Linux Cheatsheet

Must know Linux concepts and commands

Last updated: November 25, 2024 | Categories: tools, productivity

Linux Comprehensive Cheatsheet

Linux Basics
File System Navigation
File and Directory Management
File Permissions
Text Processing
Process Management
User and Group Management
Package Management
Network Commands
System Information
Disk and File System
Shell Scripting
Environment Variables
SSH and Remote Access
System Services
Log Management
Security
Performance Monitoring
Advanced Commands
Tips and Tricks

Linux Basics

What is Linux?

Linux is an open-source operating system kernel. Think of it as the engine of your car - it manages hardware resources and provides a platform for applications to run. Different distributions (distros) like Ubuntu, CentOS, and Arch are like different car models using the same engine.

Key Concepts

Kernel: Core of the OS that manages hardware
Shell: Command interpreter (bash, zsh, fish)
Terminal: Interface to interact with the shell
Root: Superuser with full system privileges
Home Directory: Personal directory for each user (~)
File System: Everything is a file in Linux

Basic Commands

# Get help
man command          # Manual page
command --help       # Brief help
info command         # Detailed info
type command         # Show command type
which command        # Show command location
whereis command      # Show binary, source, manual

# Command history
history              # Show command history
!n                   # Execute command n from history
!!                   # Execute last command
!string              # Execute last command starting with string
ctrl+r               # Search command history

# Clear and exit
clear                # Clear screen (or ctrl+l)
exit                 # Exit shell
logout               # Logout from system

Directory Structure

/                    # Root directory
├── bin/            # Essential user binaries
├── boot/           # Boot loader files
├── dev/            # Device files
├── etc/            # System configuration
├── home/           # User home directories
├── lib/            # Shared libraries
├── media/          # Removable media mount points
├── mnt/            # Temporary mount points
├── opt/            # Optional software
├── proc/           # Process information (virtual)
├── root/           # Root user home
├── sbin/           # System binaries
├── srv/            # Service data
├── sys/            # System information (virtual)
├── tmp/            # Temporary files
├── usr/            # User programs
└── var/            # Variable data

# Print working directory
pwd

# Change directory
cd /path/to/directory
cd ..                # Parent directory
cd ~                 # Home directory
cd -                 # Previous directory
cd                   # Also home directory

# List files
ls                   # Basic listing
ls -l                # Long format
ls -la               # Include hidden files
ls -lh               # Human-readable sizes
ls -lt               # Sort by modification time
ls -lS               # Sort by size
ls -R                # Recursive listing
ls -d */             # List directories only

# Tree view (install tree package)
tree
tree -L 2            # Limit depth to 2
tree -d              # Directories only
tree -a              # Include hidden

File and Directory Management

Creating Files and Directories

# Create files
touch file.txt
touch file{1..5}.txt    # Create file1.txt to file5.txt
echo "content" > file.txt
cat > file.txt          # Type content, ctrl+d to finish

# Create directories
mkdir directory
mkdir -p path/to/directory  # Create parent directories
mkdir -m 755 directory     # With permissions
mkdir {dir1,dir2,dir3}     # Multiple directories

# Create links
ln source_file hard_link
ln -s source_file soft_link
ln -s /full/path/to/file link  # Absolute path recommended

Copying, Moving, and Removing

# Copy files
cp source dest
cp -r source_dir dest_dir   # Recursive
cp -p source dest          # Preserve attributes
cp -i source dest          # Interactive (prompt)
cp -u source dest          # Update only
cp -a source dest          # Archive mode

# Move/rename
mv old_name new_name
mv file directory/
mv -i source dest          # Interactive
mv -n source dest          # No overwrite

# Remove files
rm file
rm -r directory            # Recursive
rm -f file                # Force
rm -rf directory          # Force recursive (DANGEROUS!)
rm -i file                # Interactive

# Safe remove with trash-cli
trash file                 # Move to trash
trash-empty               # Empty trash
trash-list                # List trash
trash-restore             # Restore from trash

File Operations

# View file content
cat file.txt              # Display entire file
less file.txt             # Page through file
more file.txt             # Simpler pager
head file.txt             # First 10 lines
head -n 20 file.txt       # First 20 lines
tail file.txt             # Last 10 lines
tail -n 20 file.txt       # Last 20 lines
tail -f file.txt          # Follow file updates

# Compare files
diff file1 file2
diff -u file1 file2       # Unified format
diff -r dir1 dir2         # Compare directories
cmp file1 file2           # Byte-by-byte comparison

# Find files
find /path -name "*.txt"
find . -type f -size +10M  # Files larger than 10MB
find . -mtime -7           # Modified in last 7 days
find . -perm 644           # With specific permissions
find . -user username      # Owned by user
find . -exec command {} \; # Execute command on results

