Linux command

Meaning

ls

Lists all files and directories in the present working directory

ll

ll= long list To show all files with details.

ls – R

Lists files in sub-directories as well

ls – a

Lists hidden files as well

ls – al

Lists files and directories with detailed information like permissions, size, owner, etc.

ls -ltr

latest file will show at last in list

ls -lrtha

a>> all files (including hidden files) l>> long list. r>> reverse order. T> h> human redable

ls li

l>> long. list i>> inode

mkdir madhu

mk= Make. dir=directory. To create new directory (folder) with directory name madhu.

rmdir madhu

rm=remove. dir=directory. To remove directory (folder) madhu.

rm -vrf filename

v >> verbose (to see which file is deleting first) f >>. forcefully. Deletes a file

cd

c=change. d=directory. change to directory.

touch

create empty files.

cat > filename

create new file and writes the content and ctrl+D to save content into the file.

cat >> filename

>> means append. To add more lines in to the same file without deleting the previous contents.

cat filename

display content of file.

cat file1 file2 > file3

Joins two files (file1, file2) and stores the output in a new file (file3)

pwd

present working directory. To show present working directory.

cp

copy a file or directory.

mv file “new file path”

moves a file or directory. Moves the files to the new location.

mv filename new_file_name

Renames the file to a new filename

head

display first 10 lines of a files.

tail

display last 10 lines of a file.

tac

display file content lines in reverse.

more

similar to cat and here we can display large content by using ENTER, SPACEBAR.

id

display id of user/group.

clear

clear the screen.

vi

text editor to write programs of text.

grep

filter to search given pattern in the file content.

diff

compares the content of two different files

ping

check the connectivity status of server.

sudo

Allows regular users to run programs with the security privileges of the superuser or root

man

Gives help information on a command

history

Gives a list of all past basic Linux commands list typed in the current terminal session

clear

Clears the terminal. You can press Ctrl+L.

pr -x

Divides the file into x columns

pr -h

Assigns a header to the file

pr -n

Denotes the file with Line Numbers

lp -nc
lpr c

Prints “c” copies of the File

lp -dlpr -P

Specifies name of the printer

apt-get

Command used to install and update packages

mail -s ‘subject’ -c ‘cc-address’ -b ‘bcc-address’ ‘to-address’

Command to send email

mail -s “Subject” to-address < Filename

Command to send email with attachment

date

to display the date

touch vin-date

To create file with date appended in the name

cal

To show the calendar

who

to show who logged in and from how long with time

last

to show all users who logged in succesfully and from how long with time

lastb

b >> bad log in's means it will show who tried to log but not loged in.

w

who is loged in and what they are eecuting or doing.

audit.log

file will show what has done and who has done.

SCP to copy file from aws's ec2 server to > local server's current directory.

Vinayak@vinayak-desktop:~/Downloads$ scp -i "terra-ubuntu-server-key.pem" ubuntu@ec2-54-89-77-131.compute-1.amazonaws.com:/home/ubuntu//terraform-practice/ .

scp= secure copy,
-i= input key location and ubuntu=server's user name, @ec2-54-89-77-131.compute-1.amazonaws.com aws's ec2 instance public ip address or endpoint.

: = for mentioning the path on the server where file is located which we need to copy.
/ = all files
. = paste in my current folder.

SCP to copy file from > local server to >> aws's ec2 server

Note:> Make sure you have the write permission (o+w) on the directory on ec-2 server

scp -i "terra-ubuntu-server-key.pem" ./vin.txt ubuntu@ec2-54-89-77-131.compute-1.amazonaws.com:/home/ubuntu/vin/

Here >> For directry vin located at > /home/ubuntu/vin/you must have write permissin to others. (chmod o+w /home/ubuntu/vin)

Only Important commands

Linux commands

Meaning

Details

sudo tail -f /var/log/nginx/access.log

To check nginx live logs

sudo tail -100 /var/log/nginx/access-new.log | grep executeapis

To grep executeapis from last 100 lines of the log file

If we specify the -n option, grep will prefix each matching line with the line number. -i option to perform a case-insensitive match. Refer>>> https://www.computerhope.com/unix/ugrep.htm

sudo tail -f /var/log/nginx/access.log | grep -v "notifications"

