# Booting Process, ACL, Processes, Process State, Alias in Linux ### **Booting Process** * The booting process refers to the series of **steps** that take place when a computer **starts up** and **loads** the operating system into **memory**, it involves several stages and those are listed below. --- ### **Stages of the Booting Process** 1. **BIOS** (Basic Input Output System) * When the computer is **powered** on, the **BIOS** is **activated**. * It performs the **Power-On Self-Test** (POST), **detects** and **initializes** essential hardware components, and **locates** the boot device (*usually the hard drive or SSD*) that contains the bootloader. 2. **MBR** (Master Boot Record) * When the computer starts up, the BIOS **executes** the code in the **MBR**. * The structure of the MBR consists of **three** primary components: ![](https://cdn.hashnode.com/res/hashnode/image/upload/v1689768763562/89d5725e-e88b-4a87-95cb-f6c550780799.png align="center") 1. **Bootloader**: The MBR contains a small piece of **executable code** called the bootloader, which is responsible for **locating** and **loading** the main bootable partition. 2. **Partition Table**: The MBR also contains a partition table that can hold **information** about up to **four primary partitions** on the storage device. 3. **Magic Number** (**Parity Bits**): It acts as a simple form of **data validation** and if any part of the MBR is **corrupted** or **altered**, including the magic number, the BIOS will not consider it a **valid** bootable device. 3. **GRUB** (GRand Unified Bootloader) * GRUB is a popular **bootloader** used in the booting process in Linux. * It plays a **critical** role in the boot process by allowing users to choose which operating system to boot when **multiple operating systems** are installed on the same computer. * The **different versions** of GRUB that have been developed over time: * **GRUB 1** --> Getting code from MBR * **GRUB 1.5** --> Allows file system to communicate with hardware * **GRUB 2.0** \--> Main GRUB image, /boot/grub2/grub.conf -->Loading of kernel 4. **Kernel** * It **initializes** & **configures** computer **memory** and **hardware**. * **Uncompressed** itself & mount it. * Loads the **required drivers** for it. * Initialize **virtual devices** into the system. * **Create** a **root device** mount into **read-only** memory. * **Freeze** unused memory. 5. **init.d / system.d** * It is the **first process** (*mother of the process*) after the kernel step. * Mounting the file system --> /etc/fstab * Starting services * Device attaching to the system * Identify the default target 6. **Run Level** ```bash #Commands: #The "who" command is used to display the current run level & system's last reboot time who -r ___________________________________________________________________ #The "init" command is used to change the system's run level, which represents a specific operating state #Syntax: init #Example: init 5 init 3 ___________________________________________________________________ #The default target determines the operating state in which the system will boot by default systemctl set-default multi-user.target systemctl set-default graphical.target ___________________________________________________________________ #This command is used to check the default target systemctl get-default ___________________________________________________________________ #List of Run Levels: 0 - Halt state (poweroff.target) 1 - Single user mode (rescue.target) 2 - Mutli user mode without networking 3 - Mulit user mode with networking 4 - User Definable 5 - GUI (graphical.target) 6 - Reboot ``` --- ### **ACL** * ACL stands for **Access Control Lists**. * It allows you to give separate permissions to **individual users** or **groups** on **files** and **directories** in a file system. * Allowing you to define more **fine-grained permissions** for specific **users** or **groups**. ```bash #To set ACL on a directory: #Syntax: setfacl -m u:: #Example: [root@localhost tmp]# setfacl -m u:testuser:rwx TestACL/ [root@localhost tmp]# ls -ld TestACL/ drwxrwxr-x+ 2 root root 41 Jul 19 12.00 TestACL/ #To show the list of users and groups with their respective permissions on the specified file or directory: #Syntax: getfacl OR getfacl #Example: [root@localhost tmp]# getfacl TestACL/ #file: TestACL/ #owner: root #group: root user::rwx user:testuser:rwx user:robot:rwx group::r-x mask::rwx other::r-x #To remove the ACL (remove a single user at a time): setfacl -x u:testuser: TestACL/ #Verifying... getfacl TestACL/ user::rwx user:robot:rwx group:r-x mask::rwx other::r-x #To remove the ACL (remove all users at once): setfacl -b TestACL/ #Verifying... getfacl TestACL/ user::rwx group::r-x other::r-x ``` --- ### **Processes** * The Process represents the **execution** of a **specific task** or **job** on the system. * Every action you perform on a computer involves one or more processes. ### **Types of Processes** * **Parent and Child Processes** : * When a new process is created, it is referred to as the "**child process**" and the process that creates it is called the "**parent process**". * Parent processes are processes that **create** and **manage** other processes. * If the process creates **additional processes** as **part** of its **execution**, those are considered **child processes**. * **Zombie Processes** : * The processes that have **completed** their **execution** but **still** have an **entry** in the **process table**. * Zombie processes **do not consume** system resources and are usually **cleaned up automatically** by the system. * **Orphan Processes** : * Orphan processes are child processes whose parent processes have **terminated unexpectedly** before collecting their **exit status.** ### **Process State** ![](https://cdn.hashnode.com/res/hashnode/image/upload/v1689780128576/3a5f97e6-2305-4c3d-af2f-06a1cb3bcff0.jpeg align="center") * A process state refers to the **current condition** or **status** of a **running process**. * The process state indicates what the process is currently **doing** or **waiting** for in the system. --- ### **Commands Related to Processes in Linux** **Top Command** ![](https://cdn.hashnode.com/res/hashnode/image/upload/v1689788400317/2b9f0c69-715e-4f36-b379-5dc3c2cae6da.png align="center") ```bash #This command provides real-time dynamic monitoring of system processes, #CPU usage, memory utilization, and other system statistics in an interactive shell [root@localhost ~]# top ``` **To list the processes that have "<word>" in it** ```bash #Listing the processes having word "top" in it #Used to list all running processes and # filter the output to display only the lines containing word "top" using grep command. [root@localhost ~]# ps -ef|grep top ``` **To assign higher or lower priority to a new process** ```bash #Syntax: nice -n command #Example: [root@localhost ~]# nice -n -5 top ``` * The "**nice**" command is used to execute a command to modify a niceness value. * The nice command is used when starting a new process to set its **initial priority.** * The niceness value is an integer that ranges from -**20** to +**20**. * The **lower values** indicate **higher priority**. A process with a higher priority (*lower niceness value*) will be running more frequently compared to processes with a lower priority. **To assign higher or lower priority to an existing process using its PID** ```bash #Syntax: renice -n #Example: [root@localhost ~]# renice -5 3030 #Here, 3030 is pid for command top ``` **To terminate, kill, resume or suspend a process** ```bash #Syntax: kill #Signals: Terminate : -15 Kill : -9 Resume : -18 Suspend : -19 #Example: kill -9 3030 OR kill -15 3745 ``` **Jobs command** * The command "**jobs**" is useful when you are running multiple background processes and need to **keep track** of their **status** or **interact** with them. ```bash #Command [root@localhost ~]# jobs ``` --- ### **Alias** * An alias is a **user-defined shortcut** or **nickname** for a **command** or a **sequence** of commands. * Aliases allow users to create their **shorthand notation** for **frequently** used or complex commands, making it **easier** and **faster** to execute them. ```bash #Defining an Alias (Syntax): alias aliasName="command_or_commands" #Example: alias viraj="ls -lrt" #You can define the alias in the .bashrc or .bash_aliases file #Defining the alias in the interactive terminal/shell is temporary, to define it permanently write the alias in .bashrc file ``` ---