# Locate (faster, uses database)
locate filename
updatedb                   # Update locate database

# File type
file filename              # Determine file type
stat filename              # Detailed file statistics

File Permissions

Understanding Permissions

-rwxr-xr--  1 user group 1234 Jan 1 12:00 file.txt
│├─┼─┼─┼─┘
││ │ │ └── Other (r--) = 4
││ │ └──── Group (r-x) = 5  
││ └────── User  (rwx) = 7
│└──────── File type (- = file, d = directory)

Permission values:
r (read)    = 4
w (write)   = 2
x (execute) = 1

Managing Permissions

# Change permissions
chmod 755 file             # rwxr-xr-x
chmod u+x file             # Add execute for user
chmod g-w file             # Remove write for group
chmod o=r file             # Set other to read only
chmod a+x file             # Add execute for all
chmod -R 755 directory     # Recursive

# Change ownership
chown user file
chown user:group file
chown -R user:group directory

# Change group
chgrp group file
chgrp -R group directory

# Default permissions
umask                      # Show current umask
umask 022                  # Set umask (755 for dirs, 644 for files)

# Special permissions
chmod u+s file             # Setuid
chmod g+s directory        # Setgid
chmod +t directory         # Sticky bit

# Access Control Lists (ACL)
getfacl file               # Get ACL
setfacl -m u:user:rwx file # Set ACL for user
setfacl -x u:user file     # Remove ACL
setfacl -b file            # Remove all ACLs

Text Processing

Text Manipulation

# grep - Search text
grep "pattern" file
grep -i "pattern" file     # Case insensitive
grep -v "pattern" file     # Invert match
grep -n "pattern" file     # Show line numbers
grep -r "pattern" dir/     # Recursive
grep -E "regex" file       # Extended regex
grep -P "perl-regex" file  # Perl regex
grep -A 3 "pattern" file   # Show 3 lines after
grep -B 3 "pattern" file   # Show 3 lines before
grep -C 3 "pattern" file   # Show 3 lines around

# sed - Stream editor
sed 's/old/new/' file      # Replace first occurrence
sed 's/old/new/g' file     # Replace all occurrences
sed -i 's/old/new/g' file  # In-place edit
sed -n '10,20p' file       # Print lines 10-20
sed '5d' file              # Delete line 5
sed '/pattern/d' file      # Delete matching lines

# awk - Pattern processing
awk '{print $1}' file      # Print first column
awk '{print $1, $3}' file  # Print columns 1 and 3
awk -F: '{print $1}' file  # Use : as delimiter
awk '$3 > 100' file        # Print if column 3 > 100
awk '{sum+=$1} END {print sum}' file  # Sum column 1

# cut - Extract columns
cut -d' ' -f1 file         # First field, space delimited
cut -d':' -f1,3 file       # Fields 1 and 3
cut -c1-10 file            # Characters 1-10

# sort - Sort lines
sort file
sort -n file               # Numeric sort
sort -r file               # Reverse sort
sort -k2 file              # Sort by 2nd field
sort -u file               # Unique sort

# uniq - Remove duplicates
uniq file                  # Remove adjacent duplicates
uniq -c file               # Count occurrences
uniq -d file               # Show only duplicates
sort file | uniq           # Remove all duplicates

# tr - Translate characters
tr 'a-z' 'A-Z' < file      # Convert to uppercase
tr -d '0-9' < file         # Delete digits
tr -s ' ' < file           # Squeeze spaces

Advanced Text Processing

# Join files
join file1 file2           # Join on first field
join -t: -1 2 -2 1 f1 f2  # Custom delimiter and fields

# Paste files
paste file1 file2          # Side by side
paste -d: file1 file2      # Custom delimiter

# Split files
split -l 100 file          # Split every 100 lines
split -b 10M file          # Split every 10MB

# Column formatting
column -t file             # Align columns
column -s: -t file         # Use : as delimiter

# Word count
wc file                    # Lines, words, characters
wc -l file                 # Line count only
wc -w file                 # Word count only
wc -c file                 # Byte count
wc -m file                 # Character count

Process Management

Process Commands

# View processes
ps                         # Current shell processes
ps aux                     # All processes
ps aux | grep process      # Find specific process
ps -ef                     # Full format listing
ps -u username             # Processes by user
pstree                     # Process tree
pgrep process_name         # Get PID by name

# Real-time monitoring
top                        # Interactive process viewer
htop                       # Better process viewer
atop                       # Advanced system monitor

# Kill processes
kill PID                   # Send SIGTERM
kill -9 PID               # Send SIGKILL (force)
kill -l                    # List signals
killall process_name       # Kill by name
pkill pattern              # Kill by pattern
pkill -u username          # Kill user processes

