User permission and first commands

Permissions, Users and File System in Linux

This document summarizes key concepts about how permissions, users and directories work in Linux. It also explains useful commands and how to interpret binary and decimal representations.

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Types of Users

In Linux there are 3 main types:

• User (owner): who created the file

• Group (group): set of users with shared permissions

• Others (others): the rest of the world (literally)

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How Permissions Work (rwx)

Each file or folder has permissions assigned in three blocks:

-rwxr-xr--

• r → read

• w → write

• x → execute

This example means:

• The owner can read, write and execute

• The group can read and execute

• Others can only read

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Octal Notation

Binary	Decimal	Permissions
111	7	rwx
110	6	rw-
101	5	r-x
100	4	r--
011	3	-wx
010	2	-w-
001	1	--x
000	0	---

Examples:

chmod 777 file    # Maximum permissions for everyone (rwx)
chmod 754 file    # rwx for owner, r-x for group, r-- for others  
chmod 666 file    # rw- for everyone (nobody can execute it)

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Useful Commands

Groups and Users

groups                  # View current user's groups
sudo adduser user1      # Create user
sudo deluser user1      # Delete user  
sudo addgroup grupo     # Create group

Permissions

chmod 755 file              # Change permissions
chown user:group file       # Change owner and group

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File System in Linux

Linux has a tree-like hierarchical structure. Key examples:

Directory	Description
/bin	Essential system commands
/etc	Configuration files
/home	User folders
/root	Superuser folder
/tmp	Temporary files
/var	Logs and variable files

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What is a Binary File?

A binary is a file that contains code that can be executed directly by the system. It's compiled from source language (like C or C++).

How does binary work in Linux?

• It's stored in locations like /bin, /usr/bin

• It executes if it has execution permissions (x)

• We use chmod to make it executable if it's not

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Binary and Decimal System

Linux uses numerical systems to handle permissions.

The binary system is based on powers of 2.

Example:

Binary: 01100111

Equals to:

64 + 32 + 4 + 2 + 1 = 103

Each bit position (from right to left) is worth: 1, 2, 4, 8, 16, 32, 64, 128...

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What relation does it have with IPs?

An IPv4 address is a sequence of 4 bytes → each byte goes from 0 to 255.

Example:

192.168.1.1  →  4 blocks of 8 bits each

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Recommendations

• Use chmod, chown and ls -l to experiment with permissions

• Play around changing permissions on test files (not on system files!)

• Look at the output of ls -l /home and analyze who has what permissions

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Quick Reference Commands

ls -l                    # List files with detailed permissions
ls -la                   # Include hidden files  
chmod +x script.sh       # Add execute permission
chmod -w file.txt        # Remove write permission
chown user:group file    # Change ownership
stat filename            # Show detailed file information