Linux File Editor(vi)

• A text editor is a program which enables you to create and manipulate data (text) in a Linux File

  • vi supplies commands for:

    • Inserting and Deleting text

    • Replacing text

    • Moving around the file

    • Finding and substituting strings

    • Cutting and pasting text

  • Most common keys-

    • i = insert

    • esc = escape out of any mode

    • r = replace

    • d = delete

    • dd = delete current line

    • x = delete the current character

    • u = undo

    • :q! = quit without saving

    • /<name> = to search

    • :wq! OR SHIFT+Z,Z\= quit and save

    • :set number = display no. of lines

    • :set nonumber = turns off numbering of lines

    • :%s/aditya/shiv = replace aditya with shiv

vi vs vim editor

• Vi is known for its modal editing, where the meaning of typed keys depends on which mode the editor is in.

• Vim is an improved clone of Vi that offers additional features and functionality.

• Vim is designed for use both from a command-line interface and as a standalone application in a graphical user interface.

• Vi is typist-friendly, with short commands that don't require removing hands from the keyboard.

• Vim is the most commonly used implementation of the Vi standard.

“sed” Command

• Replace a string in a file with a newstring

• Find and delete a line

• Remove empty lines

• Remove the first or n lines in a file

• To replace tabs with spaces

• Show defined lines from a file

• Substitute within vi editor

1. sed ‘s/aditya/shiv/g’ seinfeld-characters - by this cmd aditya will get replace with shiv but this reflection will not occur in actual file

   

2. sed -i ‘s/aditya/shiv/g’ seinfeld-characters - this will reflect in the file

   

3. sed ‘/aditya/d’ seinfeld-characters - It will delete the aditya keyword from file and it will just display it on the screen, not inside the file.

   

4. sed ‘/^$/d’ seinfeld-characters - To delete the empty lines inside the file

5. sed ‘1d’ seinfeld-characters - To delete 1st line

6. sed ‘1,2d’ seinfeld-characters - to delete 1st & 2nd line

7. sed ‘s/\t/ /g’ seinfeld-characters - Replace tab from space

8. sed -n 12,18p seinfeld-characters - Display define lines from bottom

9. sed 12,18d seinfeld-characters - Display define lines from top

10. sed G seinfeld-characters - Enter after every line

    

11. sed ‘8!s/shiv/RAM/’ seinfeld-characters - Substitute the RAM over shiv after line 8.

User Account Management

Commands

• useradd

• groupadd

• userdel

• groupdel

• usermod

Files

• /etc/passwd

• /etc/group

• /etc/shadow

Example

• useradd -g superheros -s /bin/bash -c “user desc“ -m -d /home/spiderman spiderman

  

Hands-On

1. Create a super using useradd-

   

2. Create a group-

   

3. Delete a user-

   

4. Delete the group-

   

5. Modify the user-

   

   NOTE: use the passwd <filename> to setup the password.

Enable Password Aging

The /etc/login.defs File

• The chage command - per user

  • Example

    chage [-m mindays] [-M maxdays] [-d lastday] [-I inactive] [-E expiredate] [-W warndays] user

• File = /etc/login.def

  • PASS_MAX_DAYS 99999

  • PASS_MIN_DAYS 0

  • PASS_MIN_LEN 5

  • PASS_WARN_AGE 7

Switch Users and sudo Access

• Commands

  • su - username

  • sudo command

  • visudo

• File

  • /etc/sudoers

Monitor Users(who, last, w, id)

• Commands

  • who - Used to check how many persons are logged into the system.

    

  • last - Check for the login history

    

  • w - Just like who with some extra details

    

  • finger OR pinky

    

  • id

    

Talking to Users

• Commands-

  • users - print the user names of users currently logged into the current host

  • wall - write a message to all users…..cntl+d to send the msg.

