1. Basic Commands
ls # List files
ls -l # Long format
ls -a # Show hidden files
ls -lh # Human-readable sizes
ls -lt # Sort by modified time
pwd # Print working directory
cd /path/to/dir # Change directory
cd .. # Go up one level
cd ~ # Go to home directory
cd - # Go to previous directory
mkdir dirname # Create directory
mkdir -p parent/child # Create nested directories
touch file.txt # Create empty file
touch file1 file2 file3 # Create multiple files
cp file.txt /tmp/ # Copy file
cp -r dir/ /tmp/ # Copy directory recursively
cp -p file.txt backup.txt # Preserve timestamps/perms
mv file.txt newname.txt # Rename file
mv file.txt /tmp/ # Move file
rm file.txt # Remove file
rm -f file.txt # Force remove (no prompt)
rm -r dir/ # Remove directory recursively
rm -rf dir/ # Force remove directory
echo "text" # Print to terminal
echo "text" > file.txt # Write/overwrite to file
echo "text" >> file.txt # Append to file
cat file.txt # Display file
cat file1 file2 # Concatenate files
ln -s /path/target link # Create symbolic link
ln file.txt hardlink # Create hard link
clear # Clear terminal
history # Show command history
history | grep ssh # Search history
!100 # Re-run command #100
!! # Re-run last command
alias ll='ls -lah' # Create alias
unalias ll # Remove alias
man ls # Manual page for command
which nginx # Location of command
whereis nginx # Locate binary + man page
type ls # Show command type
2. File Viewing & Editing
cat file.txt # Print entire file
less file.txt # Paginate (q=quit, /=search)
more file.txt # Paginate (older)
head file.txt # First 10 lines
head -n 20 file.txt # First 20 lines
head -c 50 file.txt # First 50 bytes
head -n -5 file.txt # All except last 5 lines
tail file.txt # Last 10 lines
tail -n 50 file.txt # Last 50 lines
tail -c 100 file.txt # Last 100 bytes
tail -f /var/log/syslog # Follow live (log monitoring)
tail -F /var/log/app.log # Follow + retry if file recreated
tail -f file1 file2 # Follow multiple files
wc -l file.txt # Count lines
wc -w file.txt # Count words
wc -c file.txt # Count bytes
nano file.txt # Simple editor (Ctrl+O save, Ctrl+X exit)
vim file.txt # Advanced editor
# vim: i=insert, Esc=normal, :wq=save+quit, :q!=quit without save
# vim: dd=delete line, yy=copy line, p=paste, /=search
diff file1.txt file2.txt # Compare two files
diff -u file1 file2 # Unified diff format
3. Search & Filter
# grep — Search in files
grep "error" app.log
grep -i "error" app.log # Case-insensitive
grep -r "config" /etc/ # Recursive search
grep -n "error" app.log # Show line numbers
grep -v "debug" app.log # Invert match (exclude)
grep -c "error" app.log # Count matching lines
grep -l "error" *.log # Show filenames only
grep -A 3 "error" app.log # 3 lines After match
grep -B 3 "error" app.log # 3 lines Before match
grep -E "error|warn" app.log # Extended regex (OR)
grep -w "error" app.log # Whole word match
# find — Locate files/dirs
find . -name "file.txt"
find /var/log -type f -name "*.log"
find /path -type d -name "config" # Directories only
find /path -mtime -7 # Modified last 7 days
find /path -mtime +30 # Older than 30 days
find /path -size +100M # Larger than 100MB
find /path -size -1k # Smaller than 1KB
find /path -name "*.tmp" -delete # Find and delete
find /path -name "*.sh" -exec chmod +x {} \; # Find + exec
find /path -perm 644 # By permission
find /path -empty # Empty files
find /path -user root # By owner
find . -name "*.log" | xargs rm # Pipe to xargs
# locate — Fast indexed search
sudo apt install plocate -y
locate filename
sudo updatedb # Refresh index
4. Text Processing
AWK
awk '{print $1}' file.txt # Print column 1
awk '{print $1, $3}' file.txt # Multiple columns
awk -F',' '{print $1, $2}' file.csv # CSV delimiter
awk -F':' '{print $1, $7}' /etc/passwd # Parse /etc/passwd
awk '/error/ {print}' file.log # Pattern match
awk '{print NR, $0}' file.txt # With line numbers
