these default users. In fact, we can use this code to see all users (Root etc.).

First, we open terminal and




We are writing.





Then




We are writing this. Here “passwd” is the file where the users are located. We can get the necessary information here.





All default users and users that have Root permissions are shown.





You saw “root:x:0:0”. If you say what this is; it means username(root), password(x), user ID(0), group ID(0). If you have noticed that users at the bottom, it says “/nologin” on the right side. This means that they can’t login in the background. If you scroll down, you will see your own username on above the bottom side. We have made a little entrance with you and we have looked what is and what isn’t. Now let’s get to our lesson.


Group Management


We use some commands to perform operations on group. I will show you their usage but first let me talk you about where these groups are.


Groups




are located in this.

Now let’s see, but first




We are typing this.





Then all the groups appear.


Creating Groups


Let’s create a group together.




We are writing this.





After typing, our group is created. Let’s look at our group. For this;




we are writing this.





At the bottom, our group came out. Now we have seen our group at the bottom but, we may not always want to go to the bottom side.
For this first;




we are writing this. Grep is a command that gives use results according to the criteria we are looking for in a particular section.





As you can see, it showed us our group and our GIP and UID.





As you can see, we have seen how to create a group and learn how to find it. Now we have come to perform through this group we have created.


Changing the Group ID

First from the terminal




We are taking root authorization with this. Then




we log in to the etc directory with this code. After that




we are typing this.





We finished. Now to see our group




we are writing this.








Whatever group ID you have typed, that number will appear.

Now let’s assume we want to change the name of the group. What are we gonna do? For this;




We are writing this.





Now our group name has changed. To see




we are writing this.








As you can see at the bottom after writing, there is a group called “New” and we see that the group ID is “2002”.

Now let’s move on to the Add,Remove and set password commands to the group.


Add User to the Group

First,




We are writing this. “-a” parameter means “add” here.





After writing this, we received a notification that the user had been added.





We received a notification that the user had been added. Let’s see.




We are writing this.





As you can see, the user we added appears at the end of the group.





Removing User From Group

First,




we are writing this. “-d” parameter here means “delete”.





It shows us that the user has been deleted. Let’s check it.







We are writing this.





As you can see, when we added user, the user name appeared on the right side but now we couldn’t see.

We learned how to add and remove users. Now we will learn how to set password to the group.


Setting Password to Group


First,




We are writing this.





It asks us for the new password. What you write here won’t appear on your screen so don’t worry about it.





Then it wants us to write password again.





After writing, our group has a password protection.


Delete Group


We learned how to add, remove and setting password in group. Now we will learn how to delete a group.


First,




We are doing this.





Then it finishes the process without warning us.





Let’s check if group is deleted. For this,




We are writing this.








As you see, group has been deleted.
Now let’s get to the User Management.


User Management


We use some commands for user management, I will talk about them. First, let’s talk about where these users are.

User information is found in “etc/passwd”.




With this command, you can list user information.








Creating User


First,




we are writing this.





It created without warning.





Let’s check if the user has been added. For this,





we are writing this and we can see the user that we created at the bottom.






Setting Password to User


First of all, like we did in the group




we are writing this.





After that it will ask for a password.





It asks for a password again after that process will be completed successfully.





Changing User ID


First,




we are writing this.





It accepted without warning.





Let’s check what happened.




We are writing this to see “passwd”.





After writing, as you can see at the bottom, our user and ID value appear.





Deleting User


To delete user,




we are writing this.





After that, let’s check if the user has been deleted.







As you can see, our user has been deleted.





Adding User to a Group While Creating


First,




we are writing this.





And that’s it. Our user was formed at the id value that we identified in the group called “New”.


Getting Information About User


To get information about user,




we are writing this.





Then it gives us information about the user.





View Files After Creating A User


Informations about users after creating them




are written in this file (information about the authorities). Now to learn and edit the information,




we write this.








Shadow Directory


Shadow Directory




is defined as this. Shadow directory has te same functions as “passwd”, but it is more reliable directory. Because it is encrypted in MD5. On the other hand, “Passwd” isn’t like this. It is a text file.


Difference Between Su and Sudo


The difference between Sudo and Su is, in the “su” command, when you exit the terminal or when you write “exit”, it also exits the root user but, in the “sudo” command let’s say that you are doing an operation when this operation is finished, the “sudo” command loses its function and switches to the normal user.


Switching Root User With Su Command


To switch root user,




we are writing this and asks us for our password. After writing password, we are switching to root.





Switching Root User With Sudo Command


To switch root user with sudo command




we are writing this.





It asks for our password.





After writing, you will switch to the root user.


Switching Root User With Sudo Su Command


The feature of Sudo su command is this, it switches directly to the root user without asking for any password.







Switching to Another User


To switch to another user




we are writing this and if it doesn’t have a password, it will switch directly.





File Ownership and Permissions


To change File Ownership and Permissions, we are using “chmod”, “chown” and “unmask” command.


Chmod Command





The chmod command is a slightly more complex command than others. First, let me tell what is this command. It is a command used to determine Linux users’ access rights to files.


2 : w => Writing Permission
4 : r => Reading Permission
1 : x => Running Permission


Example of Chmod;

rwx : All read, write and run Access permissions exist.
rw- : It has read and write permissions, no run permission.
r-x : It has read and run permissions, no write permission.
wx : It has write and run permission, no read permission.
-x : Only has run permission.
-w- : Only has write permission.
r- : Only has read permission.
-- : has no access.


If you say how do we know if to a file is allowed to us,




If you write this, the file’s permissions will be seen.








For example, its permissions appeared.







If we write this, we have no rights in that file. So no right to read, write or view.







If we write this, we have no right to read and write but we have the right to run.







If we write this, we have no read and run permission but we have write permission.








If we write this, we have no read permission, but we have write and run permissions.







If we write this, we have no write and run permissions but we have read permission.








If we write this, we have read and run permissions but have no write permission.







If we write this, we have read and write permissions but we have no run permission.








If we write this, we have all the permissions.


Chown Command





Chmod command is a command used to change a file’s permission system. With this command, you have the right to use and manage the file among other users.







To do that we are writing this. Which user you want to give authority to, you write that username.





Unmask Command





Unmask command is a command that is used as a permission restriction. On Linux systems, files and directories have default permissions. These permissions are set as 022 in unmask.





You can see like that. This number system comes from the “x, r, w” that I explained in chmod. We can change these values and we will do it like this;







We will write this.

For example, let’s create a file and directory;







We are writing this and looking at the permissions.





As you can see, the values “r, x, w” have also changed because we have changed the values of the permissions.


That’s all friends. If you learned something, I am very happy about that.