top - Linux

Overview

The top command in Linux provides a dynamic, real-time view of the processes running on a system. It displays a comprehensive overview of system resources, including CPU usage, memory utilization, and detailed information about individual processes. Primarily used for system monitoring, performance analysis, and managing tasks, it is a vital tool for system administrators and power users who need to oversee system health and optimize performance.

Syntax

The basic syntax of the top command is:

top [options]

Common Usage:

top

This will start top with its default settings and display.

Options/Flags

-d : Specifies the update delay; the time interval between screen updates. By default, it is set to 3 seconds.
-c: Displays the full command path alongside each process.
-i : Toggles whether idle tasks are shown; useful for focusing on active processes.
-n : Limits top to a set number of iterations before exiting, allowing for snapshots of the system.
-u : Filters the processes to those owned by the specified user.
-p : Targets the monitoring to specific processes based on their process ID.
-H: Displays threads as if they were separate processes.

Examples:

Default launch:
```
top
```
Refresh rate set to 1 second:
```
top -d 1
```
Display only processes belonging to a specific user (alice):
```
top -u alice
```
Monitoring specific processes by PID (1234 and 5678):
```
top -p 1234,5678
```
Run top for 10 iterations then exit:
```
top -n 10
```

Common Issues

High CPU usage by top itself: This can happen if the refresh interval (-d) is set very low. Adjust the interval higher to mitigate this.
Misinterpretation of CPU percentages: Remember that top shows CPU usage as a percentage of a single CPU. On multi-core systems, percentages can exceed 100% for processes using multiple cores.
Processes not visible: If you’re using user or PID filters, ensure they are correctly specified to see expected processes.

Integration

top can be combined with other commands for automated system monitoring or batch tasks:

Log top output every 10 seconds:

top -b -d 10 -n 6 >> top_output.txt

Kill a high-memory process using awk and xargs:

top -b -n 1 | awk '/^  PID USER/{y=1;next}y && $10 > 90 {system("kill -9 " $1)}'

Here, if the memory usage ($10) exceeds 90%, the process is killed.

htop – An interactive process viewer, considered an enhanced version of top.
vmstat – Reports virtual memory statistics.
ps – Reports a snapshot of current processes.

For further reading and more detailed information, consult the man page for top:

man top