Linux

Unix vs Linux

• Unix developed in mid-1970 for multi-user, multi-tasking
• Mostly used by SUN and their operating system is Solaris, HP-UX, AIX 
• Supports fewer file systems
  
• Linux in 1991 by Linus Torvalds
• Linux is mostly free
• Open source
• Used mostly by Developer communities, or companies(REDhad, Centos, Debian and many)
• Can be installed on variety of computer hardware, mobiles, tablets, video games, super computers, main frames
• Can be installed on any hardware that is running by Intel & AMD processor

Oracle Virtual Box

1. Free and Open source Hypervisor for x86 computers developed by Oracle
   

Different Linux distributions

• 200+ OS derived from linux like.. Redhat, Ubuntu, Fedora, suse, Debian, Cent os, 
  

• Client
• Windows
• Cygwin 
• mac/Linux client
  
• ssh -V
• Client Server architechture
• ssh client on client system & ssh daemon on Server
• commands
• (sudo) yum -y install nano
• nano /etc/ssh/sshd_config
• edit line #PermitRootLogin yes to no
  
• systemctl restart sshd
• Navigation commands
• ~   short cut for home directory
• write file
• echo > a.txt
• touch b.txt   to create empty file
  
• running as admin
  
• sudo <command>   run single command as administrator
  
• sudo su    to become administrator
• Install new package
• yum install java        yum is package manager for Centos systems
  
• java -version
• Bash & Shell
• Shell is a processor to process what ever we type in terminal(bash, ksh, fish..)
• Bash is a default processor
• Bash script
• #!/bin/bash                -> so say use bash shell
  for i in 1 2 3 4 5; do
  echo "Hello $i";
  sleep 2;
  done;
• bash <script name>
• Background process
• & at the end of command
• stopping background process
• ps aux
• kill -9 <pid>
• start HTTP server
• sudo python -m SimpleHTTPServer 80
• Environment Variables
• env    to list all env variables
• echo $PATH
• export  - adds new environment variable
•  export abc="asdfasdf"
  
• Strings
• "   -> interpolating variables
  
• '   -> preserving strings exactly
  
• `  -> interpolating command output
• echo "today is `date`"
• Linux commands demistified
• linux commands are executables
• which <command> gives the location

SSH Fundamentals

• Interactive ssh
• ssh user@[hostname/ipaddress]
• long lived processes
• start process and keep it running after terminating ssh session
  
• nohup <command>
• It separates process from terminatl
• non-interactive ssh
• run multiple commands at a time
• ssh user@[hostname/ipaddress]  <command>

Creating Server Alias

• ~/.ssh/config add following
  Host [server_nickname]
  hostname [Server IP address]
  user [username]

How SSH works

• SSH daemon listens at port 22
• ECDSA is unique signature of server which can't be tampered. ESDSA is sent to client
• Encrypted communication channel gets created between both parties
• Client sends authentication details on encrypted channel
• Server verified 
• communication channel gets established. 

Uploading files(SCP)- uses ssh internally

• upload
• scp [local source path] user@[hostname/ipaddress]:[server_target_path]
•  download
•  
• scp user@[hostname/ipaddress]:[server_source_path]  [local target path] 

SFTP - ssh file transfer protocol

•  download cyberduck

SSH Key Authentication

• Avoid typing password
• More secure
• Problem with passwd authentication
• Secret information is sent over communication channel which can compromise security
• Process
• Generate public and private key (ssh key pair)
• Upload public key on server
• Tell client to use SSH authentication
• Private & Public key
• AssyPublic key can be distributed
• Private key to be kept confidential
• Data encrypted with public key can only be decrypted with private key
• Private key cannot be figured out from public key
• Public key can be figured/derived out from Private key
•  Generate ssh keys
• ssh-keygen -t rsa -b 4086 -C "just comments"
• Connect to server ~/.ssh/authorized_keys file
• copy content of public key to authorized_keys on a new line
• Test ssh keys
• ssh -i [privatekey] [user]@[server]

