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Summary of "Cert Prep: LPI Linux Essentials (010-160)"

https://www.linkedin.com/learning/cert-prep-lpi-linux-essentials-010-160/

General

• 96% servers use Linux
• Open source: code avialable for public use or modification, distribution under a license.
• Licenses: 45% of github is MIT
• Distribution: kernel + config + libraries + programs
• OS differences: market share pcs (77 windows, 18 macOs, 2 linux)
• Package installs and repositories: each distro has its package manager (.deb, .rpm(red hat, fedora, openSUSE). tar(universal, arch linux))
• Package manager: deb (dpkg, apt-get, apt), rpm (rpm, yum, dnf)

CLI

• Shell: program that takes commands form the keyboard and gives them to the kernel to execute
• Bash: type of shell (there are alternatives like zsh)
• Terminal: Gui window to access to shell
• \ : scapes chars. echo "I have \$1200"
• FHS: Filesystem Hierarchy Standard defines the directory structure and directory contents in Linux distributions. It is maintained by the Linux Foundation.

• Character names:

  • / : lash
  • - : dash
  • ~ : tilde

• Commands: ls, pwd, mv, rm, mkdir, touch, locale, hostname, date, uptime, df

• ls -l(permission, ownership, size)a(show hidden)

• wildcard: characters of regexp: ? * []

  • ls -la b[ao][wl]??.txt

• Hard Link vs Symbolic Link

  • hardlink: duplicate directory entry where both entries point to the same file
  • symbolic link: a file that refers to another file by name. ln -s originame linkname

Piping

https://linuxhint.com/linux_pipe_command/

https://ryanstutorials.net/linuxtutorial/piping.php

https://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html#tag_02_07

Every single program in the UNIX/Linux system has 3 built-in data streams.

• STDIN (0): Standard input
• STDOUT (1): Standard output
• STDERR (2): Standard error

pipe redirection (|): send data from a program to another. file redirection (>): sends or gets data to/from a file

• | : stdout to stdin
• |& (shorthand for 2>&1): stderr and stdout to stdin
• > : redirects to a file, overwrittes it
• \2> : redirects errors to a file, overwrittes it
• \&> : redirects errors to a file, overwrittes it
• >> : redirects to a file, appends content
• < : redirects from a file

  user@bash: wc -l < myoutput
  8

• <>:

  user@bash: wc -l < barry.txt > myoutput
  user@bash: cat myoutput
  7

• 2> : redirects errors

  user@bash: ls -l video.mpg blah.foo 2> errors.txt

• 2>&1: errors redirected to the stdouput (&1) redirect errors to stdout and then redirect everyting to a file. When Bash sees several redirections it processes them from left to right.

  ls asdf.txt > myoutput 2>&1 

  the 2>&1 output goes to myoutput

https://stackoverflow.com/a/40319372/1034806

-command > output is just a shortcut for command 1> output

Scripts

#! (is a script) /... (program executed)

#!/bin/bash

#!/bin/bash
 
# variables:
# $ip=....
 
# conditional expressions
# if [ -s filename ]
 
filename=$1  
if  [  -f  "$filename"  ]; then  
echo  "File exists"  
else  
echo  "File does not exist"  
fi

execute it:

./asdfasdf.sh

Processes

https://www.tecmint.com/linux-process-management/

• Foreground processes (interactive processes) – these are initialized and controlled through a terminal session.
• Background processes (non-interactive/automatic processes) – are processes not connected to a terminal; they don’t expect any user input.
• Daemons: A program that runs as a "background" process (without a terminal or user interface), commonly waiting for events to occur and offering services.
• The Init Process (/sbin/init): The parent of all processes on the system, it’s the first program that is executed when the Linux system boots up.
• pid (process id), ppid (parent pid), ps, top (interactive updating view of ps), free (check free ram)
• Kernel ring buffer (/var/log/dmesg) : is stored on memory (dmesg). A ring buffer is a special kind of buffer that is always a constant size, removing the oldest messages when new messages come in.

Networking

• DNS (Domain Name System) (Internet phonebook)
• DHCP (Dynamic Host Configuration Protocol): Is a network management protocol used on Internet Protocol networks whereby a DHCP server dynamically assigns an IP address and other network configuration parameters to each device on a network so they can communicate with other IP networks.
• Ehernet: Is a family of computer networking technologies commonly used in local area networks (LAN), metropolitan (MAN), wide (WAN).
• Hostname: The name the computer gets
• Netmask (Subnet mask): Divide the net on subnets. The masks determines all the subnet parameters.
• Gateway: A gateway is a network node that serves as an access point to another network. A default gateway is the node in a computer network using the internet protocol suite that serves as the forwarding host (router) to other networks when no other route specification matches the destination IP address of a packet
• TCP/IP : set of standards that underlie most modern network connections at the sw level
• ip address ⇒ netmask ⇒ ip address(router) ⇒ ip address (dns server)

Programs

• traceroute (track problems in connection)
• test dns: host, dig, nslookup
• netstat

Security

1. shut down servers
2. enable a firewall
3. use good passwords
4. keep sw up to date

User accounts and groups

/etc/passwd

whoami

id

adduser

useradd

usermod and groupmod to manage users and groups

who: who is logged in the system

var/log/auth.log : all info about users

Ownerships and permissions

permissions for user, group and others

chown

chgrp

umask

sticky bit

suid

sgid