awk 'END {print NR}' file.txt # Total line count
awk '{print NF}' file.txt # Number of fields per line
awk '{print $NF}' file.txt # Print last field
awk '{sum += $2} END {print sum}' file # Sum column 2
awk '$3 > 100 {print}' file.txt # Conditional filter
awk '!seen[$1]++' file.txt # Remove duplicates
awk '{gsub(/foo/, "bar"); print}' file # Replace text
awk 'NF > 5 {print}' file.txt # Lines with >5 fields
awk '/START/,/END/ {print}' file.txt # Print between patterns
awk 'BEGIN{print "START"} {print} END{print "END"}' file
sed
sed 's/old/new/' file.txt # Replace first occurrence per line
sed 's/old/new/g' file.txt # Replace all occurrences
sed 's/old/new/gi' file.txt # Case-insensitive replace
sed -i 's/old/new/g' file.txt # In-place edit
sed -n '5,10p' file.txt # Print lines 5-10
sed '3d' file.txt # Delete line 3
sed '/pattern/d' file.txt # Delete matching lines
sed -n '/error/p' file.txt # Print only matching lines
sed 's/^/# /' file.txt # Add # at start of each line
sed 's/ *$//' file.txt # Remove trailing spaces
sed '/^$/d' file.txt # Remove empty lines
sort, uniq, tr
sort file.txt # Alphabetical
sort -r file.txt # Reverse
sort -n file.txt # Numeric
sort -k2 file.txt # By column 2
sort -t',' -k3 -n file.csv # CSV column 3 numeric
sort -u file.txt # Sort + remove duplicates
sort -h sizes.txt # Human-readable (1K, 2M)
uniq file.txt # Remove consecutive duplicates
uniq -c file.txt # Count occurrences
uniq -d file.txt # Show duplicate lines only
uniq -u file.txt # Show unique lines only
sort file.txt | uniq -c | sort -rn # Full frequency count
tr 'a-z' 'A-Z' < file.txt # Lowercase → uppercase
tr -d '0-9' < file.txt # Delete digits
tr -s ' ' < file.txt # Squeeze repeated spaces
tr ':' '\n' < file.txt # Replace colons with newlines
tr -d '\n' < file.txt # Remove all newlines
cut -d',' -f1 file.csv # Extract column 1 (CSV)
cut -d':' -f1,7 /etc/passwd # Extract fields 1 and 7
cut -c1-10 file.txt # Extract characters 1-10
paste file1.txt file2.txt # Merge files side by side
paste -d',' file1 file2 # Merge with comma delimiter
5. User & Group Management
whoami # Current username
id # UID, GID, groups
id username # Specific user info
who # Logged-in users
w # Who + what they're doing
last # Login history
lastlog # Last login per user
# Create / delete users
sudo useradd -m -s /bin/bash user1 # Create with home + bash
sudo useradd -m -d /opt/tomcat -U -s /bin/false tomcat # Service user
sudo adduser user2 # Interactive create
sudo userdel user1 # Delete user
sudo userdel -r user1 # Delete + remove home dir
# Password
passwd # Change own password
sudo passwd user1 # Change user's password
sudo passwd -e user1 # Expire password (force reset)
sudo passwd -l user1 # Lock account
sudo passwd -u user1 # Unlock account
# Modify user
sudo usermod -aG sudo user1 # Add to sudo group
sudo usermod -aG docker user1 # Add to docker group
sudo usermod -s /bin/bash user1 # Change shell
sudo usermod -d /new/home user1 # Change home dir
sudo usermod -l newname oldname # Rename user
# Groups
sudo groupadd developers # Create group
sudo groupdel developers # Delete group
sudo gpasswd -a user1 developers # Add user to group
sudo gpasswd -d user1 developers # Remove user from group
groups user1 # Show user's groups
cat /etc/group # List all groups
cat /etc/passwd # List all users
# Switch users
su - user1 # Switch to user1
sudo -i # Switch to root
sudo su - # Root shell
sudo -l # List sudo permissions
6. File Permissions
ls -l # View permissions
# chmod — numeric
chmod 755 file # rwxr-xr-x
chmod 644 file # rw-r--r--
chmod 600 file # rw------- (SSH keys)
chmod 700 dir # rwx------ (private)
chmod -R 755 /dir # Recursive
# chmod — symbolic
chmod u+x file # Add execute for owner
chmod g+w file # Add write for group
chmod o-r file # Remove read from others
chmod a+x file # Add execute for all