-v >> exclude

To grep live logs by excluding the word notifications.

tail -n 4 long.txt

Last 4 lines

To view only the last four lines of file long.txt

free

Free memory. (RAM and SWAP)

The free command gives information about used and unused memory usage and swap memory of a system. By default, it displays memory in kb (kilobytes). Memory mainly consists of RAM (random access memory) and swap memory. Options -b to get outputs in bit. -k = show in Kb. -m=MB. -g==GB. -w= Buffer and cache shows separate. -lm = (l)-list down the lowest memory utilization in (m)Mb.

df -Th

disk free

To show the disc partition details. -h=human readable format. -T=Filesystem type.

du -h

disk usage

Disk usage of current diretory -h=human readable format.

du -sh | sort -rn | head

>>> To print the highst disc using files at top

aide

Advanced Intrusion Detection Environment (aide)

To show who modified the file, what is modified and when.

grep "root" /etc/passwd

Global Regular Expression Print (grep)

To find the string of characters from the particular file.

chattr +i filename

Change attributes

To protect any file from accidental delete.

chmod +rwx

Change mode

To change file permissions. +rwx to add the read write and execute permissions.

./ file name

Run script

To run a script file

apt, yum & pacman

Package managers

Used to install, update, and remove the softwares.

yum command will replace by dnf command

apt-get is a command line tool in Debian and Debian-based Linux distros that you use to install and manage packages.

apt install httpd

To install Apache server (httpd) on Ubuntu OS.

Used for Debian-based (Ubuntu, Linux Mint) OS to install packages/softwares

apt-get install Nginx

To install Nginx server.

https://linuxhint.com/diff_apt_vs_aptget/

yum install httpd

To install Apache server (https) on Redhat based OS.

For Red Hat-based (Fedora, CentOS)

yum info tree

To find out package for any commands

sudo apt-get update

To check packages that can be updated.

It'll figure out what the latest version of each package and dependency is, but will not actually download or install any of those updates.

https://www.freecodecamp.org/news/sudo-apt-get-update-vs-upgrade-what-is-the-difference/

apt list --upgradable

To see which packages can be upgraded

sudo apt-get upgrade

Only upgrades what it can without removing anything.

sudo apt autoremove

To remove unnecessary packages.

pacman -S httpd

For Arch-based (Manjaro, Arco Linux)

service nginx start or systemctl start nginx

To start the Nginx. systemctl is for Redhat 8+ & service strat is for older versions.

Note:- When we install packages using yum install we need to start the service using systemctl cammand. When we use apt in ubuntu to install packages, the service will start by default.

chkconfig nginx on or
systemctl enable nginx

To enable the nginx service.

chkconfig= For older versions and systemctl for latest versions of OS.

ps

Process show

To view the running process in the current shell session.

ps fax | grep nginx

To find the process running related to Nginx.

lsof -i -P

List Of Open File. -i >> select internet files

It will list all open files with port number, node name and PID details

kill pid

kill with process ID

To kill the processes by entering either the PID (processes ID) or the program’s binary name.

head -n 5 long.txt

First 5 ines

To view only the first five lines of file long.txt

vim file-name

To open and edit the file.
To comment the lines>> 1) :[start line],[end line]s/^/# >> :10,15s/^/#
2) Shift+V >> up-down arrow >> :s/^/#

Press ESC and navigate to the lines you want to comment out. >> Press CTRL + V to enable Visual Mode. >> Using the up and down arrow key, highlight the lines you wish to comment out. >> Once you have the lines selected, press the SHIFT + I keys to enter insert mode. >> Enter your command symbol, for example, # sign, and press the ESC key. Vim will comment out all the highlighted lines.

In vim press esc and type
:redo >>for redo the changes.
:undo >> for undo the changes.

