Linux `top` is a powerful command-line utility that provides a real-time view of your system’s processes. It displays crucial information such as running processes, memory usage, and CPU utilization, making it an essential tool for system administrators and developers for performance analysis and troubleshooting.

`top` is written in C and is a standard package in most Linux distributions, included within the `procps-ng` package along with utilities like `ps`, `free`, and `kill`.

Official page of top: https://github.com/ThomasDickey/procps

Usage

To launch `top`, simply type `top` in your terminal. By default, it lists processes sorted by CPU usage in descending order.

Sorting Processes

`top` allows sorting processes based on different criteria. Use these commands while `top` is running:

• P: Sort by CPU usage (default)
• M: Sort by memory usage
• T: Sort by process time
• N: Sort by process ID

For example, press `M` to sort by memory usage.

Changing Refresh Interval

By default, `top` refreshes every 3 seconds. Change this by pressing `d` and entering a new interval in seconds.

Filtering Processes

Filter processes based on specific criteria:

• u: Filter by user
• s: Filter by state (running, sleeping, stopped, etc.)
• o: Filter by command name

For example, press `u` and enter a username to filter processes by that user.

Killing Processes

`top` provides a way to kill processes directly:

• k: Kill a process by entering its process ID
• 15: Send the TERM signal to a process (graceful shutdown)
• 9: Send the KILL signal to a process (forceful shutdown)

Press `k` and enter the process ID to kill a specific process.

Similar Packages

Alternatives to `top` include:

• htop: An interactive process viewer with a user-friendly interface, tree view, color-coded display, and process search.
• glances: A cross-platform system monitoring tool with a comprehensive overview of CPU, memory, network, and disk usage.
• atop: A full-screen performance monitor with detailed information about system resources, process activity, disk activity, and network activity, with data logging capabilities.

Script Examples

Here are example scripts using `top` for automation:

Script 1: Monitor CPU Usage

#!/bin/bash

while true; do
  top -b -n 1 | grep "Cpu(s)" >> cpu_usage.log
  sleep 1
done

This script logs CPU usage to `cpu_usage.log` every second.

Script 2: Kill High CPU Processes

#!/bin/bash

while true; do
  top -b -n 1 | awk '$9 > 50 {print $1}' | xargs kill -9
  sleep 5
done

This script kills processes with CPU usage exceeding 50% every 5 seconds (use with caution!).

Script 3: Monitor Memory Usage

#!/bin/bash

while true; do
  top -b -n 1 | grep "KiB Mem" >> memory_usage.log
  sleep 10
done

This script logs memory usage to `memory_usage.log` every 10 seconds.

List of Top Functions and Constants

Function/Constant	Description
top	Launches the top command
P	Sorts processes by CPU usage
M	Sorts processes by memory usage
T	Sorts processes by process time
N	Sorts processes by process ID
d	Changes the refresh interval
u	Filters processes by user
s	Filters processes by state
o	Filters processes by command name
k	Kills a process by entering its process ID
15	Sends the TERM signal to a process
9	Sends the KILL signal to a process

Conclusion

Linux `top` is a fundamental command-line utility for monitoring system processes in real-time. Its insights into system performance are invaluable for identifying and resolving performance bottlenecks. It is an essential tool for system administrators, developers, and anyone needing to monitor and optimize Linux system performance.

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