# Background jobs
command &                  # Run in background
jobs                       # List jobs
fg %n                      # Bring job n to foreground
bg %n                      # Send job n to background
nohup command &            # Run immune to hangups
disown %n                  # Detach job from shell

# Process priority
nice -n 10 command         # Run with lower priority
renice -n 5 -p PID        # Change priority

Process Control

# Signals
ctrl+c                     # SIGINT (interrupt)
ctrl+z                     # SIGTSTP (suspend)
ctrl+d                     # EOF
ctrl+\                     # SIGQUIT

# System calls
strace command             # Trace system calls
strace -p PID             # Attach to process
ltrace command             # Trace library calls

# Resource limits
ulimit -a                  # Show all limits
ulimit -n 1024            # Set open files limit
ulimit -u 100             # Set process limit

User and Group Management

User Management

# User information
whoami                     # Current username
id                         # User and group IDs
id username                # IDs for specific user
finger username            # User information
who                        # Logged in users
w                          # Who and what they're doing
last                       # Login history
lastlog                    # Last login times

# User management (root required)
useradd username           # Add user
useradd -m -s /bin/bash username  # With home and shell
usermod -aG group username # Add user to group
usermod -l newname oldname # Rename user
usermod -L username        # Lock account
usermod -U username        # Unlock account
userdel username           # Delete user
userdel -r username        # Delete with home directory

# Password management
passwd                     # Change your password
passwd username            # Change user password (root)
chage -l username          # Password aging info
chage -E 2024-12-31 username  # Set expiry date

Group Management

# Group information
groups                     # Your groups
groups username            # User's groups
getent group              # List all groups

# Group management (root required)
groupadd groupname         # Add group
groupmod -n newname oldname # Rename group
groupdel groupname         # Delete group
gpasswd -a user group      # Add user to group
gpasswd -d user group      # Remove user from group

# Switch user/group
su                         # Switch to root
su - username              # Switch user with environment
sudo command               # Execute as root
sudo -u username command   # Execute as user
newgrp groupname          # Switch primary group

Package Management

Debian/Ubuntu (APT)

# Update package list
sudo apt update

# Upgrade packages
sudo apt upgrade           # Upgrade installed
sudo apt full-upgrade      # Upgrade with removals
sudo apt dist-upgrade      # Distribution upgrade

# Install packages
sudo apt install package
sudo apt install -y package # Auto yes
sudo apt install package1 package2

# Remove packages
sudo apt remove package    # Keep config files
sudo apt purge package     # Remove everything
sudo apt autoremove        # Remove unused dependencies

# Search packages
apt search keyword
apt show package           # Package details
apt list --installed       # List installed

# Package information
dpkg -l                    # List installed packages
dpkg -L package           # List package files
dpkg -S /path/to/file     # Find package owning file

Red Hat/CentOS (YUM/DNF)

# Update packages
sudo yum update            # Or dnf update
sudo yum upgrade           # Or dnf upgrade

# Install packages
sudo yum install package
sudo yum groupinstall "Group Name"

# Remove packages
sudo yum remove package
sudo yum autoremove

# Search packages
yum search keyword
yum info package
yum list installed

# Package information
rpm -qa                    # List all packages
rpm -ql package           # List package files
rpm -qf /path/to/file     # Find package owning file

Arch Linux (Pacman)

# Update system
sudo pacman -Syu           # Sync and upgrade

# Install packages
sudo pacman -S package
sudo pacman -U package.pkg.tar.xz  # Local package

# Remove packages
sudo pacman -R package     # Remove only
sudo pacman -Rs package    # Remove with dependencies
sudo pacman -Rns package   # Remove with config files

# Search packages
pacman -Ss keyword         # Search in repos
pacman -Qs keyword         # Search installed
pacman -Si package         # Package info
pacman -Qi package         # Installed package info

Universal Package Managers

# Snap
snap list                  # List installed
snap find keyword          # Search
snap install package
snap remove package

# Flatpak
flatpak list
flatpak search keyword
flatpak install package
flatpak uninstall package

# AppImage
chmod +x app.AppImage
./app.AppImage

Network Commands

Network Configuration

# IP configuration
ip addr show               # Show all interfaces
ip addr add 192.168.1.100/24 dev eth0
ip addr del 192.168.1.100/24 dev eth0
ip link set eth0 up        # Enable interface
ip link set eth0 down      # Disable interface

# Legacy commands
ifconfig                   # Show interfaces
ifconfig eth0 up/down      # Enable/disable

# DNS configuration
cat /etc/resolv.conf       # DNS servers
nslookup domain.com        # DNS lookup
dig domain.com             # Detailed DNS query
host domain.com            # DNS lookup

# Hostname
hostname                   # Show hostname
hostname -f                # Fully qualified
hostnamectl set-hostname name  # Set hostname

