In 1991 when Linus Torvalds for the first time publicly shared his newly made kernel completely free from minix code, then even he was not aware of its capabilities in bringing the revolution in the software industry at that time. The open source we hear today was not much popular back then, not until linux was founded. You see, Git a very popular VCS(Version Control System) was founded by the same guy to maintain the linux kernel. People loved this operating system not just because it was free but because it brought freedom to the computer and software industry. Linux can be customized for specific users and for specific hardware requirements.
According to a survey by w3techs.com in 2020, Almost 29.7% of the websites uses linux operating system. Around 49% of all the developers uses linux as their daily drive for development purpose according to survey from 2018-20 by statista.com. Linux is a popular choice among developers and tech enthusiasts. So today i bring to you these essential linux commands that you should know no matter you are an experienced developer or a newbie. Lets have a look -

1) ls [OPTION]... [FILE]...

ls stands for list. ls command is used to list the contents of a directory or information about a file. By default ls(without any options) lists the contents of the directory you are currently in.
usage:ls to list contents of the current directory

usage:ls -a to list all the contents of the directory including hidden ones

usage:ls -A to list almost all the files (ignoring . and .. implied files)

usage:ls -Al to list almost all the files (ignoring . and .. implied files) together with their author name

usage:ls -bl to list files according to the size together with their author name

2) pwd [LP]

pwd stands for present working directory. This command is used to print the path of the current working directory. This command has only two options -L and -P
usage:pwd to print the path of the current working directory

3) mkdir [OPTION]... DIRECTORY...

mkdir stands for make directory. This command is used to create one or more than one directories if they do not already exists.
usage:mkdir john to create a directory named john

usage:mkdir john doe to create two directories named john and doe

usage:mkdir -m [FILE MODE] john to create a directory named john with a specified file mode

usage:mkdir -v [DIRECTORY NAME] to create a directory and print a message.

4) echo [SHORT-OPTION]... [STRING]...

This command is used to print a piece of text on the screen.
usage:echo [MESSAGE] to print the message on a new line

usage:echo -n [MESSAGE] to print the message on the same line as the pointer

5) whoami [OPTION]...

This command is used to print the name of the user associated with the current user ID. This command take only two arguments - --help and --version.
usage:whoami to print the username of the current user

6) cd [-L|[-P [-e]] [-@]] [dir]

cd stands for change directory. This command is used to change the working directory. By default this command changes from current directory to home directory.
usage:cd Downloads to move from current directory to Download directory

usage:cd to move from current directory to home directory

7) cat [OPTION]... [FILE]...

cat stands for concatenate. This command is used to print the contents of a file to the screen. Without any option cat is used to print output from the standard input.
usage:cat john.txt to print the content of file john.txt to the screen

usage:cat -s john.txt to print the content of john.txt by suppressing repeated empty output lines

8) top -hv|-bcEHiOSs1 -d secs -n max -u|U user -p pid -o fld -w [cols]

This command is used to display processes of a linux system. This command takes several options. Refer man page for a full description.
usage:top to display linux processes to the screen

9) man [man options] [[section] page ...] ...

man stands for manual. This command is probably the most useful but unfortunately a bit neglected command. A beginner should always first refer to the man pages for a total description,options and usage of a specific command.
usage:man ls to the open manual page of ls command

10) kill [-s sigspec | -n signum | -sigspec] pid | jobspec ... or kill -l [sigspec]

This command as it sounds is used to kill a specific process by its PID, signal name or signal number.
usage:kill -15 [PID] to soft kill the process of the following PID

usage:kill -9 [PID] to immediately kill the process of the following PID

usage:killall [PROCESS NAME] to kill all the instances of the following process owned by the current user

11) more [options] ...

This command is used to limit the contents of a file printed to the screen.
usage:more /etc/passwd to print the contents of passwd file to the screen. Press ENTER to advance one line, SPACE to advance a full page and q to quit

12) less [options] ...

This command is used to limit the contents of a file printed to the screen. It has more options than more command.
usage:less /etc/passwd to print the contents of passwd file to the screen. Press ENTER to advance one line, SPACE to advance a full page but does not quits when reaches the end.

13) ifconfig [-a] [-v] [-s] <=interface=> [[<=AF=>] ]

This command is used to list all the network interfaces of the linux machine. This command can also be used to change an interface's IP address, assign IP address to a specific interface, take an interface offline,online and more.
usage:ifconfig to list all the network interfaces together with information like IP address, MAC address, etc.
Note:ifconfig can be installed by sudo apt-get install net-tools in case of debian based distros.

14) grep [OPTION]... PATTERNS [FILE]...

This command is used to search for patterns in a specific file.
usage:grep -i 'john doe' menu.h main.c to search for john doe in the following files

15) who [OPTION]... [ FILE | ARG1 ARG2 ]

This command is used to print the information of the users who are currently logged in.
usage:who to the know the information about the users currently logged in

usage:who -b to print the time of the last system boot

16) alias [-p] [name[=value] ... ]

This command is used to give name to a command or sequence of commands. By default Without arguments, alias' prints the list of aliases in the reusable formalias NAME=VALUE' on standard output.

usage:alias cls=clear to execute clear command by typing cls.

17) chmod [OPTION]... MODE[,MODE]... FILE...

This command is used to set permissions for a file or to change the file mode.
usage:chmod 765 john.txt to set the permission of john.txt as read, write and execute(7) for the owner, read and write(6) for the group and read and execute(5) for others

18) curl [options...]

This command is used to retrieve information and files from or to a server using one of the supported protocols.
usage:curl [URL] to retrieve information from the following url
*Note:*The curl tool can be installed with sudo apt-get install curl in case of debian based distros.

This was just some of the important linux commands. For a full list of linux commands or linux cheat sheet please visit this link:Linux-Cheat-Sheet
Note:If you know some commands then please take a minute and contribute to the link above.

Thanks for Reading :)