Introduction

This comprehensive guide covers everything you need to know about Linux user management, file permissions, system navigation, and file operations. Whether you’re a beginner or an experienced user, this reference will help you master Linux system administration. Also a good application that you can use after you grasp the basics is Midnight Commander - https://midnight-commander.org

A highly regarded tool to make file management easier and more like working within a regular desktop environment.

Part 1: Linux Users, Groups, and Permissions

User Management

Viewing User Information

# Current user information
whoami
id

# All logged-in users
who
w

# User account information
cat /etc/passwd
getent passwd username

# User password/account status
sudo cat /etc/shadow
chage -l username

Creating and Modifying Users

# Create user with home directory and specific shell
sudo useradd -m -c "John Doe" -s /bin/bash johndoe

# Add user to supplementary groups
sudo useradd -m -g developers -G sudo,docker johndoe

# Modify user properties
sudo usermod -aG sudo johndoe      # Add to sudo group
sudo usermod -s /bin/zsh johndoe   # Change shell
sudo usermod -L johndoe            # Lock account

Group Management

Viewing and Managing Groups

# Current user's groups
groups
id -Gn

# Group management
sudo groupadd developers
sudo usermod -aG developers johndoe
sudo gpasswd -a johndoe developers

File Permissions Basics

Understanding Permission Types

  • Read (r) = 4: View file contents or list directory
  • Write (w) = 2: Modify file or directory contents
  • Execute (x) = 1: Run file or traverse directory

Permission Classes

  • User (u): File owner
  • Group (g): Group owner
  • Others (o): All other users

Viewing Permissions

ls -l file.txt
# -rw-r--r-- 1 johndoe developers 1024 Dec 10 14:30 file.txt

# With numeric permissions
stat -c "%a %n" file.txt

Permission Modification

Changing Permissions with chmod

# Symbolic notation
chmod u+x file.txt                 # Add execute for user
chmod g-w file.txt                 # Remove write for group

# Numeric notation
chmod 755 file.txt                 # rwxr-xr-x
chmod 644 file.txt                 # rw-r--r--

# Recursive changes
chmod -R 755 directory/

Changing Ownership

# Change owner and group
sudo chown johndoe:developers file.txt

# Change group only
sudo chgrp developers file.txt

# Recursive ownership changes
sudo chown -R johndoe:developers directory/

Special Permissions

SetUID, SetGID, and Sticky Bit

# SetUID - executes with owner's privileges
chmod u+s file
chmod 4755 file

# SetGID - executes with group's privileges, directories inherit group
chmod g+s directory/
chmod 2755 directory/

# Sticky Bit - users can only delete their own files
chmod +t directory/
chmod 1777 directory/

Access Control Lists (ACL)

Advanced Permission Management

# Install ACL tools
sudo apt install acl        # Debian/Ubuntu

# View and set ACLs
getfacl file.txt
setfacl -m u:username:permissions file.txt
setfacl -m g:groupname:permissions file.txt

# Examples
setfacl -m u:bob:rw file.txt
setfacl -m g:developers:rx directory/

# Default ACLs for directories
setfacl -m d:u:bob:rwx shared_directory/

Sudo and Privilege Escalation

Sudo Configuration

# Edit sudoers file safely
sudo visudo

# Test sudo access
sudo -l

Sudoers File Examples

# User specifications
username    ALL=(ALL:ALL) ALL
%groupname  ALL=(ALL:ALL) ALL

# Specific command permissions
johndoe     ALL=(root) /usr/bin/apt, /usr/bin/systemctl
%developers ALL=(ALL) NOPASSWD: /usr/bin/docker

Part 2: Linux Navigation and File Location

Basic Navigation

Directory Operations

# Current directory
pwd
pwd -P                    # Physical location (resolve symlinks)

# Changing directories
cd /path/to/directory     # Absolute path
cd ~                      # Home directory
cd -                      # Previous directory
cd ..                     # Parent directory

# Directory history
pushd /path/to/directory  # Save current and change
popd                      # Return to saved directory
dirs                      # Show directory stack

Linux Filesystem Hierarchy

Essential Directory Structure

/
├── bin/          # Essential user binaries
├── etc/          # System configuration files
├── home/         # User home directories
├── opt/          # Optional application software
├── tmp/          # Temporary files
├── usr/          # User utilities and applications
└── var/          # Variable data (logs, cache)

File System Navigation

Listing Files and Directories

# Basic listing with details
ls -la                    # Long format with hidden files
ls -lh                    # Human readable sizes
ls -ltr                   # Sorted by time (oldest first)

# Tree structure view
tree -L 2                 # Limit depth to 2 levels
tree -d                   # Directories only

File Information Commands

# File type and information
file filename
stat filename              # Detailed file statistics

# Path resolution
realpath filename
basename /path/to/file     # Extract filename
dirname /path/to/file      # Extract directory path

Searching for Files

Using find Command

# Basic find syntax
find [path] [options] [expression]

# By name and type
find /home -name "*.txt"
find /etc -iname "*.conf"          # Case insensitive
find /var -type f                  # Files only

# By size and time
find /var -size +10M               # Larger than 10MB
find /var/log -mtime -7            # Modified in last 7 days

# By permissions and owner
find /home -perm 644
find /var -user root

# Actions on found files
find /tmp -name "*.tmp" -delete
find . -name "*.old" -exec rm {} \;

Fast Search with locate

# Update file database
sudo updatedb

# Fast filename search
locate filename
locate -i "readme"          # Case insensitive
locate -l 20 "*.jpg"        # Limit to 20 results

