Top Command Output Explained: An Introduction for Beginners

The `top` command is a Linux utility that is used to display real-time information about running processes on a system. It provides a summary of the current state of the system, including the CPU, memory, and swap usage, as well as a list of running processes and their resource utilization.

Using the top command is simple: just enter `top` at the command prompt and press Enter. The top command will display a list of running processes, along with their process IDs, CPU and memory usage, and other information. By default, the list is sorted by CPU usage, with the most CPU-intensive processes at the top.

Advertisement

• Sysstat – Linux System Performance Monitoring Tool
• Glances – A Real-Time Linux System Monitoring Tool

Using this article, I am trying to explain understanding and interpreting the Output of the Linux `top` Command

Understanding and Interpreting the Ouptut of the Linux `top` Command

The top command is a Linux utility that is used to display real-time information about running processes on a system. When you run the top command, it displays a list of running processes, along with their process IDs, CPU and memory usage, and other information. Here’s how to read and interpret the output of the top command:

Result Row #1

The first row (Row 1) typically displaying the overall system summary. This summary includes the current time, uptime of the system (how long the system has been running since the last boot), the total number of user sessions, and the load average over the last 1, 5, and 15 minutes. The load average represents the average system load during these intervals, giving an indication of how busy or idle the system resources are. This row serves as a quick snapshot of the system’s health and performance.

Result Row #2

The second row of the top command output in Linux displays information about processes or tasks being managed by the system. It shows the total number of tasks and breaks them down into different states: running, sleeping, stopped, and zombie. Running tasks are currently executing, sleeping tasks are waiting for an event or time, stopped tasks have been halted (usually by a signal), and zombie tasks have completed execution but still have an entry in the process table. This row provides a quick overview of the system’s process activity, helping users understand the current workload and system behavior.

Zombie process or defunct process is a process that has completed execution but still has an entry in the process table. This entry is still needed to allow the parent process to read its child’s exit status.

Result Row #3

The third row of the Linux top command output details CPU usage, breaking it down into various categories: user processes, system processes, nice tasks, idle time, wait time, hardware interrupts (HI), software interrupts (SI), and steal time (ST). This information helps in identifying how the CPU resources are being allocated—whether they’re consumed by user applications, system processes, or are idly waiting, providing insights into the system’s performance and potential bottlenecks.

Steal time is the time that a virtual CPU waits for a real CPU while the hypervisor is servicing another virtual processor.
Definition from access.redhat.com

Result Row #4

The fourth row of the Linux top command output presents memory usage statistics, including total physical memory (RAM), used memory, free memory, and buffers/cache memory. This row offers a snapshot of memory allocation and availability, helping users understand how much memory is being actively used by processes versus how much is available for new processes. It also shows memory used for buffering and caching, which can improve performance by reducing disk access times.

Result Row #5

The fifth row in the Linux top command output focuses on swap space usage, showing total swap, used swap, and free swap space. Swap space is a form of virtual memory that allows the system to use disk space as an extension of RAM. This row is crucial for understanding how much of the disk space is being used to supplement physical memory and whether the system is heavily relying on swapping, which can impact performance.

Result Row #6 ( Running Processes )

In sixth row in top command it shows all the running processes on servers with additional details. These process listings include details such as PID, user, priority, CPU and memory usage, and command name, providing real-time insights into which processes are consuming the most resources.

Short Results Based on Specific Criteria

To customize how results are sorted, use the key combination SHIFT + F. This allows you to prioritize the display of processes based on specific criteria. Simply select your preferred field from the options presented and press ENTER to apply the new sorting order. This feature enhances the way you analyze and monitor process metrics.

• A : PID = Process Id
• b : PPID = Parent Process Pid
• d : UID= User Id
• e : USER = User Name
• f : GROUP = Group Name
• g : TTY= Controlling Tty
• h : PR = Priority
• i : NI = Nice value
• j : P = Last used cpu (SMP)
• k : %CPU = CPU usage
• l : TIME = CPU Time
• n : %MEM = Memory usage (RES)
• o : VIRT = Virtual Image (kb)
• p : SWAP = Swapped size (kb)
• q : RES= Resident size (kb)
• r : CODE = Code size (kb)
• s : DATA = Data+Stack size (kb
• t : SHR= Shared Mem size (kb)
• w : S = Process Status
• x : COMMAND= Command name/line

Wrap Up

In conclusion, a top command is a powerful tool that is used to display real-time information about running processes on a Linux system. When you run the top command, it displays a list of running processes, along with their process IDs, CPU and memory usage, and other information. By understanding and interpreting the output of the top command, you can get a better understanding of the current state of the system and the resources being used by various processes. With this knowledge, you can use the top command to monitor and manage processes on your Linux system more effectively.