Linux Classroom Series – 05/Aug/2020

systemd-journald (journald)

• journald runs a system service that collects & stores logging data. It has ability to collect logs from user processes, the kernel logs, STDOUT & STDERR
• journalctl: The primary tool for interacting with journal files collected & maintained by journald is called journalctl
• Command-line:
  1. command to view all the journal logs

  journalctl
  

  2. Display most recent 10 journal logs

  journalctl -n 10
  

  3. Detailed logs (verbose )

  sudo journalctl -o verbose -n 2
  

  4. if you want to view the logs as they are collected

  sudo journalctl -f
  

  5. view journal logs since last 5 days

  sudo journalctl --since="5 days ago"
  sudo journalctl --since="5 days ago" --until="2 days ago"
  sudo journalctl --since=2019-12-31
  

  6. To view the logs generated by certain service

  sudo journalctl -u httpd
  

  

BOOT LOADERS

• For any os to boot , you need boot loader.
• Boot loader is the first software program that runs when your computer starts.
• Boot Loader is responsible for handing over the control of the system to Operating system.
• Typically boot loader will reside in the Master Boot Record (MBR) of the disk.
• In Linux distros most widely used boot loader is called as GRUB
• GRUB comes in 2 versions
  • GRUB Legacy
  • GRUB Version 2 (GRUB 2)
• GRUB aims to compliant with Multiboot Specifications & offers many features.
• The GRUB Legacy boot process happens in stages. Each stage is take care by special GRUB image files

Conventions Used in GRUB

• GRUB has its own special way of referring to devices (optical disc drives, hard disk drives & so on). The device name has to be enclosed in paranthesis ()
• GRUB starts numbering its devices & partitions from 0. EG: Hard drive on the controller (hd0)

GRUB 2

• GRUB 2 is most widely used boot loader now.
• Configuration files:
  • Primary configuration file is now named as grub.cfg (In legacy grub we had menu.lst)
  • Multiple files are used to Configure GRUB’s menu & these files are stored in /etc/grub.d/
  • 00_header => Sets the default values for some GRUB variables
  • 10_linux => Helps to find all the kernels on the root device of current OS.
  • 40_custom => This is where users can edit and store custom menu entries
• Partition numbers:
  • Partition numbers in GRUB2 device names start at 1. However the physical device/name conventions are same.
• Configuring GRUB2:
  • Primary configuration file of GRUB2 is grub.cfg which is generally located in /boot/grub2/grub.cfg

How Linux System Starts

• GRUB loader located in Master Boot record is started
• GRUB loader will find kernel files and load into RAM
• Then systemd or SYSVinit (Whatever is configured by distro) will be started which will be PID1

Topics left in Linux

• File Systems
• Shell/bash scripting
• Regular expressions (grep, awk & sed)
• Cron tab