Network Testing

# Connectivity
ping host                  # Test connectivity
ping -c 4 host            # Send 4 packets
ping6 ipv6-host           # IPv6 ping

# Trace route
traceroute host           # Trace packet route
tracepath host            # Similar to traceroute
mtr host                  # Combined ping/traceroute

# Port testing
telnet host port          # Test TCP port
nc -zv host port          # Netcat port scan
nc -l -p 1234             # Listen on port
nmap host                 # Port scanning

# Network statistics
netstat -tuln             # Listening ports
netstat -an               # All connections
ss -tuln                  # Modern netstat
lsof -i :80               # Process on port 80

# Bandwidth
iftop                     # Real-time bandwidth
nethogs                   # Bandwidth by process
vnstat                    # Network statistics

File Transfer

# wget - Download files
wget URL
wget -c URL               # Continue download
wget -r URL               # Recursive download
wget -O filename URL      # Save as

# curl - Transfer data
curl URL
curl -o filename URL      # Save to file
curl -O URL               # Save with remote name
curl -I URL               # Headers only
curl -X POST -d "data" URL # POST request

# scp - Secure copy
scp file user@host:/path
scp user@host:/file .
scp -r dir user@host:/path # Recursive
scp -P 2222 file user@host:/path # Custom port

# rsync - Sync files
rsync -av source/ dest/    # Archive mode
rsync -avz source/ user@host:/dest/ # With compression
rsync --delete source/ dest/ # Mirror directories
rsync --exclude='*.tmp' source/ dest/

System Information

System Details

# System information
uname -a                   # All system info
uname -r                   # Kernel version
lsb_release -a            # Distribution info
cat /etc/os-release       # OS information
hostnamectl               # System details

# Hardware information
lscpu                     # CPU information
lsmem                     # Memory information
lsblk                     # Block devices
lspci                     # PCI devices
lsusb                     # USB devices
dmidecode                 # DMI/SMBIOS info

# Resource usage
free -h                   # Memory usage
df -h                     # Disk usage
du -sh directory          # Directory size
du -h --max-depth=1      # Subdirectory sizes
iostat                    # I/O statistics
vmstat                    # Virtual memory stats

System Monitoring

# Real-time monitoring
top                       # Process monitor
htop                      # Better top
iotop                     # I/O monitor
iftop                     # Network monitor
glances                   # System monitor

# System load
uptime                    # Uptime and load
cat /proc/loadavg         # Load average
w                         # Who and load

# Memory
free -h                   # Human readable
cat /proc/meminfo         # Detailed memory info
vmstat 1                  # Update every second

# CPU
mpstat                    # CPU statistics
sar -u 1                  # CPU usage history

Disk and File System

Disk Management

# Disk information
fdisk -l                  # List disks
lsblk                     # Block devices tree
blkid                     # Block device IDs
df -h                     # File system usage
df -i                     # Inode usage

# Partition management
fdisk /dev/sda            # Partition disk (MBR)
gdisk /dev/sda            # Partition disk (GPT)
parted /dev/sda           # Advanced partitioning

# File system
mkfs.ext4 /dev/sda1       # Create ext4 filesystem
mkfs.xfs /dev/sda1        # Create XFS filesystem
fsck /dev/sda1            # Check filesystem
e2fsck -f /dev/sda1       # Force check ext filesystem
resize2fs /dev/sda1       # Resize ext filesystem

# Mount/unmount
mount /dev/sda1 /mnt      # Mount device
mount -t ext4 /dev/sda1 /mnt
mount -o ro /dev/sda1 /mnt # Read-only
umount /mnt               # Unmount
umount -l /mnt            # Lazy unmount

# Swap
mkswap /dev/sda2          # Create swap
swapon /dev/sda2          # Enable swap
swapoff /dev/sda2         # Disable swap
swapon -s                 # Show swap usage

File System Management

# Disk usage
du -sh *                  # Size of files/dirs
du -h --max-depth=1       # One level deep
ncdu                      # NCurses disk usage

# Find large files
find / -size +100M        # Files over 100MB
find / -size +1G          # Files over 1GB

# File system table
cat /etc/fstab            # Filesystem mounts
mount -a                  # Mount all from fstab

# Quotas
quota -u username         # User quota
repquota -a               # Report quotas

# LVM (Logical Volume Manager)
pvcreate /dev/sdb         # Create physical volume
vgcreate vg0 /dev/sdb     # Create volume group
lvcreate -L 10G -n lv0 vg0 # Create logical volume
lvextend -L +5G /dev/vg0/lv0 # Extend volume

