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System Operations

Process Management​

Viewing Processes​

# List all processes
ps aux                  # All processes with detailed info
ps -ef                  # All processes with full format
ps -u username          # Processes for specific user

# Process tree
pstree                  # Show process hierarchy
pstree -p               # Include PIDs
pstree username         # Tree for specific user

# Top processes
top                     # Interactive process viewer
htop                    # Enhanced top (if installed)
top -u username         # Top for specific user
top -p 1234             # Top for specific PID

# Find processes
pgrep process_name      # Find PID by name
pgrep -f pattern        # Find by full command line
pidof process_name      # Get PID of running process

Controlling Processes​

# Kill processes
kill PID                # Terminate process
kill -9 PID             # Force kill
kill -TERM PID          # Graceful termination
kill -HUP PID           # Hangup signal (often reload config)

# Kill by name
killall process_name    # Kill all instances
pkill process_name      # Kill by name pattern
pkill -f pattern        # Kill by full command line

# Background and foreground
command &               # Run in background
jobs                    # List background jobs
fg %1                   # Bring job 1 to foreground
bg %1                   # Send job 1 to background
disown %1               # Detach job from shell

# Suspend and resume
Ctrl+Z                  # Suspend current process
fg                      # Resume suspended process
bg                      # Resume in background

Process Information​

# Detailed process info
ps -p PID -o pid,ppid,cmd,vsz,rss,time
lsof -p PID             # Files opened by process
strace -p PID           # System calls made by process
/proc/PID/              # Process information directory

# Process resource usage
time command            # Time command execution
/usr/bin/time -v command # Verbose timing info

System Information​

System Status​

# System uptime and load
uptime                  # System uptime and load average
w                       # Who is logged in and what they're doing
who                     # Currently logged in users
last                    # Login history
lastlog                 # Last login for all users

# System information
uname -a                # All system information
uname -s                # Kernel name
uname -r                # Kernel release
uname -m                # Machine architecture
hostname                # System hostname
hostname -I             # IP addresses

Hardware Information​

# CPU information
lscpu                   # CPU architecture info
cat /proc/cpuinfo       # Detailed CPU info
nproc                   # Number of processing units

# Memory information
free -h                 # Memory usage (human readable)
free -m                 # Memory usage in MB
cat /proc/meminfo       # Detailed memory info
vmstat                  # Virtual memory statistics
vmstat 1 5              # Update every 1 second, 5 times

# Disk information
df -h                   # Disk space usage
df -i                   # Inode usage
du -h directory         # Directory size
du -sh *                # Size of all items in current directory
lsblk                   # List block devices
fdisk -l                # List disk partitions (root required)

System Resources​

# I/O statistics
iostat                  # CPU and I/O statistics
iostat -x 1             # Extended I/O stats every second
iotop                   # I/O usage by process

# Network statistics
netstat -tuln           # Network connections
netstat -i              # Network interface statistics
ss -tuln                # Socket statistics (modern netstat)
ss -s                   # Summary statistics

# System calls and performance
strace command          # Trace system calls
ltrace command          # Trace library calls
perf top                # Performance monitoring

File System Operations​

Disk Usage​

# Disk space
df -h                   # Human-readable disk usage
df -T                   # Show filesystem type
df /path                # Usage for specific mount point

# Directory size
du -h directory         # Human-readable directory size
du -sh directory        # Summary only
du -h --max-depth=1     # One level deep
du -h | sort -hr        # Sorted by size
ncdu                    # Interactive disk usage analyzer

# Find large files
find /path -type f -size +100M    # Files larger than 100MB
find /path -type f -exec ls -lh {} \; | sort -k5 -hr

Mount Operations​

# View mounted filesystems
mount                   # Show all mounted filesystems
mount | column -t       # Formatted output
findmnt                 # Tree view of mounted filesystems
cat /proc/mounts        # Kernel view of mounts

# Mount filesystem
mount /dev/sdb1 /mnt/usb    # Mount USB drive
mount -o ro /dev/sdb1 /mnt  # Mount read-only
mount -t ext4 /dev/sdb1 /mnt # Specify filesystem type

