Linux Top is a command-line utility that allows users to monitor the system’s processes in real-time. It provides a dynamic view of the system’s performance, displaying information about the running processes, memory usage, CPU utilization, and more. Top is a powerful tool for system administrators and developers to analyze and troubleshoot system performance issues.

Top is written in C programming language and is available as a standard package in most Linux distributions. It is a part of the procps-ng package, which also includes other useful utilities like ps, free, and kill.

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

Usage

Once installed, top can be launched by simply typing “top” in the terminal. By default, it displays a list of processes sorted by their CPU usage in descending order.

Sorting Processes

Top provides various options to sort the processes based on different criteria. The following commands can be used to change the sorting order:

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

For example, to sort the processes by memory usage, press “M” while top is running.

Changing Refresh Interval

By default, top updates the displayed information every 3 seconds. This interval can be changed by pressing the “d” key and entering a new value in seconds.

Filtering Processes

Top allows users to filter the displayed processes based on specific criteria. The following commands can be used to filter the processes:

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

For example, to filter the processes by a specific user, press “u” and enter the username.

Killing Processes

Top provides a convenient way to kill processes directly from the interface. The following commands can be used to send signals to processes:

• k: Kill a process by entering its process ID
• 15: Send the TERM signal to a process
• 9: Send the KILL signal to a process

For example, to kill a process with a specific process ID, press “k” and enter the process ID.

Similar Packages

While top is a powerful tool for monitoring system processes, there are other similar packages available that serve the same purpose. Some of the popular alternatives to top are:

• htop: Htop is an interactive process viewer for Linux that provides a more user-friendly interface and additional features like tree view, color-coded display, and process search.
• glances: Glances is a cross-platform system monitoring tool that provides a comprehensive overview of the system’s performance. It displays information about CPU, memory, network, disk usage, and more.
• atop: Atop is a full-screen performance monitor that provides detailed information about system resources, process activity, disk activity, and network activity. It also allows users to log the performance data for later analysis.

Script Examples

Here are three examples of scripts that utilize the top command 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 continuously monitors the CPU usage and appends the output of the top command to a log file named “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 identifies processes with CPU usage greater than 50% and kills them using the kill command. It runs in a loop every 5 seconds.

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 monitors the memory usage and appends the output of the top command to a log file named “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 powerful command-line utility that allows users to monitor system processes in real-time. It provides valuable insights into the system’s performance and helps in identifying and troubleshooting performance issues. Top is widely used by system administrators, developers, and anyone who needs to monitor and optimize system performance. With its rich set of features and flexibility, top is an essential tool in the Linux ecosystem.

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