Linux

Linux is a Unix-like, open-source operating system kernel first released by Linus Torvalds in 1991. When combined with GNU utilities and other software, it forms complete operating systems (distributions) like Ubuntu, Debian, Fedora, and Arch Linux. Linux dominates the server market, powers most of the internet’s infrastructure, and is the foundation for Android and countless embedded systems.

See also: Systemd, Command Line (CLI), Containers, Kubernetes

File System Hierarchy

The Filesystem Hierarchy Standard (FHS) defines the directory structure on Linux systems.

Directory	Purpose
/	Root filesystem, contains everything
/bin	Essential user binaries (ls, cp, cat)
/sbin	Essential system binaries (init, fsck, mount)
/etc	Host-specific configuration files
/home	User home directories
/root	Home directory for root user
/var	Variable data (logs, mail, spool, cache)
/var/log	Log files
/tmp	Temporary files (cleared on reboot)
/usr	Secondary hierarchy for read-only user data
/usr/bin	Most user commands
/usr/lib	Libraries for /usr/bin and /usr/sbin
/usr/local	Locally installed software
/opt	Optional/third-party application packages
/lib	Essential shared libraries and kernel modules
/dev	Device files (everything is a file)
/proc	Virtual filesystem for process and kernel info
/sys	Virtual filesystem for device and driver info
/mnt	Temporary mount point
/media	Mount point for removable media
/boot	Boot loader files and kernel
/srv	Data for services provided by the system

Process Management

Understanding Processes

Every running program is a process with a unique PID (Process ID). Processes have a parent-child relationship forming a tree structure, with PID 1 (init/systemd) at the root.

fork()

The fork system call clones the current process to create a new process. It creates a new process (the child process) by duplicating the state of the existing process with a few minor differences. The child process does not start from main. Instead it returns from fork() just as the parent process does.

After fork(), the parent receives the child’s PID while the child receives 0. This is how each process knows its role:

pid_t pid = fork();
if (pid == 0) {
    // Child process
} else {
    // Parent process (pid contains child's PID)
}

Process Commands

Command	Description
ps aux	List all processes with detailed info
ps -ef	List all processes in full format
top	Interactive real-time process viewer
htop	Improved interactive process viewer (needs install)
pgrep <name>	Find PIDs by process name
pidof <name>	Find PID of a running program
kill <pid>	Send signal to process (default: SIGTERM)
kill -9 <pid>	Force kill (SIGKILL, cannot be caught)
killall <name>	Kill all processes by name
pkill <pattern>	Kill processes matching pattern
nice -n 10 cmd	Run command with adjusted priority
renice -n 5 -p <pid>	Change priority of running process
nohup cmd &	Run command immune to hangups
jobs	List background jobs
fg %1	Bring job 1 to foreground
bg %1	Resume job 1 in background

Nice values range from -20 (highest priority) to 19 (lowest). Only root can set negative nice values.

Common Signals

Signal	Number	Description
SIGHUP	1	Hangup (often used to reload config)
SIGINT	2	Interrupt (Ctrl+C)
SIGKILL	9	Force kill (cannot be caught)
SIGTERM	15	Graceful termination (default)
SIGSTOP	19	Pause process (cannot be caught)
SIGCONT	18	Resume paused process

Users and Permissions

User Management

Command	Description
whoami	Display current username
id	Display user and group IDs
who	Show who is logged in
useradd <user>	Create new user
usermod -aG <group> <user>	Add user to group
userdel <user>	Delete user
passwd <user>	Change password
su - <user>	Switch user with login shell
sudo <cmd>	Run command as root
visudo	Edit sudoers file safely

File Permissions

Permissions are represented as rwxrwxrwx (read, write, execute) for owner, group, and others.

-rwxr-xr-x 1 user group 4096 Jan 24 12:00 file.sh
│└┬┘└┬┘└┬┘
│ │  │  └── Others: r-x (read + execute)
│ │  └───── Group: r-x (read + execute)
│ └──────── Owner: rwx (read + write + execute)
└────────── File type (- = file, d = directory, l = symlink)

Command	Description
chmod 755 file	Set permissions using octal (rwxr-xr-x)
chmod u+x file	Add execute permission for owner
chmod g-w file	Remove write permission for group
chmod o=r file	Set others permission to read only
chmod -R 644 dir/	Recursively set permissions
chown user:group file	Change owner and group
chown -R user dir/	Recursively change owner
chgrp group file	Change group

Octal permission values: r=4, w=2, x=1 (e.g., 755 = rwxr-xr-x)

Special Permissions

Permission	Octal	Description
setuid	4000	Execute as file owner
setgid	2000	Execute as group owner / inherit directory group
sticky bit	1000	Only owner can delete files (common on /tmp)

Package Management

Debian/Ubuntu (apt)

apt update                 # Update package lists
apt upgrade                # Upgrade installed packages
apt install <package>      # Install package
apt remove <package>       # Remove package
apt search <term>          # Search packages
apt show <package>         # Show package details
apt autoremove             # Remove unused dependencies

RHEL/Fedora (dnf/yum)

dnf update                 # Update all packages
dnf install <package>      # Install package
dnf remove <package>       # Remove package
dnf search <term>          # Search packages
dnf info <package>         # Show package details

Arch Linux (pacman)

pacman -Syu                # Sync and upgrade
pacman -S <package>        # Install package
pacman -R <package>        # Remove package
pacman -Ss <term>          # Search packages
pacman -Qi <package>       # Query installed package info