chmod u=rwx,go=rx file # Set exact permissions
# chown / chgrp
chown user file # Change owner
chown user:group file # Change owner + group
chown -R user:group /dir # Recursive
chgrp group file # Change group only
# umask
umask # View current (default 022)
umask 022 # Files=644, Dirs=755
umask 027 # Files=640, Dirs=750
# Special bits
chmod u+s file # SUID — run as file owner
chmod g+s dir # SGID — inherit group
chmod +t dir # Sticky bit — only owner deletes
# Permission table
# 7=rwx 6=rw- 5=r-x 4=r-- 0=---
7. Process Management
ps aux # All running processes
ps aux | grep nginx # Filter by name
ps -ef # Full format listing
ps -ef --forest # Process tree format
pgrep nginx # Get PID by name
pidof nginx # PID of named process
pstree # Visual process tree
top # Live process monitor (q to quit)
htop # Enhanced monitor (if installed)
kill 1234 # Graceful stop (SIGTERM)
kill -9 1234 # Force kill (SIGKILL)
kill -15 1234 # Explicit SIGTERM
kill -l # List all signals
killall nginx # Kill all by name
pkill -f "python script.py" # Kill by full command
pkill -u username # Kill all user's processes
# Job control
command & # Run in background
jobs # List background jobs
fg # Bring last job to foreground
fg %2 # Bring job #2 to foreground
bg # Resume job in background
Ctrl+Z # Suspend foreground process
Ctrl+C # Terminate foreground process
nohup command & # Run, persist after logout
nohup ./script.sh > out.log 2>&1 & # With output redirect
disown %1 # Detach job from shell
screen # Terminal multiplexer (persistent)
tmux # Modern terminal multiplexer
8. System Info & Monitoring
uname -a # Kernel + system info
uname -r # Kernel version only
hostname # Hostname
hostnamectl # Full hostname info
hostnamectl set-hostname newname # Change hostname
lscpu # CPU info
lsmem # Memory info
lshw # Full hardware info
lsblk # Block devices (disks)
lsusb # USB devices
lspci # PCI devices
uptime # Uptime + load average
last reboot # Reboot history
free -h # Memory usage (human-readable)
vmstat 1 5 # Virtual memory stats (5 samples)
iostat # CPU + I/O stats
iostat -x 1 # Extended I/O stats every 1s
mpstat # Per-CPU stats
sar -u 1 3 # CPU usage over time
sar -r 1 3 # Memory over time
top # Live processes
htop # Enhanced live view
ulimit -a # Show all resource limits
ulimit -n 65535 # Set max open file descriptors
cat /proc/cpuinfo # CPU details
cat /proc/meminfo # Memory details
cat /proc/version # Kernel version
cat /etc/os-release # OS version info
cat /etc/hostname # Hostname file
# Logs
dmesg # Kernel ring buffer
dmesg | tail -20 # Last 20 kernel messages
journalctl # systemd journal
journalctl -f # Follow live logs
journalctl -n 100 # Last 100 entries
journalctl --since "1 hour ago"
journalctl -u nginx # Logs for specific service
journalctl -p err # Error-level only
9. Disk & Storage
df -h # Disk space (all filesystems)
df -h / # Root filesystem only
df -i # Inode usage
du -sh /var/log # Directory total size
du -sh * # Size of each item in current dir
du -h --max-depth=1 /var # One level deep
du -sh /path | sort -h # Sort by size
lsblk # List block devices
lsblk -f # With filesystem info
blkid # Block device UUIDs
fdisk -l # Disk partition info (root)
parted -l # Partition tool
mount # Show all mounts
mount /dev/sdb1 /mnt/data # Mount device
umount /mnt/data # Unmount
cat /etc/fstab # Permanent mounts config
# Check for reboot after patch
cat /var/run/reboot-required
cat /var/run/reboot-required.pkgs
10. Networking
# Connectivity
ping google.com
ping -c 4 8.8.8.8 # Send 4 packets
ping -i 2 google.com # 2 second interval
traceroute google.com # Trace route
mtr google.com # Interactive traceroute
# DNS
nslookup google.com
dig google.com
dig google.com +short # IP only
dig google.com MX # Mail records
dig @8.8.8.8 google.com # Use specific DNS server
host google.com
# IP / Interface
ip a # All interfaces + IPs
ip addr show eth0 # Specific interface
