Skip to main content

Chapter 1.5 - Fundamental Concepts

Learning Objectives​

At the end of this chapter, you will be able to:

  • Understand what a process is
  • Explain the file system
  • Understand users and permissions
  • Use the shell to interact with the system

Introduction​

Linux uses several fundamental concepts that you will constantly use. This chapter presents the most important ones: Processes, File System, Users, and Shell.

Processes: The Unit of Execution​

Definition​

A process is an instance of a program in execution.

Characteristics:

  • Each process has a unique PID
  • Parent and child processes (hierarchy)
  • Isolated memory space
  • Own files and resources

Process States​

States:

  • Running (R): Currently executing
  • Sleeping (S): Waiting
  • Stopped (T): Suspended
  • Zombie (Z): Terminated but not cleaned up

File System: Data Organization​

Hierarchical Structure​

Important directories:

  • /: System root
  • /bin, /usr/bin: System executables
  • /etc: Configuration files
  • /home: User directories
  • /var: Variable data (logs, cache)
  • /tmp: Temporary files

Inodes and Metadata​

Each file has an inode that contains:

  • File type
  • Permissions
  • Owner and group
  • Size
  • Timestamps
  • Pointers to data

Users and Permissions​

User Types​

Types:

  • Root (superuser): Full access, UID 0
  • Normal users: Limited access, UID ≥ 1000
  • Groups: Shared permissions

File Permissions​

Three levels:

  • Owner: rwx
  • Group: rwx
  • Others: rwx

Types:

  • r (read): Read
  • w (write): Write
  • x (execute): Execute

Example:

-rwxr-xr-- 1 user group 1024 Jan 1 12:00 file
# Owner: rwx (read, write, execute)
# Group: r-x (read, execute)
# Others: r-- (read only)

Shell: Interface with the System​

What is a Shell?​

The shell is the interface between the user and the system. It interprets commands.

Features:

  • Command execution
  • Scripting and automation
  • Redirection and pipelines
  • Environment variables
  • Command history

Basic Commands​

# Navigation
pwd          # Where am I?
cd /home     # Go to /home
ls -la       # List files

# Files
cat file     # Display content
cp src dst   # Copy
mv src dst   # Move/rename
rm file      # Delete

# Processes
ps aux       # View processes
top          # Real-time monitoring
kill PID     # Kill a process

Key Concepts Summarized​

Summary​

In this chapter, you learned:

Processes: Instance of a program, unique PID, states
File System: Hierarchical structure, inodes, directories
Users: Root, users, groups, permissions
Shell: Command line interface, commands, scripts
Permissions: rwx (read, write, execute)
Hierarchy: Processes and files organized in trees

Next Steps​

Chapter 1.6: Installation and First Use
Module 2: Linux System Architecture

Chapter created on: December 2024