Shell Scripting

Basic Script Structure

#!/bin/bash
# Shebang - specifies interpreter

# Variables
NAME="John"
AGE=25
echo "Hello, $NAME. You are $AGE years old."

# Command substitution
DATE=$(date)
FILES=`ls`

# Arrays
FRUITS=("apple" "banana" "orange")
echo ${FRUITS[0]}         # First element
echo ${FRUITS[@]}         # All elements
echo ${#FRUITS[@]}        # Array length

# Input
read -p "Enter your name: " USERNAME
read -s -p "Password: " PASSWORD  # Silent

# Arithmetic
NUM1=10
NUM2=5
SUM=$((NUM1 + NUM2))
let "RESULT = NUM1 * NUM2"
RESULT=$(expr $NUM1 + $NUM2)

Control Structures

# If statements
if [ "$VAR" = "value" ]; then
    echo "Match"
elif [ "$VAR" = "other" ]; then
    echo "Other match"
else
    echo "No match"
fi

# Test operators
# -eq, -ne, -lt, -le, -gt, -ge  # Numeric
# =, !=, <, >                    # String
# -z (empty), -n (not empty)     # String tests
# -f (file), -d (directory)      # File tests
# -r (readable), -w (writable), -x (executable)

# Case statement
case $VAR in
    pattern1)
        echo "Pattern 1"
        ;;
    pattern2|pattern3)
        echo "Pattern 2 or 3"
        ;;
    *)
        echo "Default"
        ;;
esac

# For loops
for i in 1 2 3 4 5; do
    echo $i
done

for i in {1..10}; do
    echo $i
done

for file in *.txt; do
    echo "Processing $file"
done

for ((i=0; i<10; i++)); do
    echo $i
done

# While loops
COUNT=0
while [ $COUNT -lt 10 ]; do
    echo $COUNT
    COUNT=$((COUNT + 1))
done

# Until loops
until [ $COUNT -eq 10 ]; do
    echo $COUNT
    COUNT=$((COUNT + 1))
done

Functions

# Function definition
function greet() {
    echo "Hello, $1!"
}

# Alternative syntax
greet() {
    local name=$1
    echo "Hello, $name!"
    return 0
}

# Call function
greet "World"

# Function with return value
add() {
    local result=$(( $1 + $2 ))
    echo $result
}

SUM=$(add 5 3)
echo "Sum: $SUM"

Script Best Practices

#!/bin/bash
set -euo pipefail         # Exit on error, undefined vars, pipe failures

# Error handling
trap 'echo "Error on line $LINENO"' ERR

# Logging
LOG_FILE="/var/log/script.log"
log() {
    echo "[$(date +'%Y-%m-%d %H:%M:%S')] $*" | tee -a "$LOG_FILE"
}

# Check root
if [[ $EUID -ne 0 ]]; then
    echo "This script must be run as root"
    exit 1
fi

# Parse arguments
while getopts "hf:v" opt; do
    case $opt in
        h) show_help; exit 0 ;;
        f) FILE="$OPTARG" ;;
        v) VERBOSE=true ;;
        *) echo "Invalid option"; exit 1 ;;
    esac
done

# Default values
: ${VAR:="default"}       # Set if unset
: ${VAR:?"Error: VAR not set"}  # Exit if unset

Environment Variables

Working with Variables

# Set variables
export VAR="value"
VAR="value" command       # For single command

# View variables
env                       # All environment variables
printenv                  # Same as env
echo $VAR                 # Specific variable
set                       # All variables (including shell)

# Common variables
echo $HOME                # Home directory
echo $USER                # Current user
echo $PATH                # Executable search path
echo $PWD                 # Current directory
echo $SHELL               # Current shell
echo $TERM                # Terminal type
echo $LANG                # Language settings

# Modify PATH
export PATH=$PATH:/new/path
export PATH=/new/path:$PATH  # Prepend

# Make permanent
# Add to ~/.bashrc or ~/.profile
echo 'export VAR="value"' >> ~/.bashrc
source ~/.bashrc          # Reload

SSH and Remote Access

SSH Basics

# Connect
ssh user@host
ssh -p 2222 user@host     # Custom port
ssh -i key.pem user@host  # With key file

# SSH config (~/.ssh/config)
Host myserver
    HostName 192.168.1.100
    User myuser
    Port 2222
    IdentityFile ~/.ssh/mykey

# Then connect with:
ssh myserver

# Key generation
ssh-keygen -t rsa -b 4096
ssh-keygen -t ed25519     # Modern, secure
ssh-keygen -t rsa -b 4096 -C "email@example.com"

# Copy key to server
ssh-copy-id user@host
ssh-copy-id -i ~/.ssh/key.pub user@host

# SSH agent
eval $(ssh-agent)
ssh-add ~/.ssh/id_rsa
ssh-add -l                # List keys
ssh-add -D                # Delete all keys

