Understanding the Linux Top Command

<span>top</span> is a very powerful real-time system monitoring tool in Linux that allows you to dynamically view the system’s operating status, including the usage of resources such as CPU, memory, and processes.

1. Starting `top`

To start top, simply enter the following command in the terminal:

top

2. Overview of the `top` Interface

The top interface is divided into two parts:

System Overview Area (Top):

Displays the overall status of the system, including uptime, number of users, load, tasks, CPU, memory, etc.

Process List Area (Bottom):

Displays detailed information about each process, sorted by CPU usage by default.

3. Analysis of the System Overview Area

3.1 First Line: System Uptime and Load

Example:

top - 14:32:01 up 10 days,  2:15,  3 users,  load average: 0.15, 0.10, 0.05

Analysis:

Load value indicates the number of tasks waiting for CPU. If the load value is close to or exceeds the number of CPU cores, it indicates high system load.
14:32:01: Current time.
up 10 days, 2:15: System uptime.
3 users: Current number of logged-in users.
load average: 0.15, 0.10, 0.05: System load (average values over 1 minute, 5 minutes, and 15 minutes).

3.2 Second Line: Task Status

Example:

Tasks: 123 total,   1 running, 122 sleeping,   0 stopped,   0 zombie

Analysis:

123 total: Total number of tasks.
1 running: Number of tasks currently running.
122 sleeping: Number of sleeping tasks.
0 stopped: Number of stopped tasks.
0 zombie: Number of zombie processes.

3.3 Third Line: CPU Usage
Example:

%Cpu(s):  1.5 us,  0.5 sy,  0.0 ni, 97.5 id,  0.5 wa,  0.0 hi,  0.0 si,  0.0 st

Analysis:

us: Percentage of CPU used by user space (running user processes).
sy: Percentage of CPU used by kernel space (running system processes).
ni: Percentage of CPU used by user processes with adjusted priority.
id: Percentage of idle CPU.
wa: Percentage of CPU waiting for I/O operations.
hi: Percentage of CPU used by hardware interrupts.
si: Percentage of CPU used by software interrupts.
st: Percentage of CPU stolen by the virtual machine.

3.4 Fourth Line: Memory Usage

Example:

MiB Mem :  16000.0 total,   2000.0 free,   5000.0 used,   9000.0 buff/cache

Analysis:

total: Total memory.
free: Free memory.
used: Used memory.
buff/cache: Memory used for caching and buffering.

3.5 Fifth Line: Swap Space Usage

Example:

MiB Swap:   2000.0 total,   1500.0 free,    500.0 used.  11000.0 avail Mem

Analysis:

total: Total swap space.
free: Free swap space.
used: Used swap space.
avail Mem: Available memory.

4. Analysis of the Process List Area

4.1 Default Columns

PID: Process ID.
USER: Process owner.
PR: Process priority.
NI: Nice value of the process (priority adjustment value).
VIRT: Total virtual memory used by the process (including swap space).
RES: Size of physical memory used by the process.
SHR: Size of shared memory used by the process.
S: Process status (R=running, S=sleeping, Z=zombie).
%CPU: Percentage of CPU used by the process.
%MEM: Percentage of memory used by the process.
TIME+: Total CPU time used by the process.
COMMAND: Command name of the process.

4.2 Sorting

By default, sorted by %CPU.
Sort by M for memory usage.
Sort by P for CPU usage.
Sort by T for uptime.

5. Common Shortcuts

k: Terminate a process. Enter PID and press enter.
r: Adjust process priority (nice value).
h: Display help information.
q: Exit top.
f: Enter field management interface to select displayed columns.
1: Display usage for each CPU core.
z: Toggle color display.
W: Save current configuration to ~/.toprc file.
u: Display processes for a specific user. Enter username and press enter.

Introduction to top shortcuts

1. Global Shortcuts

h: Display help information.
q: Exit top.
z: Toggle color display.
W: Save current configuration to ~/.toprc file (this configuration will load next time top starts).
E: Toggle memory unit in the overview area (KiB/MiB/GiB/TiB).
e: Toggle memory unit in the process list area (KiB/MiB/GiB/TiB).
l: Show or hide system uptime and load information.
t: Show or hide task and CPU state information.
m: Show or hide memory and swap space information.
1: Show usage for each CPU core (toggle between single-core/multi-core view).

2. Process Sorting
P: Sort by CPU usage (default).
M: Sort by memory usage.
T: Sort by uptime.
N: Sort by PID.
<: Sort by the left column.
>: Sort by the right column.

3. Process Operations

k: Terminate a process. Enter PID and press enter.
r: Adjust the priority of a process (nice value). Enter PID and press enter, then enter nice value (-20 to 19).
c: Display the full command path.
i: Ignore idle processes (only show active processes).
u: Display processes for a specific user. Enter username and press enter.

4. Display Column Management

f: Enter field management interface to select displayed columns.

Press d or spacebar to toggle the display status of columns.
Press s to set the sorting field.
Press q to exit the field management interface.

F: Select sorting field (similar to f, but directly enters sorting field selection).

5. Refresh and Delay

s: Set refresh interval (seconds). Enter time and press enter.
d: Set refresh interval (seconds). Enter time and press enter.
Space: Immediately refresh the interface.

6. Other Shortcuts

x: Highlight the sorting field.
y: Highlight running processes.
b: Toggle bold display.
V: Switch to tree view (show parent-child relationship of processes).
H: Show or hide threads (toggle process/thread view).
Z: Change color configuration.

1. Starting top

2. Overview of the top Interface