uname : To check which platform you are using.

uptime : To check system uptime or when machines start.

date : To check the machine date and time.

who : To check all login user details with login time.

whoami : To check the existing.

which : To find the location of an executable file.

id : To check the user uid and group gid.

sudo : To provide administrator privileges.(sudo means Super Do)

cat /etc/passwd : To check created user.

shutdown : To shutdown your machine or server.

reboot : To reboot or restart your machine.

apt : Is a command line package manager. (For any installation, you need to use this command.)

apt-get : To download from the internet. (sudo apt-get install docker)

sudo apt remove docker : this will uninstall docker from your machine.

yum : This is used for CentOS system installation.

dnf : This is used for Fedora OS installation.

rpm : This is used for Red hat OS installation.

User and Group management commands:

sudo useradd -m user_name : To create a user

sudo passwd user_name : To create passwords for created users.

su : To change user.

sudo userdel user_name : To delete the user.

groupadd group_name : To create a group.

sudo gpasswd -a user_name group_name : To add a single user to an existing group. ( -a means adding one user at a time.)

sudo gpasswd -m user_name group_name : To add multiple users to an existing group. (-m means to add multiple users at a time.)

sudo deluser user_name group_name : To remove a user from a group.

sudo groupdel group_name : To delete a group. (The user will not delete.)

Linux File Permission Commands :

r - read

w - write

x - executable

chmod : To change the permissions of files and directories.

umask : To set the default permission for files or directories.

sudo chown user_name file_name : To change the ownership of the file.

sudo chgrp group_name files_name : To change the group permission.