Advanced SSH

# Port forwarding
ssh -L 8080:localhost:80 user@host  # Local forward
ssh -R 8080:localhost:80 user@host  # Remote forward
ssh -D 1080 user@host              # SOCKS proxy

# File transfer
scp file user@host:/path
scp -r dir user@host:/path
scp user@host:/file .

# SSH tunneling
ssh -N -L 3306:localhost:3306 user@host  # MySQL tunnel
ssh -N -L 5432:localhost:5432 user@host  # PostgreSQL tunnel

# Execute remote commands
ssh user@host 'command'
ssh user@host 'ls -la'
ssh user@host < script.sh

# SSH multiplexing
# In ~/.ssh/config:
Host *
    ControlMaster auto
    ControlPath ~/.ssh/sockets/%r@%h-%p
    ControlPersist 600

# SSHFS - Mount remote filesystem
sshfs user@host:/path /local/mount
fusermount -u /local/mount  # Unmount

System Services

Systemd (Modern Systems)

# Service management
systemctl start service
systemctl stop service
systemctl restart service
systemctl reload service
systemctl status service
systemctl enable service   # Start at boot
systemctl disable service
systemctl is-enabled service
systemctl is-active service

# List services
systemctl list-units --type=service
systemctl list-unit-files --type=service
systemctl --failed        # Failed services

# Logs
journalctl -u service     # Service logs
journalctl -f             # Follow logs
journalctl --since "1 hour ago"
journalctl -b             # Boot logs

# Targets (runlevels)
systemctl get-default
systemctl set-default multi-user.target
systemctl isolate graphical.target

# Create service
# /etc/systemd/system/myservice.service
[Unit]
Description=My Service
After=network.target

[Service]
Type=simple
ExecStart=/usr/bin/myprogram
Restart=always
User=myuser

[Install]
WantedBy=multi-user.target

Init.d (Legacy Systems)

# Service management
service service_name start
service service_name stop
service service_name restart
service service_name status

# Or directly:
/etc/init.d/service_name start

# Enable at boot
chkconfig service_name on  # Red Hat
update-rc.d service_name enable  # Debian

# Runlevels
runlevel                  # Current runlevel
init 3                    # Change to runlevel 3

Log Management

System Logs

# Common log files
/var/log/syslog           # System log (Debian)
/var/log/messages         # System log (Red Hat)
/var/log/auth.log         # Authentication
/var/log/kern.log         # Kernel logs
/var/log/boot.log         # Boot logs
/var/log/dmesg            # Driver messages
/var/log/cron             # Cron logs
/var/log/mail.log         # Mail logs

# View logs
tail -f /var/log/syslog   # Follow log
tail -n 100 /var/log/syslog # Last 100 lines
less +F /var/log/syslog   # Follow in less
grep ERROR /var/log/syslog
zcat /var/log/syslog.1.gz # View compressed

# Kernel messages
dmesg                     # Kernel ring buffer
dmesg -T                  # Human readable time
dmesg -w                  # Follow

# Log rotation
cat /etc/logrotate.conf   # Config
logrotate -f /etc/logrotate.conf  # Force rotation

Journalctl (Systemd)

# View logs
journalctl                # All logs
journalctl -f             # Follow
journalctl -r             # Reverse (newest first)
journalctl -e             # Jump to end
journalctl -n 50          # Last 50 entries

# Filter logs
journalctl -u nginx       # By service
journalctl -p err         # By priority
journalctl --since "1 hour ago"
journalctl --since "2023-01-01" --until "2023-01-02"
journalctl _PID=1234      # By PID
journalctl _UID=1000      # By user ID

# Output formats
journalctl -o json        # JSON format
journalctl -o json-pretty
journalctl -o cat         # Message only

# Disk usage
journalctl --disk-usage
journalctl --vacuum-time=2w  # Keep 2 weeks
journalctl --vacuum-size=500M  # Keep 500MB

Security

File Security

# File permissions
chmod 600 private_key     # Owner read/write only
chmod 644 public_file     # Owner write, others read
chmod 755 script.sh       # Owner all, others read/execute

# Find insecure files
find / -perm -002 -type f # World-writable files
find / -nouser            # Files with no owner
find / -perm -4000        # SUID files

# File integrity
md5sum file               # MD5 checksum
sha256sum file            # SHA-256 checksum
sha256sum -c file.sha256  # Verify checksum

# Encryption
gpg -c file               # Encrypt with password
gpg file.gpg              # Decrypt
openssl enc -aes-256-cbc -in file -out file.enc

User Security

# Password policies
chage -l username         # Password aging info
passwd -l username        # Lock account
passwd -u username        # Unlock account
passwd -S username        # Password status

# Sudo configuration
visudo                    # Edit sudoers safely
# Allow user to run specific command:
# username ALL=(ALL) NOPASSWD: /path/to/command