  • write - write a message to specific user — CMD- write <username>

Linux Account Authentication

• Types of Accounts

  • Local accounts

  • Domain/Directory accounts

• Active Directory = Microsoft

• IDM = Identity Manager

• WinBIND = Used in Linux to communicate with Windows(Samba)

• OpenLDAP(open source) = it’s a protocol

• IBM Directory Server

• JumpCloud

• LDPA = Lightweight Directory Access Protocol

System Utility Commands

• date = shows date and time

• uptime = tell from how much time the system is running

• hostname = shows hostname

• uname = print system information

• which = shows full path of cmd

• cal = display a calendar

• bc = binary calculator

Processes and Jobs

• Application = Service

• Script = Shell Scripts or cmd are list of instructions e.g., adduser, cd, pwd etc

• Process = Every Service has Process

• Daemon = Runs till interrupted

• Threads = Every Service has Process and it has threads

• Job = Job or workorder- Run a service or process at a schedule time

Commands-

• systemctl or service

  • systemctl command is a new tool to control system services

    • Examples:

      • systemctl start|stop|status servicename.service

      • systemctl enable servicename.service

      • systemctl restart|reload servicename.service

      • systemctl list-units --all

  • To add a service under systemctl management:

    • Create a unit file in /etc/systemd/system/servicename.service

  • To control system with systemctl

    • systemctl poweroff

    • systemctl halt

    • systemctl reboot

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• ps

  • ps command stands for process status and it displays all the currently running process in the Linux system.

    • Example:

      • ps = Shows the processed of the current shell

      • ps -e = Shows all running processes

      • ps aux = Shows all running processes in BSD format

      • ps -ef = Shows all running process in full format listing

      • ps -u username = shows all processes by the username

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• top

  • top command is used to show the Linux processes and it provides a real-view of the running system

  • This command shows the summary information of the system and the list of processes or threads which are currently managed by the Linux Kernel.

  • When the top command is executed then it goes into interactive mode and you can exit out by hitting q.

  • Example:

    • top -u username = shows tasks/processes by user owned

    • top then press c = shows commands absolute path

    • top then press k = kill a process by PID within top session

    • top then M and P = to sort all Linux running processes by Memory usage

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• kill

  • this command is used to terminate processes manually

  • It sends a signal which ultimately terminates or kills a particular process or group of processes

    • Example:

      • kill [OPTION] [PID]

        • OPTION\= Signal name or signal number/ID

        • PID = Process ID

    • kill PID = Kill a process with default signal

    • kill -1 = Restart

    • kill -2 = Interrupt from the keyboard just like ctrl+C

    • kill -9 = Forcefully kill the process

    • kill -15 = Kill a process gracefully

    • killall = Kill All

    • pkill = Kill by process name

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• crontab

  • Crontab command is used to schedule tasks

  • Usage:

    • crontab -e = edit the crontab

    • crontab -l = list the crontab entries

    • crontab -r = remove the crontab

    • crond = crontab daemon/service that manages scheduling

    • systemctl status crond = to manage the crond service

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• at

  • at command is like crontab which allows you to schedule jobs but only once

  • When the command is run it will enter interactive mode and you can get out by pressing CTRL+D

  • Usage:

    • at HH:MM PM = Schedule a job

    • atq = List the at entries

    • atrm # = Remove at entry

    • atd = at daemon/service that manages scheduling

    • systemctl status atd = To manage the atd service

  • Other future scheduling format:

    • at 2:45 AM 101621 = schedule a job to run on Oct 16th, 2021 at 2:45am

    • at 4PM + 4 days = schedule a job at 4pm four days from now

    • at now +5 hours = schedule a job to run five hours from now

    • at 8:00 AM Sun = schedule a job to 8am on coming Sunday

    • at 10:00 AM next month = schedule a job to 10am next month

Additional Cron Jobs

• by default there are 4 different types of cronjobs

  • Hourly

  • Daily

  • Weekly

  • Monthly

• All the above crons are setup in

  • /etc/cron.____ (directory)

• The timing for each are set in

  • /etc/anacrontab - - except hourly

• For hourly

  • /etc/cron.d/0hourly

Process Signals in Linux

Types-

• SIGNIT (Signal Interrupt) —> Used to interrupt process: Kill -SIGINT PID

• SIGTERM (Signal Terminate) —> Process to terminate gracefully: kill -SIGTERM PID

• SIGKILL (Signal Kill) —> Forcefully kill process: kill -SIGKILL PID

• SIGSTOP (Signal Stop) —> Pause Running process: kill -SIGSTOP PID

• SIGCONT (Signal Continue) —> Resume Stopped Process: kill -SIGCONT PID

• SIGSEGV (Signal Segmentation Fault) —> Access Invalid Memory Location: kill -SIGSEGV PID