Linux System Commands

Essential Linux commands for monitoring system resources, managing disk space, and maintaining the Ubuntu server.

🟡 LOW RISK

Most commands here are read-only monitoring commands. Deletion commands are marked with warnings.

Disk Space Management
Memory Monitoring
CPU & Process Monitoring
System Logs Cleanup
Network Monitoring
System Information

Disk Space Management

Check Disk Usage (df)

bash

# Human-readable format
df -h

# Show only specific filesystem
df -h /

# Show all filesystems including tmpfs
df -ha

Example Output:

Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       100G   45G   50G  47% /

Warning Levels:

< 70% - Normal
70-85% - Monitor closely
> 85% - Take action to free space
> 95% - Critical! System may fail

Check Directory Sizes (du)

bash

# Check specific directory
du -sh /var/www/html

# Check with subdirectories
du -h --max-depth=1 /var

# Find largest directories
du -h --max-depth=1 / 2>/dev/null | sort -rh | head -20

Find Large Files

bash

# Files larger than 100MB
find / -type f -size +100M 2>/dev/null | head -20

# Files larger than 1GB
find / -type f -size +1G 2>/dev/null

# With sizes
find / -type f -size +100M -exec ls -lh {} \; 2>/dev/null | head -20

Find Large Directories

bash

# Top 10 largest directories
du -h --max-depth=2 / 2>/dev/null | sort -rh | head -20

Clean Package Cache

🟠 CAUTION

This removes downloaded package files. Safe but irreversible.

bash

# Clean apt cache
sudo apt clean

# Remove old packages
sudo apt autoremove -y

# Check freed space
df -h

Clean Journal Logs

🟠 CAUTION

This removes old system logs.

bash

# Check journal size
journalctl --disk-usage

# Keep only last 7 days
sudo journalctl --vacuum-time=7d

# Keep only 500MB
sudo journalctl --vacuum-size=500M

Memory Monitoring

Check Memory Usage (free)

bash

# Human-readable
free -h

# With totals
free -ht

# In megabytes
free -m

Example Output:

              total        used        free      shared  buff/cache   available
Mem:           16Gi       8.5Gi       2.1Gi       512Mi       5.4Gi       6.8Gi
Swap:          4.0Gi       1.2Gi       2.8Gi

Key Values:

available - Memory available for new applications
buff/cache - Memory used for caching (can be freed)
swap used - If high, you may need more RAM

Memory Usage by Process

bash

# Top memory consumers
ps aux --sort=-%mem | head -15

# With specific columns
ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%mem | head -15

Check Swap Usage

bash

# Swap summary
swapon --show

# Detailed swap info
cat /proc/swaps

Clear Cache (Emergency)

🟠 USE WITH CAUTION

Only use if system is critically low on memory.

bash

# Clear page cache
sudo sync; echo 1 > /proc/sys/vm/drop_caches

# Clear dentries and inodes
sudo sync; echo 2 > /proc/sys/vm/drop_caches

# Clear all
sudo sync; echo 3 > /proc/sys/vm/drop_caches

CPU & Process Monitoring

Real-time Monitoring (top)

bash

top

Key Commands in top:

q - Quit
M - Sort by memory
P - Sort by CPU
k - Kill a process
1 - Show individual CPUs

Better Monitoring (htop)

bash

htop

Install if not available:

bash

sudo apt install htop

htop Features:

Color-coded display
Mouse support
Tree view (F5)
Search (F3)
Filter (F4)
Kill process (F9)

Check CPU Info

bash

# CPU details
lscpu

# Number of cores
nproc

# CPU model
cat /proc/cpuinfo | grep "model name" | head -1

Check Load Average

bash

# Quick view
uptime

# Detailed
cat /proc/loadavg

Understanding Load Average:

Three numbers: 1min, 5min, 15min averages
Load = 1.0 per CPU core is 100% utilization
Example: 4-core server, load 4.0 = 100% utilized

Find Process by Name

bash

ps aux | grep nginx
ps aux | grep php
ps aux | grep mysql

Kill Process

🔴 CAUTION

Killing wrong process can crash services.

bash

# Graceful kill
kill <PID>

# Force kill
kill -9 <PID>

# Kill by name
pkill nginx
killall php-fpm8.3

System Logs Cleanup

Check System Log Sizes

bash

du -sh /var/log/*
du -sh /var/log/

Clear System Logs

🟠 CAUTION

This removes log history. Back up if needed.

