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Boot Process of Linux

Phase 1 : BIOS/UEFI Initialization

The process begins when we press the power botton, the BIOS(Basic Input/Output System) or UEFI (Unified Extension firmware Interface) initializes the computer's hardware components (keyboard,screen etc) UEFI is a newer,fasteralternative to BIOS, offering improved security features like Secure Boot and support for disks larger than 2TB (unlike BIOS's MBR limitation)

Phase 2: POST and Boot Loader

Next, the BIOS/UEFI performs a Power-On-Self-Test(POST) to verfiy hardware functionality. If POST is successful, the BIOS/UEFI locates and loads the boot loader software, typically checking the hard drive first then USB or CDs. the Bootloader (GRUB2) locates the operating system kernal and loads it into memory (RAM).

Phase 3: Kernal initialization and Systemd

The kernal then takes over,decompressing itselft,checking hardware, and loading devices drivers and kernal modules. the init process(Systemd in modern linux system) starts, managing all other processes. Systemd handles various tasks: loadng remainng drives,mouting file systems, starting background services(networking,sound,power management),managing user logins and loading the desktop environment. Systemd uses target configuration files to determine the boot mode.

Linux FileSystem

Types of files

  • Regular files : it contains programs, executable files and text files
  • Directory : it is shown in blue color, it contains list of files.
  • Special File :
    • Blocked file (b)
    • Character devices file (c)
    • Named pipe file (p)
    • Symbolic link file (l)
    • socket file (s)

Linux file system Hierarchy

  1. Root Directory (/) : The top-level directory in the Linux file system. All other directories are subdirectories of this root.
  2. Important Directory:
  • /bin: Contains essential executable programs and core operating system commands. This directory is often linked to /usr/bin for convenience
  • /boot: Holds files needed by the bootloader, including the kernel and initial RAM file system
  • /dev: Contains device files that represent hardware devices connected to the system
  • /etc: Stores critical configuration files and startup scripts. For example, SSH settings are found in /etc/ssh/sshd_config
  • /home: The home directory for individual users, similar to "My Documents" in Windows. Each user has a separate directory here
  • /lib: Contains shared libraries needed by programs
  • /media: The mount point for removable media, such as USB drives
  • /mnt: Used for temporarily mounting devices
  • /proc: A pseudo file system that provides information about running processes and system information
  • /usr: Contains most of the user commands and utilities
  • /var: Holds variable files such as logs and temporary files
  1. Important Directory for Docker
  2. /var/lib/ : this directory stores persistent application data. - /var/lib/docker : This stores peristent data related to docker, such as images layers , volumes, networks , engine-id also information about running contaners and temp files. - NOTE : lib directory stores shared libraries needed by programs thats why this directory var/lib/docker stores files which are important to share among containers such as image layers, volumes and info about running containers and temp files.
  3. /usr/lib/systemd/system/docker: This directory stores two files docker.service and docker.socket. These are used by systemd to manage the service of docker daemon
  4. /run/: File system for system packages to place runtime data, socket files and similar.
    • /run/docker: this is used by docker to store runtime data. This data is temp and exits as long as the docker service.
    • it stores information such as containerd, libnetwork, netns, swarm and most important **runtime-runc which stores logs of running containers
    • /run/docker/runtime-runc/: It has a folder moby which stores state.json file of running container. This file stores information such as PID of parent process which started the container, linux capabilities used , system call etc
  5. /usr/bin: This stores essential executable programs and os commands . That is why it store docker which is cli also it has dockerd as a cli .

Inodes , Soft Links, Hard Links

Inodes

  • An inode is a data structure in a filesystem that stores metadata about a file or directory (e.g., file size, permissions, timestamps, and pointers to the actual data blocks).
  • Each file or directory has a unique inode number in its filesystem.
    ls -i filename

Hard Links

  • A hard link is a direct reference (alias) to the same inode as another file. Multiple filenames can point to the same data on disk.
  • Shares the same inode number as the original file.
  • Changes to the content of the file reflect across all hard links.
  • Deleting a hard link does not delete the data until all links are removed.
  • Cannot link across different filesystems or partitions.
    ln original.txt hardlink.txt

Soft link

  • A soft link is a special type of file that points to the path of another file or directory (like a shortcut).
  • Has a different inode than the original file.
  • Deleting the original file leaves the soft link broken ("dangling link").
  • Can link across different filesystems or partitions.
    ln -s originalfile.txt softlink.txt

Permissions

chmod, chown, chgrp, umask,SUID,SGID,sticky bit

chmod

  • modifies the permissions of files or directory

chown

  • changes the owernship of the file or directory to a specificied user or user:group.

chgrp

  • changes the group owernship of a file or directory

umask

  • umask is like a default setting for file and directory permission in linux. when we create a new file or directory it decides what permissions should not be given to others .

SUID

  • Allows a file to be executed with the permissions of the file's owner, regardless of who runs it.

SGID

  • Ensure that the file is executed with the permissions of the file's group.

sticky bit

  • Restricts file deletion in a directory to only the file owner,directory owner or root.even if others have write permission.

Systemd

  • Systemd is a modern init system and service manager for Linux, designed to improve boot performance and manage services, processes, and system states. Its architecture is modular and composed of multiple components working together.