ip link show # Link state
ip route show # Routing table
ip route add default via 192.168.1.1 # Add default gateway
ip link set eth0 up/down # Enable/disable interface
ifconfig # Interfaces (older)
ifconfig eth0 up/down
# Public/local IP
curl ifconfig.me # Public IP
curl -s ifconfig.me
hostname -I # All local IPs
# Ports & connections
ss -tuln # Listening TCP/UDP ports
ss -tulnp # With process name/PID
ss -s # Summary
netstat -tuln # Listening ports (older)
netstat -anp # All connections + PID
lsof -i :80 # Process using port 80
lsof -i :8080
lsof -i TCP # All TCP connections
# Port scanning
nmap localhost
nmap -sV 192.168.1.1 # Version detection
nmap -p 22,80,443 host # Specific ports
# HTTP
curl -O https://example.com/file.zip # Download (keep name)
curl -o out.zip https://example.com/file.zip
curl -I https://example.com # Headers only
curl -v https://example.com # Verbose
curl -L https://example.com # Follow redirects
curl -X POST -d "key=val" https://api.example.com
curl -H "Authorization: Bearer TOKEN" https://api.example.com
curl -u user:pass https://example.com # Basic auth
wget https://example.com/file.zip
wget -c https://example.com/large.iso # Resume download
wget -r https://example.com/ # Recursive
wget -q https://example.com/file.zip # Quiet mode
wget -P /tmp/ https://example.com/file.zip # Save to dir
11. SSH & Remote Access
ssh user@host # Connect
ssh -p 2222 user@host # Custom port
ssh -i ~/.ssh/id_rsa user@host # With key file
ssh -v user@host # Verbose (debug)
ssh -X user@host # X11 forwarding (GUI)
ssh -L 8080:localhost:80 user@host # Local port forward
ssh -R 9090:localhost:3000 user@host # Remote port forward
ssh -N -L 3306:db-host:3306 user@jump # SSH tunnel (no shell)
# Key management
ssh-keygen -t rsa -b 4096 # Generate RSA key
ssh-keygen -t ed25519 # Generate Ed25519 key (modern)
ssh-copy-id user@host # Copy public key to remote
cat ~/.ssh/id_rsa.pub # View public key
ssh-add ~/.ssh/id_rsa # Add key to agent
ssh-agent bash # Start SSH agent
# Config file (~/.ssh/config)
# Host myserver
# HostName 192.168.1.10
# User ubuntu
# IdentityFile ~/.ssh/id_rsa
# Port 22
# Known hosts
cat ~/.ssh/known_hosts
ssh-keyscan host >> ~/.ssh/known_hosts # Add host key
# Authorized keys (on remote server)
cat ~/.ssh/authorized_keys
12. SCP & SFTP
# SCP — Secure Copy
scp file.txt user@host:/home/user/ # Local → Remote
scp user@host:/home/user/file.txt /local/path/ # Remote → Local
scp -r /local/dir/ user@host:/remote/dir/ # Directory
scp -P 2222 file.txt user@host:/home/user/ # Custom port
scp -i ~/.ssh/id_rsa file.txt user@host:/tmp/ # With key
scp -p file.txt user@host:/tmp/ # Preserve timestamps
scp -l 500 largefile.tar.gz user@host:/tmp/ # Limit bandwidth
scp user1@server1:/path/file user2@server2:/path/ # Server to server
# SFTP — Interactive
sftp user@host
sftp -P 2222 user@host
sftp -i ~/.ssh/id_rsa user@host
# SFTP session commands
ls / lls # Remote / local listing
cd /path / lcd /path # Remote / local cd
put file.txt # Upload file
put -r local_dir/ # Upload directory
get remote.txt # Download file
get -r remote_dir/ # Download directory
pwd / lpwd # Remote / local path
mkdir newfolder # Create remote dir
rm remotefile.txt # Delete remote file
rename old.txt new.txt # Rename remote file
exit # Quit SFTP
13. Firewall (UFW / iptables)
# UFW — Uncomplicated Firewall (Ubuntu)
ufw status # Show status + rules
ufw status verbose # Detailed status
ufw enable # Enable firewall
ufw disable # Disable firewall
ufw allow 22 # Allow SSH
ufw allow 80/tcp # Allow HTTP
ufw allow 443/tcp # Allow HTTPS
ufw allow 8080/tcp # Allow custom port
ufw deny 3306 # Block MySQL
ufw deny from 192.168.1.5 # Block IP
ufw allow from 10.0.0.0/8 # Allow subnet
ufw delete allow 80 # Remove rule
ufw reset # Reset all rules
ufw reload # Reload rules
# iptables (lower level)
iptables -L # List all rules
