iptables command blocks or allows traffic on a Linux host, similar to a network firewall. This iptables command may prevent certain applications from receiving or transmitting requests.

cd- command go to the previous directory.

df -I command shows free inodes on mounted filesystems.

cd dir && command go to the dir, executes the command and returns to the current directory.

pushd command put current dir on the stack so you can pop back to it.

Linux is an open-source operating system, nowadays it is rapidly growing in the technology market. We have a few reasons why Linux is more secure than other OS.

• The perk of accounts: Linux allows only a few users to access the system. Thus, the virus cannot attack the whole system, it may cause only a few files in the system. 
• Strong Community: Linux users first accomplished the files before they open. So they can save their systems from vulnerabilities. 
• Iptables: Iptables are used by the Linux because it checks the security circle of the system. 
• Different Working Environment: Linux system has different working environments like Linux Mint, Debian, Arch, and many more, these working environments protect from the virus.
• Recording in Linux: It maintains log history because later it can view the details of the system files easily. 
• Few User: Linux users are less compared to others, due to this security will be more.

This command shows top disk users in the current dir.

This command sort path by easy to interpret disk usage.

This command shows free space on mounted file systems.

fdisk -I command show disks partitions sizes and types (run as root).

Use this command to kills the program using one port: sudo fuser -k 8000/tcp

Use these commands:

•	tar tf file.tgz
•	tar xf file.tgz filename

Use this command to find who logged in: w

It is useful to detect permissions errors, for example when configuring a web server.

namei -l /path/to/file.txt

Use this command to do:

while inotifywait -e close_write document.tex
do
make
done

Use this command: cat file.txt | xclip -selection clipboard

Use this command to check resource usage: /usr/bin/time -v ls

Use this command: timeout 10s ./script.sh

# Restart every 30 minutes
while true; do timeout 30m ./script.sh; done

Use this command: comm file1 file2.

It is a form of redirection that is used in Linux, it is used to combine more than two commands and the output of one command can take as input to the next command. 

Syntax:

command_1 | command_2 | command_3 | .... | command_N

The parent process communicates with the child process by using pipes, sockets, messages queues, and more. 

It is a centralized server that does not have any exists states on the working station. It may have scenarios when a state of a particular system takes a snapshot then, the user wants all other machines to be in that particular state.

Features of Stateless Linux Server

1. Stores the prototype of every system.
2. Stores the snapshot was taken. 
3. Stores the home directories.
4. Uses LDAP, which contains the information about which snapshot should run on which system. 

It is a process whose execution is completed but even the information exists in the process table. It occurs for the child process because the parent process needs to read the child process status. Once it is completed using the wait system call, then the zombie process is removed from the process table. This is known as Zombie Process.

In Linux, the Ctrl+Alt+Del key is used to restart the computer, and it does not display any confirmation message before rebooting the system.