# Login history
last                      # Login history
lastb                     # Failed logins
who                       # Current logins
w                         # Detailed who

# Account auditing
cat /etc/passwd           # User accounts
cat /etc/shadow           # Password hashes (root)
cat /etc/group            # Groups

Firewall

# iptables (legacy)
iptables -L               # List rules
iptables -A INPUT -p tcp --dport 80 -j ACCEPT
iptables -A INPUT -s 192.168.1.0/24 -j ACCEPT
iptables-save > /etc/iptables/rules.v4

# firewalld (Red Hat)
firewall-cmd --state
firewall-cmd --get-zones
firewall-cmd --get-default-zone
firewall-cmd --add-service=http --permanent
firewall-cmd --add-port=8080/tcp --permanent
firewall-cmd --reload

# ufw (Ubuntu)
ufw status
ufw enable
ufw allow 22/tcp
ufw allow from 192.168.1.0/24
ufw deny 3306/tcp
ufw delete allow 80/tcp

SELinux

# Status
getenforce                # Current mode
sestatus                  # Detailed status

# Set mode
setenforce 0              # Permissive (temporary)
setenforce 1              # Enforcing (temporary)

# File contexts
ls -Z file                # Show context
chcon -t httpd_sys_content_t /var/www/html
restorecon -R /var/www    # Restore contexts

# Troubleshooting
ausearch -m avc -ts recent # Recent denials
sealert -a /var/log/audit/audit.log

Performance Monitoring

Resource Monitoring

# CPU
top                       # Interactive monitor
htop                      # Better top
mpstat 1                  # CPU stats per second
sar -u 1 10              # CPU usage (1s x 10)

# Memory
free -h                   # Memory usage
vmstat 1                  # Virtual memory stats
sar -r 1 10              # Memory usage history

# Disk I/O
iostat -x 1               # Extended I/O stats
iotop                     # I/O by process
dstat                     # Combined stats

# Network
iftop                     # Network bandwidth
nethogs                   # Bandwidth by process
ss -s                     # Socket statistics
sar -n DEV 1             # Network device stats

# Process tracing
strace -p PID            # System calls
ltrace -p PID            # Library calls
perf top                 # Performance analysis

System Tuning

# Kernel parameters
sysctl -a                 # All parameters
sysctl net.ipv4.ip_forward  # Specific parameter
sysctl -w net.ipv4.ip_forward=1  # Set temporarily
echo "net.ipv4.ip_forward=1" >> /etc/sysctl.conf  # Permanent

# Process limits
ulimit -a                 # All limits
ulimit -n 65536          # Open files limit
ulimit -u 4096           # Process limit

# CPU governor
cpupower frequency-info
cpupower frequency-set -g performance

# I/O scheduler
cat /sys/block/sda/queue/scheduler
echo noop > /sys/block/sda/queue/scheduler

Advanced Commands

Text Processing Power Tools

# xargs - Build command lines
find . -name "*.txt" | xargs grep "pattern"
find . -name "*.log" -print0 | xargs -0 rm
echo "file1 file2 file3" | xargs -n 1 command
cat urls.txt | xargs -n 1 -P 10 wget  # Parallel

# parallel - GNU parallel
parallel -j 4 gzip ::: *.log
find . -name "*.jpg" | parallel -j 8 convert {} {.}.png
parallel echo ::: A B C ::: 1 2 3

# jq - JSON processor
curl api.example.com | jq '.'
cat file.json | jq '.items[]'
jq '.name' file.json
jq -r '.users[] | .name' file.json  # Raw output

Archive and Compression

# tar
tar -cvf archive.tar files/   # Create
tar -xvf archive.tar          # Extract
tar -tvf archive.tar          # List
tar -czf archive.tar.gz files/ # Gzip compression
tar -cjf archive.tar.bz2 files/ # Bzip2
tar -cJf archive.tar.xz files/  # XZ
tar -xzf archive.tar.gz -C /dest # Extract to directory

# zip/unzip
zip archive.zip file1 file2
zip -r archive.zip directory/
unzip archive.zip
unzip -l archive.zip          # List contents
unzip archive.zip -d /dest    # Extract to directory

# Compression tools
gzip file                     # Compress (replaces file)
gzip -d file.gz              # Decompress
gzip -k file                 # Keep original
bzip2 file                   # Better compression
xz file                      # Best compression
7z a archive.7z files/       # 7-zip

System Rescue

# Recovery mode
# Boot with init=/bin/bash
mount -o remount,rw /
passwd                        # Reset root password

# File recovery
extundelete /dev/sda1 --restore-all
photorec                      # Recover various files
testdisk                      # Partition recovery