iptables -L -v -n # Verbose + numeric
iptables -F # Flush (clear all rules)
iptables -A INPUT -p tcp --dport 22 -j ACCEPT # Allow SSH
iptables -A INPUT -p tcp --dport 80 -j ACCEPT # Allow HTTP
iptables -A INPUT -j DROP # Drop everything else
iptables-save # Save current rules
iptables-restore < rules # Restore rules
14. Archive & Compression
# tar
tar -czvf archive.tar.gz folder/ # Create .tar.gz
tar -xzvf archive.tar.gz # Extract .tar.gz
tar -cjvf archive.tar.bz2 folder/ # Create .tar.bz2
tar -xjvf archive.tar.bz2 # Extract .tar.bz2
tar -tvf archive.tar.gz # List contents
tar -xzvf archive.tar.gz -C /target/dir/ # Extract to dir
tar -czvf out.tar.gz --exclude='*.log' dir/ # Exclude files
tar -xzvf archive.tar.gz specificfile # Extract one file
# zip / unzip
zip -r archive.zip folder/ # Zip folder
zip archive.zip file1 file2 # Zip specific files
zip -e archive.zip folder/ # Password-protected
zip -9 -r archive.zip folder/ # Max compression (1-9)
unzip archive.zip # Extract
unzip archive.zip -d /target/dir/ # Extract to dir
unzip -l archive.zip # List contents
unzip -o archive.zip # Overwrite without prompt
unzip -n archive.zip # Never overwrite
# gzip / bzip2
gzip file.txt # Compress → file.txt.gz
gzip -d file.txt.gz # Decompress
gunzip file.txt.gz # Same as gzip -d
gzip -k file.txt # Keep original
bzip2 file.txt # Compress → file.txt.bz2
bunzip2 file.txt.bz2 # Decompress
15. Date & Time
date # Current date + time
date +"%Y-%m-%d" # 2026-05-30
date +"%H:%M:%S" # 14:30:00
date +"%d/%m/%Y %T" # 30/05/2026 14:30:00
date +"%A, %B %d %Y" # Saturday, May 30 2026
date +"%s" # Unix timestamp
date -d "2 days ago" # Past date
date -d "next Monday" # Future date
date -d "@1700000000" # Convert timestamp
date -s "2026-05-30 14:00:00" # Set date/time (root)
timedatectl # Full time/timezone info
timedatectl set-timezone Asia/Kolkata # Set timezone
timedatectl set-timezone UTC # Set to UTC
timedatectl list-timezones # All timezones
timedatectl list-timezones | grep India
hwclock # Hardware clock time
hwclock --systohc # Sync hardware from system
hwclock --hctosys # Sync system from hardware
cal # Current month calendar
cal 2026 # Full year calendar
cal 5 2026 # Specific month/year
uptime # Uptime + load average
16. Package Management
APT (Ubuntu / Debian)
sudo apt update # Refresh package index
sudo apt upgrade -y # Upgrade all packages
sudo apt full-upgrade -y # Upgrade + handle deps
sudo apt install nginx -y # Install package
sudo apt install --only-upgrade curl # Upgrade specific package
sudo apt remove nginx # Remove package
sudo apt purge nginx # Remove + config files
sudo apt autoremove -y # Remove unused deps
sudo apt autoclean # Clean cache
sudo apt search nginx # Search packages
sudo apt show nginx # Package info
apt list --installed # List installed
apt list --upgradable # List upgradable
sudo dpkg -i package.deb # Install .deb file
dpkg -l | grep nginx # Check if installed
YUM / DNF (RHEL / CentOS)
sudo yum update -y # Update all
sudo yum install nginx -y # Install
sudo yum remove nginx # Remove
sudo yum search nginx # Search
sudo yum info nginx # Package info
sudo yum list installed # List installed
sudo dnf update -y # DNF (CentOS 8+)
sudo dnf install nginx -y
sudo dnf remove nginx
sudo rpm -ivh package.rpm # Install .rpm file
17. Shell Scripting
Variables & Types
#!/bin/bash
name="devops" # String variable
age=30 # Integer
pi=3.14
fruits=("apple" "banana" "orange") # Array
echo ${fruits[0]} # Access element
echo ${fruits[@]} # All elements
echo ${#fruits[@]} # Array length
readonly MY_VAR="constant" # Read-only variable
export PATH=$PATH:/new/path # Export to child processes
# Special variables
echo $0 # Script name
echo $1 $2 # Arguments 1, 2
echo $@ # All arguments
echo $# # Number of arguments
echo $$ # Script PID
echo $? # Exit status of last command
echo $! # PID of last background job