Key Components of the Systemd Architecture

  1. PID 1 (Primary Init Process)

    Role: Systemd is the first process (PID 1) started by the kernel during system boot. Responsibilities:

    • Initializes the system.
    • Spawns and monitors services.
    • Handles system shutdown and reboot.

  2. Unit Files

    Role: Define how system resources are managed. These files represent services, devices, mount points, and more. Types: - Service Units (.service): Define services (e.g., nginx.service). - Socket Units (.socket): Handle network or IPC sockets. - Timer Units (.timer): Schedule tasks (cron-like functionality). - Target Units (.target): Represent states (e.g., multi-user.target for multi-user mode).

  3. The Journal (Logging System)

    Role: Centralized logging for all system events. Features: - Binary format for efficient storage. - Queryable using journalctl.

  4. Dependency Management

    Role: Services and units have defined dependencies, ensuring the proper order of execution. Key Directives:

    • After=: Start after a specific unit.
    • Requires=: Depend on another unit.

Core Systemd Utilities

  1. systemctl

    • Central command for controlling services and units.

  2. journalctl

    • Query logs from the journal.

  3. systemd-analyze

    • Analyze system performance and boot time.

Boot Process with Systemd

  1. Kernel Initialization:

    • The Linux kernel loads and starts systemd as PID 1.

  2. Target Activation:

    • Systemd loads the default target (e.g., multi-user.target).

  3. Service Initialization:

    • Services are started based on their dependencies.

  4. System Ready:

    • When all critical units are active, the system reaches the desired target state.

Systemd Architecture Diagram

Kernel
⬇
Systemd (PID 1)
⬇
Unit Manager
    Unit Files: .service, .socket, .timer, .target.
    Dependencies: Manages relationships between units.
    ⬇
The Journal
    Logs for services, kernel messages, and more.
    ⬇
User Interaction
    Tools: systemctl, journalctl, systemd-analyze.

Basic commands for SystemD

  1. Starting a service sudo systemctl start <serivce name>
  2. Stoping a service sudo systemctl stop <service name>
  3. Checking a services sudo systemctl status <service name>
  4. Restarting a service sudo systemctl restart <service name>
  5. Enabling and disabling a serivce sudo systemctl enable/disable <service name>
  6. Reloading configuration if you change a serive's configuration file,use sudo systemctl daemon-reload to apply the changes
  7. Editing and creating service files sudo systemctl edit <services_name>

Understanding Units in Systemd

Definition of Units

  • units are the objects that systemd manages,including services,timers,mounts and more. A service is a specific type of unit

Service Files

  • Service files are text files that define how systemd manages a service.they contains sectiosn [Unit],[service], and [install],each serving different purpose.

Structure of a service file

  1. Unit section
  • contains general information about the unit,such as its description and dependencies.
  1. service section
  • defines how the service operates, including the command to start it (ExecStart) and its type(eg, simple ,notify)
  1. Install section
  • configures how the unit is enabled or disabled, specifying dependencies and run levels

Common Directories for Unit files

  • /etc/systemd/system/
    • highest priority directory for custom unit files
  • /run/systemd/system/
    • contains runtime units
  • /usr/lib/systemd/system
    • from locally installed packages (via. apt-get)
  • /lib/systemd/system
    • standard unit files location (your distro maintainers)

RunLevel and Targets

RunLevels

  1. Defination
  • A runlevel is a state of init that defines which system serivces are operating.
  • it a mode or state in which a linux system operates.
  • Runlevels are represented by single-digit integes(0-6).
  1. Initialization.
  • when a linux systemd boots,the init process starts first.
  • the init process is responsible for:
    • running startup scripts
    • initializing hardware.
    • bringing up the network
    • starting the graphical interface
  1. Default Runlevel
  • the init process determines the default runlevel from system configuration
  • it executes the start scripts associated with the
  1. purpose of runlevel
  • define the system's operational state.
  • allow access to different combinations of processes and system configurations.

Logging In Linux

  • Logging in linux uses rsyslog daemon. This serivce is responsible for listening to log messages from different parts of linux system and routing the message to an appropriate log file in the /var/log/ directory.
  • rsyslog.conf daemon gets its configuration information from the rsyslog.conf file.Located under /etc directory.

Rotating Log files

  • Rotating log files allows log files instead of purging or deleting to override them and and optionally compressed.
  • A log file can thus have multiple old versions remaining online. These files will go back over a period of time and will represent the backlog.
  • The rotation is initiated through the logrotate utility. Located under /etc.

Essential log files

  1. Boot log
  • located in /var/log/boot.log, it contains messages related to the boot process, useful for troubleshooting boot issues.
  1. apt log
  • found in /var/log/apt this log tracks packages installations and updates on debian ssytem,helping to identify issues with installed packages.
  1. wtmp log
  • a binary log located /var/log/wtmp, it records login and logout events,use the last command to view its contents.
  1. authorization log
  • found in /var/log/auth.log on debain/ubuntu systems,it records login attempts and is useful for troubleshooting authentication issues.
  1. system log(syslog)
  • located at /var/log/syslog, it logs system events and is helpful for troubleshooting hardware issues
  1. D Message Log(dmesg)
  • this log contains kernel-related messages and is useful for hardware troubleshooting. Access it using the dmesg command

using journalctl

  • the journalctl command is specific to systemd and allows inspection of logs for specific services. for examples journalctl -u ssh shows logs related to the ssh services.