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.

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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:

     

     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

#Commands:

#The "who" command is used to display the current run level & system's last reboot time
who -r 
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#The "init" command is used to change the system's run level, which represents a specific operating state
#Syntax:
init <runlevel>
#Example:
init 5 
init 3
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#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
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#This command is used to check the default target
systemctl get-default
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#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

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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.

#To set ACL on a directory:
#Syntax:
setfacl -m u:<user>:<perm> <dirname>

#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 <dirname>
OR
getfacl <filename>

#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

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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

• 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.

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Commands Related to Processes in Linux

Top Command

#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

#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

#Syntax:
nice -n <nicenessValue> 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

#Syntax:
renice -n <niceness_value> <PID>

#Example:
[root@localhost ~]# renice -5 3030
#Here, 3030 is pid for command top

To terminate, kill, resume or suspend a process

#Syntax:
kill <signal> <PID>

#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.

#Command
[root@localhost ~]# jobs

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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.

#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

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