# Unmount filesystem
umount /mnt/usb         # Unmount by mount point
umount /dev/sdb1        # Unmount by device
umount -l /mnt/usb      # Lazy unmount
fuser -km /mnt/usb      # Kill processes using mountpoint

File System Checking​

# Check filesystem
fsck /dev/sdb1          # Check filesystem
fsck -y /dev/sdb1       # Automatically fix errors
e2fsck /dev/sdb1        # Check ext2/3/4 filesystem

# Bad blocks
badblocks -v /dev/sdb1  # Check for bad blocks
badblocks -w /dev/sdb1  # Write test (destructive)

Network Operations​

Network Configuration​

# Interface information
ip addr show            # Show IP addresses
ip link show            # Show network interfaces
ifconfig                # Network interface configuration
iwconfig                # Wireless interface configuration

# Routing
ip route show           # Show routing table
route -n                # Numeric routing table
ip route add default via 192.168.1.1    # Add default route
ip route del 192.168.1.0/24             # Delete route

# DNS
nslookup example.com    # DNS lookup
dig example.com         # DNS lookup with more detail
host example.com        # Simple DNS lookup
systemd-resolve --status # DNS configuration (systemd)

Network Testing​

# Connectivity testing
ping -c 4 example.com   # Ping 4 times
ping6 -c 4 example.com  # IPv6 ping
traceroute example.com  # Trace route to destination
mtr example.com         # Combined ping and traceroute

# Port testing
telnet example.com 80   # Test TCP connection
nc -zv example.com 80   # Test port with netcat
nmap -p 22,80,443 example.com # Port scan

# Network monitoring
netstat -tuln           # Network connections
ss -tuln                # Socket statistics
lsof -i                 # Files opened by network processes
lsof -i :80             # Processes using port 80

Network Transfer​

# Download files
wget https://example.com/file.tar.gz
wget -c https://example.com/file.tar.gz    # Continue download
curl -O https://example.com/file.tar.gz
curl -L https://example.com/file.tar.gz    # Follow redirects

# Upload files
scp file.txt user@server:/path/
rsync -avz local_dir/ user@server:/remote_dir/

Service Management​

Systemd Services​

# Service status
systemctl status service_name
systemctl is-active service_name
systemctl is-enabled service_name

# Service control
systemctl start service_name
systemctl stop service_name
systemctl restart service_name
systemctl reload service_name

# Service configuration
systemctl enable service_name    # Enable at boot
systemctl disable service_name   # Disable at boot
systemctl mask service_name      # Prevent service from starting
systemctl unmask service_name    # Allow service to start

# List services
systemctl list-units --type=service
systemctl list-units --type=service --state=active
systemctl list-units --type=service --state=failed

Service Logs​

# View logs
journalctl -u service_name       # Logs for specific service
journalctl -u service_name -f    # Follow logs
journalctl -u service_name --since="2023-01-01"
journalctl -u service_name --until="2023-01-02"

# System logs
journalctl -b                    # Boot logs
journalctl -k                    # Kernel logs
journalctl --disk-usage          # Log disk usage

User and Group Management​

User Information​

# Current user info
whoami                  # Current username
id                      # User and group IDs
groups                  # Groups for current user
groups username         # Groups for specific user

# User details
finger username         # User information
getent passwd username  # User from passwd database
last username           # Login history for user

User Management (requires root)​

# Add user
useradd username
useradd -m -s /bin/bash username    # Create home directory and set shell
adduser username                    # Interactive user creation (Debian/Ubuntu)

# Modify user
usermod -aG group username          # Add user to group
usermod -s /bin/zsh username        # Change shell
usermod -l newname oldname          # Rename user

# Delete user
userdel username                    # Delete user
userdel -r username                 # Delete user and home directory

# Password management
passwd username                     # Change password
passwd -l username                  # Lock account
passwd -u username                  # Unlock account
chage -l username                   # Password aging info

Group Management (requires root)​

# Group operations
groupadd groupname                  # Create group
groupdel groupname                  # Delete group
groupmod -n newname oldname         # Rename group

