Comprehensive Guide to All Essential Linux Commands

date
- Used to print the current date and time in the terminal.
echo "hello world"
- Used to display lines of text or strings that are passed as arguments on the command line.
ls
- Lists the contents of a directory.
pwd
- Prints the present working directory.
whoami
- Displays the username of the user currently logged in.
mkdir
- Creates directories.
touch
- Used to update the access and modification times of files, and to create new, empty files.
cd <directory_name>
- Changes the current directory to the specified directory.
cd
- Changes to the user's home directory.
cd /

Changes to the root directory (/), which contains all important system files in Linux.

df -h

Shows the amount of free disk space on each mounted disk.

man

Provides detailed information about a command or function, including its syntax, options, and examples.

cd ..

Moves up one directory level.

ping

Used to check the network connectivity between the host and another server or host.

touch file{2..10}.txt

Creates files named file2.txt through file10.txt.

rm -rf file*

The rm command is used to remove files or directories. The -rf option removes files recursively and forces the operation without prompting.

sudo

The sudo command stands for "Super User Do". It allows a permitted user to execute a command as the superuser or another user.

sudo apt-get update

Updates the package list on Ubuntu-based systems.

sudo apt-get install docker.io

Installs Docker on a Linux system (Ubuntu).

sudo apt purge docker

Uninstalls Docker from the system.

vi <filename>

Opens the vi command-line text editor to create or edit a file.

uptime

Shows how long the system has been running.

cd home

Changes to the /home directory.

mkdir -p /Rajan/file/myfile

Creates multiple directories along the specified path.

cd ~

Changes to the home directory of the user.

ls -a

Lists all files, including hidden files (those starting with a dot).

echo "hello-world" > hello.txt

Redirects the string "hello-world" into the file hello.txt.

vi

A command-line text editor used to create, edit, and modify files.

:wq!

Saves the file and exits the vi or vim editor.

`Common vi/vim Commands:`

i — Switch to insert mode.
Esc — Switch to command mode.
:w — Save the file and continue editing.
:wq or ZZ — Save and quit.
:q! — Quit without saving changes.
yy — Yank (copy) a line of text.
p — Paste the copied text.
o — Open a new line below the current line.
O — Open a new line above the current line.
A — Append text at the end of the line.
a — Append text after the cursor's position.
I — Insert text at the beginning of the line.
b — Move to the beginning of the current word.
e — Move to the end of the current word.
x — Delete a single character.
dd — Delete the entire current line.
Xdd — Delete a specified number of lines.
Xyy — Yank a specified number of lines.
G — Go to the last line of the file.
XG — Go to line X in the file.
gg — Go to the first line of the file.
:num — Display the current line's line number.
h — Move left by one character.
j — Move down by one line.
k — Move up by one line.
l — Move right by one character.

head hello.txt -n 2

Displays the first two lines of hello.txt.

cat <filename>

Displays the contents of the specified file.

head -n 4 example1.txt

Displays the first 4 lines of example1.txt.

tail

Displays the last ten lines of a file (default behavior).

tail hello.txt -n 3

Displays the last 3 lines of hello.txt.

sudo apt install nginx

Installs the NGINX web server.

Additional Steps:

To access the public IP of an EC2 instance, copy the public IP and paste it into another browser window.
Change to the NGINX directory: cd /var/www/html
Edit the default index file: sudo vi index.nginx-debian.html (Modify the file as required).
Again, copy the public IP of the EC2 instance and paste it in another browser. It will open the page of NGINX .
========================================================= User Management in Linux
1. sudo su
  - Use this command to log in as the superuser (root). The root user has all privileges.
2. sudo useradd -m Devra
  - Adds a user named Devra and the -m option creates the user's home directory.
3. sudo passwd Devra
  - This command sets a password for the user Devra.
4. su Devra
  - Switches to the user Devra.
5. cat /etc/passwd
  - Displays the contents of the /etc/passwd file, where you can see the list of users.

`Create Multiple Users and Groups`

Create users:
```
 sudo useradd -m user-1
 sudo useradd -m user-2
 sudo useradd -m user-3
```
- Creates multiple users user-1, user-2, and user-3, each with their own home directory.
Create groups:
```
 sudo groupadd devops
 sudo groupadd Tester
```
- Creates two groups: devops and Tester.
Add user to a group:
```
 sudo gpasswd -a user-1 devops
```
- Adds user-1 to the devops group.
cat /etc/group
- Displays the list of groups and users associated with each group.

`File Permissions in Linux`

File Permissions:
- r - Read: Anyone can read this file.
- w - Write: Anyone can modify this file.
- x - Execute: Anyone can execute the file programmatically.
Example: -rw-rw-r--
- First three (rw-): User has read and write permissions.
- Second three (rw-): Group has read and write permissions.
- Last three (r--): Others have read-only permission.

sudo chown Devra hello.txt

Changes the ownership of the file hello.txt to user Devra.

`SSH (Secure Shell)`

Launch an Ubuntu EC2 instance.

Go to the path /home/ubuntu/.ssh on your local machine.

Generate a .pem file and copy the private key's content.
SSH into another EC2 instance:

Go to the other instance and run the SSH command to connect using the copied private key.

`Package Management in Linux`

sudo apt-get update

Updates the package list for the system.

sudo apt upgrade

Upgrades installed packages to their latest available versions.

sudo apt install nginx

Installs the NGINX package.

sudo apt purge nginx

Uninstalls the NGINX package.

systemctl status nginx

Checks the current status of the NGINX service.

sudo systemctl stop nginx

Stops the NGINX service.

sudo systemctl start nginx

Starts the NGINX service.

`GREP (Global Regular Expression Print)`

grep junoon -r /home/ubuntu/

Searches for the string "junoon" recursively under the directory /home/ubuntu/.

grep junoon -ir /home/ubuntu/

Performs a case-insensitive search for "junoon" under /home/ubuntu/.

`AWK`

awk '/WAR/' Zookeeper_2k.log

Searches for the string "WAR" in the Zookeeper_2k.log file.

awk '/WAR/ {print $1}' Zookeeper_2k.log

Prints the first column for each line containing "WAR".

awk '/WAR/ {print NR $1$5}' Zookeeper_2k.log | grep ZooKeeperServer@793]

Prints specific fields from Zookeeper_2k.log and filters with grep for the key "ZooKeeperServer@793]".

awk '$2>="19:16:14" && $2<="19:18:14" && /WAR/ {print $1$2$5}' Zookeeper_2k.log

Filters and prints logs between specific timestamps containing the string "WAR".

`SED (Stream Editor)`

sed 's/Rajan@1234/****/g' password.txt

Replaces all occurrences of "Rajan@1234" with "****" in password.txt.

sed -i 's/Rajan@1234/RK/g' password.txt

Directly modifies password.txt, replacing "Rajan@1234" with "RK".

`Find`

find . -name *.log

Finds all .log files in the current directory and its subdirectories.

find /home/ubuntu -name *.log

Searches for .log files under the /home/ubuntu directory.