1) :%s/^#/
2) Shift+V >> up-down arrow >> Press x to remove the comments

Navigate to the line you wish to comment out and press CTRL + V to enter Visual mode. >> Use the up and down arrow keys to highlight the lines you wish to uncomment. Once selected, press x to remove the comments.

vim editor's Short cuts

Working >>>>

For indentation try >> Press "SHIFT + v" to enter VISUAL LINE mode.
Select the text you wish to indent but using either the cursor keys or the "j" and "k" keys.
To indent right press "SHIFT + dot" (> character). To indent left press "SHIFT + comma" (< character).

whoami

Print current user name

To print the Current user name

whatis command name

To prints a single-line description of any other command

which command name

To show the full path of the shell of commands.

wc

word count

To show the “word count,”. It returns the number of words in a text file.

uname -a

Unix name

Prints the operating system's all information

netstat

network statistics https://www.supportsages.com/understanding-the-netstat-command/

netstat command to list all open ports, including those of TCP, UDP, which are the most common protocols for packet transmission in the network layer.

To find out which perticular process running on which perticular port >> We need to use netstat -anp command and then we need to use ps -ef | grep 457 (457> is PID)

netstat -tulnp

network statistics

Options== -l= tells netstat to only show listening sockets.. -n= instructs it to show numerical addresses. (-n). -t= tells it to display tcp connections.. -u= UDP ports. -p= enables showing of the process ID and the process name.

ss -tulnp

show socket / socket statistics

Just to ensure that we are getting consistent outputs, let’s verify this using the ss command to list listening sockets with an open port.

ss -punta | grep "51903" | wc -l

ss is used to dump socket statistics. It allows showing information similar to netstat. It can display more TCP and state information than other tools.

curl ifconfig.me

to get IP

To get our own machine's public IP address.

cp -i source destination

copy interactive mode

-i for interactive mode= it will check file is already present or not. If file is present then it will ask whether to overwrite it or not?.

Network troubleshooting Commands

Linux troubleshooting Commands Video duration 01 H :40 min :41 sec

https://www.tecmint.com/linux-networking-commands/

ping

to check the network connectivity.

to check the network connectivity.

yum install bind-utils

Package for network troubleshooting Commands

bind-utils package has many commands which are used for network troubleshooting. Such as dig, nslookup, host etc commands will get installed with this package.

host www.google.com

To print ip address

To print the ipv4 & ipv6 address for google.com

host -a www.google.com

print all details with ip address

ssh admin@ip

to check connection

nmap -A ip address

To find out port open on the remote server.

It is used for hacking purpose.

Fingerprints are used to provide authenticity between two servers

To provide authorizaton that what the server is and which server is trying to coneect.

mtr ip address of ec2/vm

To show details of packets sent and latency % lost packets

To check which firewall is blocking connection. yum install mtr= to install the package for the mtr command.

nslookup google.com

To check latency

To check latency between our machin and google server.

dig google.com

More details with header

To check more details of network connections with server ip, time etc.

tracepath www.google.com

To find out where our trafic went down.

tracepath=for linux 7+ versions and traceroot=for older versions. Tracepath is used to find out at exact what point our trafic went down. Whichever is the last hope that ip address should be our own ip. or if its other third party ip then that third party vendor blocking the connection. no reply in result means that perticular isp or router hiding their identity.

Process to copy or replace key file

ssh-keygen -t rsa

ssh key generation

To generate ssh keys, -t is for type and type is rsa. The result will show where identification and key has been saved in with location. The public key will be saved to /root/.ssh/id_rsa.pub

cat /root/.ssh/id_rsa.pub >> /home/ec2-user/.ssh/authorized_keys

To append keys into ec2-user's authorized keys.

cat .ssh/id_rsa

Open file select the entire key and then go to cd Downloads/ then create one file.

vi vin.pem

Paste the entire copied content and save it.

chmod 400 vin.pem

To change the permissions.

We can import keys in aws. Go to key pairs >> action >> import key >> give name to key and then go to the server and open file from location cat /home/ec2-user/.ssh/authorized_keys >> copy the appended contents starting from ssh-rsa till the end and paste it in to blank box and >> click on import key. We can use this key for creating new ec2 machine.

ssh -i "vin.pem" ec2-user@all copied command from aws

We have successfully replaced the key file with new one and logged in to the server using new .pem file.