Amazon EC2

• sudo su -    to become a root user
  
• Dir structure
• /   ->ROOT directory similar to  C:\ in windows
  
• /root -> home directory for root user- similar to C:\Document and Setting\Administrator
  
• /home  -> home directory for other users- similar to C:\Document and Setting\username
• /usr -> softwares are installed- similar to c:\program files
• /bin -> commands used by all users
  
• /sbin -> commands used by super user(root)
  
•  /var -> container variable data like logs, mails..
• Basic commands
• User Management
• Super or root user -> most powerful & admin user
• /root   is home directory with shell /bin/bash
  
• Sytem user ->Created by applications or softwares
• /var/ftp  is home directory with shell /sbin/no login
  
• Normal user ->
• /home/<user> is home directory with shell /bin/bash
• user creation
• useradd <option> <name>
• UID & GID will be assigned to user
• Software Management
• yum for REDHAT
• yum install <package>
• yum remove 
• yum update
• yum info
• yum list
• yum install wget
• Networking
• hostname    ->servername
• init 6   or reboot -> restart system
• ping  -> to check if target is available or not
• wget -> download files/packages
• ifconfig -> to get ipaddress of your system
  
• telnet ->to check if a particular port is used or not
  
• curl ->access appln as from browser
  
• port numbers
• ftp - 21
  
• ssh -22
  
• telnet - 23
  
• smtp - 25
  
• dns - 53
  
• http - 80
  
• https - 443
• ports usage
• netstat -tulpn
• Services(systemctl)
• Service
• Controls starting and stopping of services
• service <name> <status | start | stop | reload | restart>
• yum install httpd   -> command to install http
  • service httpd status
  • service httpd start
  • telnet localhost 80
  • curl localhost
• chkconfig
• Check availability of service
• chkconfig --list
• chkconfig <service> <on | off> -> to make service available/unavailable after restart(boot time)
  

SSH(Secure shell)

• Method for secure remote login from one computer to another. 
• Provides alternative options for authentication
• Protects communications security and integrity with strong encryption
• port - 22
• Daemon - sshd
• conf file - /etc/ssh/sshd_config
•  HTTP
  
  • port - 80
  • Daemon - httpd
  • conf file - /etc/httpd/conf/httpd_config
  • default file - /var/www/html 
   

Process Management

• ps -ef  -> list of processes which are running in system
  
• kill -9 -> kill a process or service
• jobs -> gives list of jobs getting executed
  
• fg & %<job id> -> run process in foreground
  
• bg -> run in background
  
• top -> list top 20 process which are consuming more cpu

Archiving

• gzip
• Create compressed file 
• gzip <file>  -> file.gz gets created
  
• gunzip
•  gunzip <file.gz>
  
• tar
• extract tar file
• works on directories
• tar -cvf <directory> dir1.tar directory1
• tar -xvf  dir1.tar   ->  tar is a file
  
• gzip dir1.tar ->dir1.gz
• tar -xvzf dir1.gz  to extrat tar & directories -> does 2 steps

CronTab

• daemon called crond
• job scheduling
• crontab -l
• List crons
  
• crontab -e
• Min  Hr Day_Of_Month Month  Day_of_week  <command>
  
• Execute job at 8.30 in mornring
• 30 8 * * * 
• Execute job at 2 pm every saturday
• 0 14 * * 6
• Execute job at 12am on Jul1st
• 00 00 1 6 *
• Execute job at 3.30pm every month 25th
• 30 15 25 * *

Link files

• Softlink
• shortcut file
• ln -s <source file> <dest file>
• used to create short cut of shell script at /usr/local/bin/hello
  
• Harklink
• Backup file
• ln <source file> <dest file>

Windows to Linux

• Mobaxterm
• winscp

Copy from LINUX to LINUX server

• scp