Finding Commands

# Find executables
which ls
which -a python3            # Show all instances

# Locate binaries and manuals
whereis bash
whereis -b ls               # Only binaries

# Command documentation
whatis ls

Finding File Content

grep for Text Search

# Basic text search
grep "pattern" file.txt
grep -i "pattern" file.txt        # Case insensitive
grep -n "pattern" file.txt        # Show line numbers

# Recursive search
grep -r "pattern" /path/
grep -r --include="*.py" "import" .  # Specific file types

# Context around matches
grep -A 3 -B 2 "pattern" file.txt # 3 lines after, 2 lines before

Modern Alternatives

# ripgrep (faster)
rg "pattern"
rg -t py "import"                 # Python files only

# Silver Searcher
ag "pattern"
ag -G "\.js$" "function"          # JavaScript files

File Compression and Archiving

tar Commands

# Create archives
tar -czvf archive.tar.gz files/   # Create gzip compressed
tar -cjvf archive.tar.bz2 files/  # Create bzip2 compressed

# Extract archives
tar -xzvf archive.tar.gz          # Extract gzip
tar -xjvf archive.tar.bz2         # Extract bzip2

# List contents
tar -tzf archive.tar.gz           # List gzip archive

Other Compression Tools

# Individual file compression
gzip file.txt                     # Creates file.txt.gz
gunzip file.txt.gz                # Decompress

bzip2 file.txt                    # Creates file.txt.bz2
xz file.txt                       # Creates file.txt.xz

Disk Usage Analysis

Analyzing Space Usage

# Disk usage by directory
du -sh directory/                 # Summary total
du -h --max-depth=1 /var         # One level deep

# Sorting by size
du -h /var/* | sort -hr           # Sort by size (largest first)

# Free space
df -h                             # Human readable filesystem usage
df -i                             # Inode usage

Interactive Analysis

# NCurses disk usage
ncdu /path
ncdu -x /                         # Stay on filesystem

# Find large files
find / -type f -size +100M -exec ls -lh {} \; 2>/dev/null

File Operations

Copying and Moving

# Copy with preservation
cp -p file1 file2                # Preserve attributes
cp -a dir1/ dir2/                # Archive mode (preserve all)

# Safe moves
mv -i file* dest/                # Prompt before overwrite
mv --backup=numbered file1 file2 # Numbered backups

Creating Files and Directories

# Create nested directories
mkdir -p path/to/nested/dirs     # Create parent directories

# Create files with content
cat > file.txt << EOF
line 1
line 2
EOF

# Create large files
dd if=/dev/zero of=largefile.bin bs=1M count=100  # 100MB file

Text File Operations

# View files
cat file.txt
less file.txt                    # Interactive pager
head -20 file.txt                # First 20 lines
tail -f logfile.txt              # Follow (real-time)

# File comparison
diff file1.txt file2.txt
vimdiff file1.txt file2.txt      # Visual diff

# Text processing
sort file.txt
sort -n file.txt                 # Numerical sort
uniq -c file.txt                 # Count occurrences

Part 3: Essential Applications and Tools

Terminal File Managers

Midnight Commander (mc)

# Installation
sudo apt install mc        # Debian/Ubuntu
sudo yum install mc        # RHEL/CentOS

# Features: Two-pane interface, built-in FTP, file operations

Ranger

# Installation
sudo apt install ranger

# Features: Vim-like navigation, three-pane layout, file previews

Other Terminal Managers

  • Vifm: Dual-pane with Vim keybindings
  • nnn: Fast and minimalistic
  • Yazi: Modern, asynchronous (Rust-based)

Enhanced Search Tools

ripgrep (rg)

# Installation
sudo apt install ripgrep

# Usage: Faster than grep, better default experience
rg "pattern"
rg -t js "function"              # JavaScript files only

fd-find

# Installation  
sudo apt install fd-find

# Usage: Simpler alternative to find
fd "*.txt"
fd -e py                         # Python files

Disk Analysis Tools

ncdu

# Installation
sudo apt install ncdu

# Usage: Interactive disk usage analyzer
ncdu /path/to/scan

dust

# Installation (via cargo)
cargo install du-dust

# Usage: More intuitive disk usage
dust /var

Permission Management Tools

ACL Tools

# Installation
sudo apt install acl

# Essential commands
getfacl /path/to/file
setfacl -m u:user:perms /path/to/file

Best Practices and Security

Security Guidelines

Principle of Least Privilege

chmod 750 sensitive_directory/
chown root:root /etc/shadow
chmod 600 /etc/shadow

Regular Audits

# Find world-writable files
find / -perm -o+w ! -type l 2>/dev/null

# Find SetUID/SetGID files
find / -perm /6000 2>/dev/null

Secure Defaults

# Set secure umask
umask 027

# Directory shortcuts
export CDPATH=.:~:/var/www:/opt
alias proj='cd /home/user/projects/current'
alias logs='cd /var/log/apache2'

# Enhanced prompt showing path
PS1='\u@\h:\w\$ '

File Management Tips

# Safe operations
cp -i file* dest/                # Always prompt before overwrite
rm -I file*                      # Prompt if deleting >3 files

# Efficient archiving
tar -czf backup.tar.gz --exclude='*.tmp' /important/data/

# Regular cleanup
find /tmp -type f -atime +7 -delete
find ~/.cache -type f -mtime +30 -delete

Conclusion

Mastering Linux user management, permissions, and file navigation is essential for effective system administration. This guide provides comprehensive coverage of:

  • User and group management for multi-user systems
  • File permissions for security and access control
  • Efficient navigation techniques and shortcuts
  • Powerful search tools for locating files and content
  • Essential applications that enhance productivity

Regular practice with these commands and tools will significantly improve your efficiency as a Linux system administrator.