bash

# Clear syslog
sudo truncate -s 0 /var/log/syslog

# Clear auth log
sudo truncate -s 0 /var/log/auth.log

# Clear kern log
sudo truncate -s 0 /var/log/kern.log

# Clear all old logs
sudo find /var/log -type f -name "*.log" -mtime +30 -delete
sudo find /var/log -type f -name "*.gz" -delete
sudo find /var/log -type f -name "*.1" -delete
sudo find /var/log -type f -name "*.old" -delete

Clear Specific Service Logs

bash

# Nginx logs
sudo truncate -s 0 /var/log/nginx/access.log
sudo truncate -s 0 /var/log/nginx/error.log

# MySQL logs
sudo truncate -s 0 /var/log/mysql/error.log

# PHP logs
sudo truncate -s 0 /var/log/php8.3-fpm.log

Rotate Logs Now

bash

sudo logrotate -f /etc/logrotate.conf

Network Monitoring

Check Network Connections

bash

# All connections
netstat -an

# Listening ports
netstat -tlnp

# Established connections
netstat -an | grep ESTABLISHED | wc -l

Check Specific Port

bash

# Who's using port 80
sudo lsof -i :80
sudo netstat -tlnp | grep :80

Check Bandwidth (if iftop installed)

bash

sudo iftop

System Information

System Overview

bash

# OS version
cat /etc/os-release

# Kernel version
uname -r

# All system info
uname -a

Uptime

bash

uptime

Last Reboot

bash

last reboot | head -5

Check Running Services

bash

systemctl list-units --type=service --state=running

Quick Disk Cleanup Script

bash

#!/bin/bash
# Save as /root/cleanup.sh

echo "=== Disk Usage Before ==="
df -h /

echo ""
echo "=== Cleaning apt cache ==="
sudo apt clean
sudo apt autoremove -y

echo ""
echo "=== Cleaning journal logs ==="
sudo journalctl --vacuum-time=7d

echo ""
echo "=== Cleaning old Laravel logs ==="
find /var/www/html/storage/logs -name "*.log" -mtime +7 -delete

echo ""
echo "=== Cleaning old system logs ==="
sudo find /var/log -type f -name "*.gz" -delete
sudo find /var/log -type f -name "*.1" -delete

echo ""
echo "=== Disk Usage After ==="
df -h /

Run:

bash

sudo chmod +x /root/cleanup.sh
sudo /root/cleanup.sh

Quick Reference Card

Action	Command
Disk usage	`df -h`
Directory size	`du -sh /path`
Memory usage	`free -h`
CPU/Process monitor	`htop` or `top`
Load average	`uptime`
Find large files	`find / -type f -size +100M`
Clear apt cache	`sudo apt clean && sudo apt autoremove`
Clear journal	`sudo journalctl --vacuum-time=7d`
Check ports	`netstat -tlnp`
Kill process	`kill <PID>`
System info	`uname -a`
OS version	`cat /etc/os-release`

Linux System Commands ​

Table of Contents ​

Disk Space Management ​

Check Disk Usage (df) ​

Check Directory Sizes (du) ​

Find Large Files ​

Find Large Directories ​

Clean Package Cache ​

Clean Journal Logs ​

Memory Monitoring ​

Check Memory Usage (free) ​

Memory Usage by Process ​

Check Swap Usage ​

Clear Cache (Emergency) ​

CPU & Process Monitoring ​

Real-time Monitoring (top) ​

Better Monitoring (htop) ​

Check CPU Info ​

Check Load Average ​

Find Process by Name ​

Kill Process ​

System Logs Cleanup ​

Check System Log Sizes ​

Clear System Logs ​

Clear Specific Service Logs ​

Rotate Logs Now ​

Network Monitoring ​

Check Network Connections ​

Check Specific Port ​

Check Bandwidth (if iftop installed) ​

System Information ​

System Overview ​

Uptime ​

Last Reboot ​

Check Running Services ​

Quick Disk Cleanup Script ​

Quick Reference Card ​

Linux System Commands

Table of Contents

Disk Space Management

Check Disk Usage (df)

Check Directory Sizes (du)

Find Large Files

Find Large Directories

Clean Package Cache

Clean Journal Logs

Memory Monitoring

Check Memory Usage (free)

Memory Usage by Process

Check Swap Usage

Clear Cache (Emergency)

CPU & Process Monitoring

Real-time Monitoring (top)

Better Monitoring (htop)

Check CPU Info

Check Load Average

Find Process by Name

Kill Process

System Logs Cleanup

Check System Log Sizes

Clear System Logs

Clear Specific Service Logs

Rotate Logs Now

Network Monitoring

Check Network Connections

Check Specific Port

Check Bandwidth (if iftop installed)

System Information

System Overview

Uptime

Last Reboot

Check Running Services

Quick Disk Cleanup Script

Quick Reference Card