Essential Commands

Navigation and File Operations

Command	Description
pwd	Print working directory
cd <dir>	Change directory
cd -	Go to previous directory
ls -la	List all files with details
ls -lh	Human-readable file sizes
mkdir -p a/b/c	Create nested directories
cp -r src/ dest/	Copy directory recursively
mv old new	Move/rename files
rm -rf dir/	Remove directory recursively (careful!)
ln -s target link	Create symbolic link
touch file	Create empty file / update timestamp

File Content

Command	Description
cat file	Display file contents
less file	View file with pagination
head -n 20 file	First 20 lines
tail -n 20 file	Last 20 lines
tail -f file	Follow file (live updates)
wc -l file	Count lines

Searching

Command	Description
grep pattern file	Search for pattern in file
grep -r pattern dir/	Recursive search in directory
grep -i pattern	Case-insensitive search
grep -v pattern	Invert match (exclude)
grep -E 'regex'	Extended regex (egrep)
find . -name "*.txt"	Find files by name
find . -type f -mtime -7	Files modified in last 7 days
find . -size +100M	Files larger than 100MB
locate filename	Fast search using database

Text Processing

Command	Description
awk '{print $1}'	Print first column
awk -F: '{print $1}'	Use : as delimiter
sed 's/old/new/g'	Replace all occurrences
sed -i 's/old/new/g' file	In-place replacement
cut -d: -f1	Cut first field, delimiter :
sort	Sort lines
sort -u	Sort and remove duplicates
uniq	Remove adjacent duplicates
tr 'a-z' 'A-Z'	Translate characters

xargs

Build and execute commands from stdin:

find . -name "*.log" | xargs rm           # Delete found files
cat urls.txt | xargs -n1 curl             # Curl each URL
find . -name "*.txt" | xargs -I {} cp {} backup/   # Copy with placeholder

Shell Basics

Bash Configuration Files

File	Purpose
/etc/profile	System-wide login shell config
~/.bash_profile	User login shell config
~/.bashrc	User interactive non-login shell config
~/.bash_logout	Executed on logout

Environment Variables

export VAR="value"       # Set variable for current session and child processes
echo $VAR                # Access variable
env                      # List all environment variables
printenv VAR             # Print specific variable
unset VAR                # Remove variable

Important Environment Variables

Variable	Description
PATH	Directories to search for executables
HOME	User’s home directory
USER	Current username
SHELL	Current shell
PWD	Current working directory
EDITOR	Default text editor
LANG	System locale

PATH

The PATH variable contains colon-separated directories where the shell looks for executables:

echo $PATH
# /usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin
 
# Add directory to PATH
export PATH="$HOME/bin:$PATH"

Useful Shell Features

!!                       # Repeat last command
!$                       # Last argument of previous command
Ctrl+R                   # Reverse search history
Ctrl+L                   # Clear screen
Ctrl+A / Ctrl+E          # Start / end of line
Ctrl+U / Ctrl+K          # Delete to start / end of line

Networking Commands

Command	Description
ip addr	Show IP addresses
ip link	Show network interfaces
ip route	Show routing table
ss -tuln	Show listening TCP/UDP ports
ss -tp	Show TCP connections with process
netstat -tuln	Show listening ports (legacy)
curl -I url	Fetch HTTP headers
curl -X POST -d 'data' url	POST request
wget url	Download file
dig domain	DNS lookup
nslookup domain	DNS lookup (legacy)
host domain	DNS lookup (simple)
ping host	Test connectivity
traceroute host	Trace packet route
mtr host	Combined ping and traceroute
nc -zv host port	Test TCP port connectivity

Firewall (iptables/nftables)

Modern systems use nftables, but many still use iptables:

iptables -L              # List rules
iptables -A INPUT -p tcp --dport 22 -j ACCEPT  # Allow SSH
ufw status               # Ubuntu firewall status
ufw allow 22             # Allow port 22

Service Management

Most modern Linux distributions use Systemd for service management:

systemctl status nginx        # Check service status
systemctl start nginx         # Start service
systemctl stop nginx          # Stop service
systemctl restart nginx       # Restart service
systemctl enable nginx        # Start on boot
systemctl disable nginx       # Don't start on boot
systemctl daemon-reload       # Reload unit files after changes
journalctl -u nginx           # View service logs
journalctl -fu nginx          # Follow service logs

Useful Developer Tools

Tool	Description
htop	Interactive process viewer
tmux / screen	Terminal multiplexers
ripgrep (rg)	Fast recursive grep
fd	Fast, user-friendly find
fzf	Fuzzy finder
bat	Cat with syntax highlighting
jq	JSON processor
yq	YAML processor
ncdu	Disk usage analyser
tree	Directory tree visualisation
watch	Run command repeatedly
strace	Trace system calls
lsof	List open files
lsof -i :8080	Find process using port

Common Tasks

Disk Usage

df -h                    # Filesystem disk space
du -sh *                 # Size of items in current directory
du -sh */ | sort -h      # Sorted directory sizes
ncdu                     # Interactive disk usage

Logs

journalctl -xe           # Recent systemd logs with context
dmesg                    # Kernel ring buffer
tail -f /var/log/syslog  # Follow syslog

Compression

tar -czvf archive.tar.gz dir/    # Create gzipped tarball
tar -xzvf archive.tar.gz         # Extract gzipped tarball
tar -cjvf archive.tar.bz2 dir/   # Create bzip2 tarball
zip -r archive.zip dir/          # Create zip
unzip archive.zip                # Extract zip

Resources

• Linux Documentation Project
• Arch Wiki - Excellent documentation applicable to most distros
• man7.org - Linux man pages online
• ExplainShell - Breaks down shell commands
• tldr pages - Simplified man pages