Conditionals
# if / elif / else
if [ $num -gt 10 ]; then
echo "Greater"
elif [ $num -eq 10 ]; then
echo "Equal"
else
echo "Less"
fi
# File tests
if [ -f file.txt ]; then echo "File exists"; fi
if [ -d /var/log ]; then echo "Dir exists"; fi
if [ -z "$var" ]; then echo "Empty string"; fi
if [ -n "$var" ]; then echo "Non-empty"; fi
if [ -r file ]; then echo "Readable"; fi
if [ -w file ]; then echo "Writable"; fi
if [ -x file ]; then echo "Executable"; fi
# Operators
# -eq -ne -lt -gt -le -ge (numeric)
# = != < > (string)
# && || (AND / OR)
Loops
# for loop
for i in 1 2 3 4 5; do
echo "Number: $i"
done
# for range
for i in {1..10}; do echo $i; done
# for with command
for file in $(ls /tmp/*.log); do
echo "Processing: $file"
done
# C-style for
for ((i=0; i<5; i++)); do
echo $i
done
# while loop
count=1
while [ $count -le 5 ]; do
echo "Count: $count"
((count++))
done
# until loop
until [ $count -gt 5 ]; do
echo $count
((count++))
done
# case statement
case $input in
"start") echo "Starting..." ;;
"stop") echo "Stopping..." ;;
"status") systemctl status nginx ;;
*) echo "Unknown option" ;;
esac
Functions
# Define and call
greet() {
echo "Hello, $1!"
}
greet "DevOps"
# With local variable + return
add() {
local result=$(( $1 + $2 ))
echo $result
}
sum=$(add 4 6)
echo "Sum: $sum"
# Check exit status
if ! command -v docker &>/dev/null; then
echo "Docker not installed"
exit 1
fi
Useful Patterns
set -e # Exit on any error
set -x # Debug — print each command
set -eo pipefail # Exit on pipe failure too
# Read input
read -p "Enter username: " username
read -sp "Enter password: " password # Silent (no echo)
# String operations
str="hello world"
echo ${str^^} # Uppercase
echo ${str,,} # Lowercase
echo ${#str} # Length
echo ${str/world/linux} # Replace
# Arithmetic
result=$((5 + 3))
result=$(expr 5 + 3)
let result=5+3
18. Cron Jobs
crontab -e # Edit crontab
crontab -l # List crontabs
crontab -r # Remove all crontabs
crontab -u user -l # List for specific user
sudo crontab -e # Root crontab
# Cron format:
# MIN HOUR DOM MON DOW COMMAND
# 0 2 * * 0 /path/script.sh
# Examples
* * * * * /root/script.sh # Every minute
0 * * * * /root/script.sh # Every hour
0 2 * * * /root/script.sh # Daily at 2 AM
0 2 * * 0 /root/script.sh # Every Sunday 2 AM
0 2 1 * * /root/script.sh # 1st of every month
*/5 * * * * /root/script.sh # Every 5 minutes
0 8,17 * * * /root/script.sh # 8 AM and 5 PM
0 9 * * 1-5 /root/script.sh # Weekdays at 9 AM
# With logging
* * * * * /root/script.sh >> /var/log/cron.log 2>&1
# Special strings
@reboot /root/script.sh # Run at boot
@daily /root/backup.sh # Daily
@weekly /root/cleanup.sh # Weekly
# Reference: https://crontab.guru/
19. systemd Service Management
# Status & Control
systemctl status nginx
systemctl start nginx
systemctl stop nginx
systemctl restart nginx
systemctl reload nginx # Reload config (no downtime)
systemctl enable nginx # Auto-start on boot
systemctl disable nginx # Remove from boot
systemctl is-active nginx # Check if running
systemctl is-enabled nginx # Check if enabled
systemctl daemon-reload # Reload unit files (after editing)
# Listing
systemctl list-units --type=service # All active services
systemctl list-units --type=service --all # Including inactive
systemctl list-unit-files # All unit files
# Logs
journalctl -u nginx # All logs for service
journalctl -u nginx -f # Follow live
journalctl -u nginx -n 100 # Last 100 lines
journalctl -u nginx --since "1 hour ago"
journalctl -u nginx --since "2026-05-30 08:00"
journalctl -n 50 -p err # Last 50 errors
journalctl -b # Since last boot
journalctl --disk-usage # Journal disk usage
# Service file location
# /etc/systemd/system/myapp.service
# /usr/lib/systemd/system/myapp.service
# Minimal service file template
cat > /etc/systemd/system/myapp.service << 'EOF'
[Unit]
Description=My Application
After=network.target
