Introduction: Why Monitoring is Crucial?

Linux system performance is like the pulse of an organism – regular checks help detect problems before they become critical. Whether you manage a production server or simply want to optimize your desktop, knowing basic real-time monitoring tools is absolutely essential.

In this article, we will present the Top 5 command-line tools that will help you quickly diagnose what’s happening with your system.

1. top and htop - The Kings of Process Monitoring

top

top is a classic tool that provides a dynamic real-time view of running processes. By default, it sorts processes by CPU usage, refreshing the view every few seconds.

top

Key things to know about top:

  • First line: Information about uptime, number of logged-in users, load average (system load - number of processes waiting for CPU).
  • CPU line: Percentage of CPU usage by users (us), system (sy), nice (processes with altered priority), idle (CPU inactivity), wa (waiting for I/O).
  • Memory lines: Information about physical RAM (Mem) and SWAP memory (Swap).
  • Process table: List of processes with PID, user, CPU usage, memory, and uptime.

htop

htop is an enhanced, interactive version of top, offering a much more user-friendly interface and additional features.

htop

Advantages of htop:

  • Colorful interface: Easier to read.
  • Progress bars: Graphical representation of CPU and memory usage.
  • Interactivity: Ability to scroll, sort (F6), filter (F4), and kill processes (F9) directly within the interface.
  • Tree view: (F5) Shows processes in a hierarchical structure.

2. vmstat - Quick Diagnosis of Memory and I/O

vmstat (virtual memory statistics) is a tool for a quick overview of virtual memory activity, processes, block I/O operations, traps, and CPU activity. It is ideal for finding “bottlenecks.”

vmstat 1 5

The command above will display 5 reports, every 1 second.

Key vmstat columns:

  • Procs: r (runnable), b (blocked, waiting for I/O).
  • Memory: swpd (SWAP used), free (free memory), buff (buffer), cache (cache).
  • Swap: si (swap-in, memory from disk to RAM), so (swap-out, memory from RAM to disk). High si/so values indicate memory problems!
  • IO: bi (blocks received), bo (blocks sent) – disk activity.
  • CPU: us (user), sy (system), id (idle), wa (waiting for I/O). High wa indicates an I/O bottleneck!

3. iostat - The Disk Performance Detective

When you suspect a slow disk is the problem, iostat (input/output statistics) is your tool.

iostat -x 1 5

The command above will display 5 reports, every 1 second, with extended statistics.

Key iostat -x columns:

  • %util: Percentage of time the I/O device was busy. Values close to 100% mean disk overload!
  • await: The average time (in ms) an I/O operation waited in the queue + its execution time. High await is a problem!
  • %iowait (from vmstat): The time the CPU spent waiting for I/O operations to complete. High values indicate a disk bottleneck.

4. ss (Socket Statistics) - Analyzing Network Connections

ss is a newer, more modern successor to netstat, used to display information about open sockets. It is much faster and offers more detail.

ss -tuln

The command above will display open TCP (t) and UDP (u) ports, in listening mode (l), with port numbers instead of service names (n).

Examples of ss usage:

  • ss -s: Summary of connection statistics.
  • ss -tnp: Active TCP connections, show process PIDs.
  • ss -lp 'sport = :ssh': Show processes listening on the SSH port.

5. nmon - All-in-one, Interactive Powerhouse

nmon (Nigel’s Monitor) is an interactive tool that combines the functionality of many others. It displays CPU usage, memory, disks, network, and much more, in a single, refreshing view.

nmon

Advantages of nmon:

  • All-in-one: You don’t have to switch between different tools.
  • Interactivity: Keys like c (CPU), m (memory), d (disks), n (network) switch views.
  • Data export: Ability to export data to a file in a format that can later be imported into a spreadsheet for analysis.

Summary

Knowing these five tools is a solid foundation for monitoring and diagnosing performance issues in Linux. Remember that every system is different, and the key is practice and understanding what each metric means. Choose the right tool for the job and start observing your system!