# System repair
fsck -f /dev/sda1            # Force filesystem check
dpkg --configure -a          # Fix broken packages (Debian)
yum-complete-transaction     # Fix broken transactions (Red Hat)

# Boot repair
grub-install /dev/sda        # Reinstall GRUB
update-grub                  # Update GRUB config

Tips and Tricks

Command Line Productivity

# Shortcuts
ctrl+a                   # Beginning of line
ctrl+e                   # End of line
ctrl+k                   # Cut to end of line
ctrl+u                   # Cut to beginning
ctrl+w                   # Cut previous word
ctrl+y                   # Paste (yank)
ctrl+l                   # Clear screen
ctrl+r                   # Search history
ctrl+d                   # Exit/EOF
alt+.                    # Last argument
alt+b                    # Back one word
alt+f                    # Forward one word

# History expansion
!!                       # Last command
!$                       # Last argument
!*                       # All arguments
!:2                      # Second argument
^old^new                 # Replace in last command

# Brace expansion
mkdir -p project/{src,bin,doc}
touch file{1..10}.txt
echo {A..Z}
cp file.txt{,.bak}       # Quick backup

# Command substitution
echo "Today is $(date)"
files=`ls *.txt`

# Process substitution
diff <(sort file1) <(sort file2)

Useful One-Liners

# Find and replace in files
find . -type f -exec sed -i 's/old/new/g' {} +

# Delete empty directories
find . -type d -empty -delete

# Show top 10 largest files
find . -type f -exec du -h {} + | sort -rh | head -10

# Monitor log file and highlight
tail -f /var/log/syslog | grep --color=auto ERROR

# Batch rename files
for f in *.jpeg; do mv "$f" "${f%.jpeg}.jpg"; done

# Create backup with timestamp
tar czf backup-$(date +%Y%m%d-%H%M%S).tar.gz directory/

# Find files modified in last day
find . -type f -mtime -1

# Show listening ports with process
sudo netstat -tlnp

# Quick HTTP server
python3 -m http.server 8080

# Watch command output
watch -n 1 'ps aux | grep process'

# Generate random password
openssl rand -base64 32
tr -dc 'A-Za-z0-9!@#$%' < /dev/urandom | head -c 20

# System backup
rsync -aAXv --exclude={"/dev/*","/proc/*","/sys/*","/tmp/*","/run/*"} / /backup/

# Find duplicate files
find . -type f -exec md5sum {} + | sort | uniq -d -w 32

Shell Customization

# ~/.bashrc additions
# Aliases
alias ll='ls -alF'
alias la='ls -A'
alias l='ls -CF'
alias ..='cd ..'
alias ...='cd ../..'
alias grep='grep --color=auto'
alias df='df -h'
alias du='du -h'
alias free='free -h'
alias vi='vim'

# Functions
mkcd() { mkdir -p "$1" && cd "$1"; }
backup() { cp "$1"{,.bak-$(date +%Y%m%d-%H%M%S)}; }
extract() {
    case "$1" in
        *.tar.gz|*.tgz) tar xzf "$1" ;;
        *.tar.bz2) tar xjf "$1" ;;
        *.zip) unzip "$1" ;;
        *.gz) gunzip "$1" ;;
        *) echo "Unknown archive format" ;;
    esac
}

# Prompt customization
PS1='\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$ '

# History settings
HISTSIZE=10000
HISTFILESIZE=20000
HISTCONTROL=ignoredups:ignorespace
shopt -s histappend

# Better tab completion
bind 'set show-all-if-ambiguous on'
bind 'set completion-ignore-case on'

System Administration Tips

# Quick system health check
echo "=== System Health Check ==="
echo "Uptime: $(uptime)"
echo "Memory: $(free -h | grep Mem | awk '{print $3 "/" $2}')"
echo "Disk: $(df -h / | tail -1 | awk '{print $3 "/" $2 " (" $5 ")"}')"
echo "Load: $(cat /proc/loadavg | cut -d' ' -f1-3)"
echo "Processes: $(ps aux | wc -l)"

# Monitor multiple log files
multitail /var/log/syslog /var/log/auth.log

# Secure file deletion
shred -vfz -n 3 sensitive_file

# Create encrypted archive
tar czf - directory/ | openssl enc -aes-256-cbc -out archive.tar.gz.enc

# Check all running services
systemctl list-units --type=service --state=running

# Find recently modified config files
find /etc -type f -mtime -7 -name "*.conf"

# Quick performance check
echo "CPU:" && top -bn1 | grep "Cpu(s)" && \
echo "Memory:" && free -h | grep Mem && \
echo "Disk I/O:" && iostat -x 1 1 | grep -A1 avg-cpu

Linux is incredibly powerful - the more you learn, the more efficient you become. Practice these commands regularly, and always be careful with destructive operations!