[Service]
Type=simple
User=appuser
WorkingDirectory=/opt/myapp
ExecStart=/usr/bin/java -jar /opt/myapp/app.jar
Restart=on-failure
RestartSec=10
[Install]
WantedBy=multi-user.target
EOF
sudo systemctl daemon-reload
sudo systemctl enable myapp
sudo systemctl start myapp
20. Maven Commands
# Install
sudo apt install maven -y
mvn --version
# Core lifecycle
mvn clean # Delete target/
mvn validate # Validate pom.xml
mvn compile # Compile source
mvn test # Run unit tests
mvn package # Create .jar/.war
mvn verify # Run integration tests
mvn install # Install to local ~/.m2
mvn deploy # Publish to remote repo
# Common combinations
mvn clean package # Clean + package
mvn clean install # Most common CI command
mvn clean install -DskipTests # Skip test execution
mvn clean install -Dmaven.test.skip=true # Skip test compile too
mvn clean package -Dmaven.test.failure.ignore=true
# Dependencies
mvn dependency:tree # Full dependency tree
mvn dependency:resolve # Resolve + list deps
mvn dependency:purge-local-repository # Clear local cache
# Debugging / info
mvn help:effective-pom # Show resolved POM
mvn help:effective-settings # Show settings
mvn help:describe -Dplugin=compiler # Plugin info
mvn versions:display-dependency-updates # Available updates
mvn site # Generate project docs
mvn clean install -X # Debug mode
mvn clean install -o # Offline mode
mvn -pl module-name clean package # Specific submodule
mvn -am -pl module-name package # With dependencies
# Build Spring PetClinic
git clone https://github.com/spring-projects/spring-petclinic.git
cd spring-petclinic
mvn clean package
java -jar target/spring-petclinic-*.jar
# Access: http://localhost:8080
21. Gradle Commands
# Install via SDKMAN
curl -s "https://sdkman.io" | bash
source "$HOME/.sdkman/bin/sdkman-init.sh"
sdk install gradle 8.14
gradle --version
# Core tasks
./gradlew build # Compile + test + package
./gradlew clean # Delete build/
./gradlew test # Run unit tests
./gradlew assemble # Package (no tests)
./gradlew check # All verifications
./gradlew run # Run app (needs 'application' plugin)
./gradlew clean build # Clean + full rebuild
# Info
./gradlew tasks # List available tasks
./gradlew dependencies # Dependency tree
./gradlew properties # Project properties
./gradlew help --task build # Help for specific task
# Build options
./gradlew build -x test # Skip tests
./gradlew build --info # Verbose logging
./gradlew build --debug # Full debug output
./gradlew build --offline # No network
./gradlew build --parallel # Parallel execution
./gradlew build --scan # Build scan (online)
./gradlew build --refresh-dependencies # Re-download deps
# Multi-project
./gradlew :moduleName:taskName # Specific module task
./gradlew :app:assembleDebug # Android example
# Wrapper
gradle wrapper --gradle-version 8.7 # Update wrapper version
./gradlew wrapper # Regenerate wrapper files
# Build Spring PetClinic
git clone https://github.com/spring-projects/spring-petclinic.git
cd spring-petclinic
./gradlew build
java -jar build/libs/spring-petclinic-*.jar
22. Linux Patching
# Ubuntu / Debian — APT
sudo apt update
apt list --upgradable
sudo apt upgrade -y
sudo apt full-upgrade -y
sudo apt install --only-upgrade curl # Single package
sudo apt autoremove -y
sudo apt autoclean
# One-liner full patch
sudo apt update && sudo apt full-upgrade -y && sudo apt autoremove -y && sudo apt autoclean
# Security patches only
sudo apt install unattended-upgrades -y
sudo unattended-upgrade --dry-run -v # Preview only
sudo unattended-upgrade -v # Apply
# Enable automatic security updates
sudo dpkg-reconfigure --priority=low unattended-upgrades
# Reboot check
cat /var/run/reboot-required
cat /var/run/reboot-required.pkgs
sudo reboot
# RHEL / CentOS
sudo yum update -y
sudo yum update --security -y # Security only
sudo dnf update -y # CentOS 8+
sudo dnf update --security -y
