Linux and Unix-style systems are built around a few simple ideas: users, files, processes, permissions and small tools that can be combined from the command line.
Everything starts with files
Linux represents many things as files or file-like objects: documents, devices, logs, configuration and system information. Learning the filesystem layout is one of the first useful steps.
The shell
The shell is a command interpreter. It lets you run programs, move around directories, redirect output, create scripts and automate repeatable work.
pwd
ls
cd /etc
whoamiUsers and permissions
Linux is a multi-user system. File permissions control who can read, write or execute files. This is central to both day-to-day use and server security.
Processes
Running programs are processes. Tools such as ps, top, kill and service managers help you inspect and control them.