# Group membership
gpasswd -a username groupname       # Add user to group
gpasswd -d username groupname       # Remove user from group

System Monitoring​

Real-time Monitoring​

# System load
uptime                  # Load average
tload                   # Load average graph
w                       # System load and user activity

# Resource usage
top                     # Process monitor
htop                    # Enhanced process monitor
iotop                   # I/O monitor
nethogs                 # Network monitor per process

System Logs​

# Log files
tail -f /var/log/syslog     # System log
tail -f /var/log/auth.log   # Authentication log
tail -f /var/log/kern.log   # Kernel log
tail -f /var/log/daemon.log # Daemon log

# Log analysis
grep "error" /var/log/syslog
grep "failed" /var/log/auth.log
zgrep "pattern" /var/log/old.log.gz    # Search compressed logs

Performance Monitoring​

System Performance​

# CPU usage
mpstat                  # CPU statistics
mpstat 1 5              # Update every 1 second, 5 times
sar -u 1 5              # CPU utilization

# Memory usage
vmstat 1 5              # Virtual memory statistics
free -h                 # Memory usage
pmap PID                # Memory map of process

# I/O performance
iostat -x 1             # Extended I/O statistics
iotop                   # I/O usage by process

Process Monitoring​

# Process statistics
ps aux --sort=-%cpu     # Sort by CPU usage
ps aux --sort=-%mem     # Sort by memory usage
ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%cpu

# Process resource usage
pidstat 1 5             # Process statistics
pidstat -r 1 5          # Memory usage
pidstat -d 1 5          # I/O usage

Environment Variables​

System Environment​

# Environment variables
env                     # All environment variables
printenv                # All environment variables
printenv PATH           # Specific variable
echo $PATH              # Variable value

# Set environment variables
export VAR=value        # Export variable
VAR=value command       # Set for single command
unset VAR               # Remove variable

# Common environment variables
echo $HOME              # Home directory
echo $USER              # Current user
echo $SHELL             # Current shell
echo $PATH              # Executable search path
echo $PWD               # Current directory
echo $OLDPWD            # Previous directory

Shell Configuration​

# Shell configuration files
~/.bashrc               # Bash configuration
~/.bash_profile         # Bash login configuration
~/.profile              # Shell-independent profile
/etc/profile            # System-wide profile
/etc/bash.bashrc        # System-wide bash configuration

# Reload configuration
source ~/.bashrc        # Reload bash configuration
. ~/.bashrc             # Alternative syntax

Security Operations​

File Security​

# File permissions
ls -l file              # View permissions
chmod 755 file          # Change permissions
chown user:group file   # Change ownership
chgrp group file        # Change group

# Special permissions
chmod +t directory      # Sticky bit
chmod +s file           # Setuid/setgid
ls -la /tmp             # View sticky bit
find /usr -perm -4000   # Find setuid files

System Security​

# Login monitoring
who                     # Current users
w                       # Current users with activity
last                    # Login history
lastlog                 # Last login per user
lastb                   # Failed login attempts

# Process security
ps aux | grep suspicious
lsof -i                 # Network connections
netstat -tuln           # Network listeners

Best Practices​

System Maintenance​

# Regular maintenance tasks
# Update package lists
apt update && apt upgrade   # Debian/Ubuntu
yum update                  # CentOS/RHEL

# Clean up
apt autoremove              # Remove unused packages
apt autoclean               # Clean package cache
yum clean all               # Clean YUM cache

# Log rotation
logrotate -f /etc/logrotate.conf

Performance Optimization​

# Monitor system regularly
uptime                      # Check load
free -h                     # Check memory
df -h                       # Check disk space
iostat                      # Check I/O

# Identify bottlenecks
top -o %CPU                 # High CPU processes
top -o %MEM                 # High memory processes
iotop                       # I/O intensive processes

Security Best Practices​

# Regular security checks
ps aux | grep -v "^\[.*\]$" | grep -v "root"  # Unusual processes
netstat -tuln | grep LISTEN                   # Network listeners
find /tmp -type f -atime +7 -delete           # Clean old temp files