# Check last update time
rpm -qa --last | head -20 # RPM history
cat /var/log/dpkg.log | grep "upgrade" # Debian history
# Scheduled patching with cron
crontab -e
# 0 2 * * 0 sudo apt update && sudo apt full-upgrade -y >> /var/log/patch.log 2>&1
23. WSL (Windows Subsystem for Linux)
# Install (PowerShell as Admin)
wsl --install # WSL2 + Ubuntu (default)
wsl --install -d Ubuntu-22.04 # Specific distro
# Management
wsl --list --online # Available distros
wsl --list --verbose # Running + version
wsl --set-default-version 2 # Set WSL2 as default
wsl --set-version Ubuntu 2 # Upgrade distro to WSL2
wsl --shutdown # Stop all WSL instances
wsl --terminate Ubuntu # Stop specific distro
wsl --unregister Ubuntu # Remove distro (deletes data)
wsl --export Ubuntu ubuntu.tar # Backup distro
wsl --import Ubuntu /path ubuntu.tar # Restore distro
# Launch
wsl # Launch default
wsl -d Ubuntu-22.04 # Launch specific
ubuntu # Launch Ubuntu directly
# Inside WSL — access Windows files
ls /mnt/c/Users/YourName/
ls /mnt/d/Projects/
cd /mnt/c/
# Patch Ubuntu in WSL
sudo apt update && sudo apt full-upgrade -y && sudo apt autoremove -y
# C:\Users\Name\.wslconfig — Resource limits
[wsl2]
memory=4GB
processors=2
swap=2GB
localhostForwarding=true
24. Environment Variables
env # List all environment variables
printenv # List all (same as env)
printenv PATH # Specific variable
echo $HOME # Print variable
echo $USER
echo $SHELL
echo $PATH
echo $PWD
export MY_VAR="value" # Set + export to child processes
export PATH=$PATH:/usr/local/bin # Add to PATH
unset MY_VAR # Remove variable
# Persistent variables — add to ~/.bashrc or ~/.bash_profile
echo 'export JAVA_HOME=/usr/lib/jvm/java-17-openjdk-amd64' >> ~/.bashrc
echo 'export PATH=$PATH:$JAVA_HOME/bin' >> ~/.bashrc
source ~/.bashrc # Reload shell config
# System-wide: /etc/environment
# /etc/profile.d/*.sh for session-wide scripts
# Common variables
echo $JAVA_HOME
echo $MAVEN_HOME
echo $GRADLE_HOME
echo $DOCKER_HOST
echo $KUBECONFIG
echo $AWS_DEFAULT_REGION
25. Redirection & Pipes
# Redirection
command > file.txt # Stdout to file (overwrite)
command >> file.txt # Stdout to file (append)
command 2> error.log # Stderr to file
command 2>> error.log # Stderr append
command &> all.log # Stdout + stderr to file
command > /dev/null # Discard stdout
command 2>/dev/null # Discard stderr
command > out.log 2>&1 # Both to same file (old syntax)
command < input.txt # Read stdin from file
command << EOF # Here document
multi line input
EOF
# Pipes
command1 | command2 # Pipe stdout to next
ps aux | grep nginx # Filter processes
cat file.txt | grep "error" | wc -l # Count errors
ls -la | sort -k5 -n # Sort by size
cat /etc/passwd | cut -d: -f1 | sort # Extract + sort
command | tee file.txt # Output to screen AND file
command | tee -a file.txt # Output + append
# xargs — build commands from stdin
find . -name "*.log" | xargs rm
cat urls.txt | xargs -I{} curl -O {}
find . -name "*.sh" | xargs chmod +x
echo "file1 file2" | xargs -n1 ls # One arg per line
# Command substitution
today=$(date +%Y-%m-%d)
files=$(ls /tmp/*.log)
count=$(wc -l < file.txt)
echo "Today is $today, files: $files"
Quick Reference — Most Used Commands
| Category |
Commands |
| Navigate |
ls -la, cd, pwd, mkdir -p, rm -rf |
| Files |
cat, less, head -n, tail -f, cp -r, mv |
| Search |
grep -rni, find -name, locate, awk, sed |
| Permissions |
chmod 755, chown user:group, umask |
| Users |
useradd -m, usermod -aG, passwd, sudo |
| Processes |
ps aux, top, kill -9, nohup & |
| System |
uname -a, free -h, df -h, uptime |
| Network |
ip a, ss -tulnp, ping, curl, ssh |
| Transfer |
scp, sftp, rsync, wget |
| Archive |
tar -czvf, tar -xzvf, zip -r, unzip |
| Packages |
apt update && apt upgrade, yum update |
| Services |
systemctl start/stop/enable/status |
| Build |
mvn clean package, ./gradlew build |
| Cron |
crontab -e, crontab -l |