OS troubleshooting Commands

We need to understand the linux bootup process in order to troubleshoot the OS.

Select the ec2 instance >> Actions >> Instance settings >> Get system logs. >> We can check what happen when linux/ OS boots up.

up to 1H 22 Min

https://www.dell.com/support/kbdoc/en-in/000123974/introduction-to-basic-troubleshooting-commands-within-ubuntu-linux

sudo dmesg > dmesg.log

To show the contents of the boot log

Use the command dmesg to show the contents of the boot log. This is a good way to find errors at startup

cat dmesg.log | grep error

To show the error word in dmesg.log file

lspci

Lists all PCI devices.

lspci

lsusb

Lists all USB devices.

lsusb

lsmod

Shows the status of modules in the Linux kernel.

ufw

Uncomplicated Firewall

Uncomplicated Firewall (UFW) is a program for managing a netfilter firewall designed to be easy to use. It uses a command-line interface consisting of a small number of simple commands, and uses iptables for configuration. UFW is available by default in all Ubuntu installations since 8.04 LTS.

ufw status

>> To check status of the firewall.

ufw enable

>> To enable the firewall.

date

to display the date

touch vin-date%F

To create file with date appended in the name

Port opening Link

https://www.digitalocean.com/community/tutorials/opening-a-port-on-linux

User management Lokendra

rpm

Redhat package manager

rpm -ql

less filename

To show file page by page. To exit need to press q to quit.

more filename

Work same like less To show file page by page but no need to quite.

vim /etc/default/useradd

To change default user's parameters like home directory. SHELL etc. Whenever we add new user the default SHELL and Directory will be defined by this (/etc/default/useradd) file

cd /etc/skel

If we add any file or folder in this directory then that all files and directory will get copied to user's home directory whenever we create new user after modifying this directory.

history

To Show command history

export HISTTIMEFORMAT='%F %T '

temp >> history format

to set HISTTIMEFORMAT variable temporarily to current user.

echo 'HISTTIMEFORMAT="%F %T "' >> ~/.bashrc

permanent >> history format

Append this HISTTIMEFORMAT variable to .bashrc or .bash_profile file to make it permanent for every user.

echo 'HISTTIMEFORMAT="%F %T "' >> ~/.bash_profile

history -i

To show history with dat in zsh shell.

iptables -I INPUT ! -s 192.168.1.2 -p tcp --dport ssh -m state --state NEW,ESTABLISHED,RELATED -j REJECT

https://www.golinuxhub.com/2014/03/how-to-allowblock-ssh-connection-from/

Block ssh connection from all the host EXCEPT 192.168.1.2

Linux command you must know:---

uname -o

To know the OS type:

uname -m

To know the CPU archit­ecture:

uname -r

To check the kernel version:

cat /etc/o­s-r­elease

To get the OS name, release, version:

lshw

To list the system hardware:

lscpu

To get the CPU details:

free -h or free -m

To check system memory:

vmstat -S m

To check the virtual memory stats:

echo 3 > /proc/­sys­/vm­/dr­op_­caches

Free memory cache, dentries and inode (with root):

ps aux --sort­=-%mem

To print the process specific memory utiliz­ations:

apt search <pa­ckage name>

To search packages for instal­lation:

e.g.: apt search python­-boto

sudo apt-get install <pa­ckage name>

To installed package:

sudo apt-get remove <pa­ckage name>

To uninstall package:

df -kh

To list the mounted disk drives:

mkdir -p <di­rectory path e..g /mount­-vo­l>

To mount the volume:

sudo mount <src path> <above created dir path>

(create the directory first to mount volume)

sudo du -a /dir/ | sort -n -r | head -n 5

To list biggest files from directory (biggest 5):

find <dir path> -name <fi­len­ame> -print

Find the file (search for a file):

find /var -name app.log –print`

e.g. to find app.log in /var directory

find /var -type f -print | xargs grep <search test>

Search the text string in a directory and print filename containing that string:

grep -rIn <search text> <di­rectory path>

File the text string from a given directory: