Master Linux top: A Complete Real‑Time Process Monitoring Guide

1. Top Command Output

The top utility displays a live snapshot of system activity, showing overall load, task summary, CPU states, memory usage, and a table of processes. The first few lines summarize system uptime, average load, number of logged‑in users, and the load averages for the past 1, 5, and 15 minutes. Press l to toggle this view.

1.1 System Uptime and Load Average

These fields correspond to the output of uptime:

Current time

System uptime

Number of logged‑in users

Load averages for the last 1, 5, and 15 minutes

Use the l command to switch the display.

1.2 Tasks Summary

The second line shows a summary of all processes, including counts of running, sleeping, stopped, and zombie tasks. Press t to toggle this view.

1.3 CPU States

The next line reports CPU usage percentages for various modes:

us – user processes (non‑nice)

sy – kernel processes

ni – user processes with a nice value

wa – I/O wait

hi – hardware interrupts

si – software interrupts

st – stolen time (virtualized environments)

Press t to toggle the CPU view.

1.4 Memory Usage

Two lines mimic the free command, showing physical memory and swap usage (total, used, free, buffers). Press m to toggle the memory view.

1.5 Process Columns

Below the system summary, each process is listed in columns. The default columns are:

PID – process identifier

USER – owning user name

PR – scheduling priority (e.g., rt for real‑time)

NI – nice value (lower = higher priority)

VIRT – virtual memory size

RES – resident (non‑swap) memory size

SHR – shared memory size

S – process state (D, R, S, T, Z)

%CPU – CPU usage since last update

%MEM – percentage of physical memory used

TIME+ – total CPU time consumed

COMMAND – command line (full path can be toggled)

2. Interactive Commands

While top runs, you can press keys to modify the display:

h / ? – show the help screen.

Enter / Space – refresh immediately.

A – switch between the four predefined field groups (Def, Job, Mem, Usr). Use a and w to move between windows, g to jump to a specific window.

B – toggle bold highlighting for important rows.

d / s – set a custom refresh interval (in seconds).

l / t / m – toggle load, task/CPU, and memory sections.

f – open the field‑management screen; * marks selected fields. Use arrow keys to navigate, < / > to move fields left/right, and Enter to confirm.

R – reverse sort order.

c – toggle display of the full command line path.

i – show or hide idle tasks.

V – switch to tree view.

Z – open the color‑scheme selector (up to eight colors for the four windows).

z – enable/disable colored output.

x / y – highlight the sorted column (x) or the running process row (y); requires color mode.

u – filter processes by a specific user (blank shows all).

n / # – set the maximum number of tasks displayed.

k – send a signal to terminate a task (the most important interactive command).

r – change a task’s scheduling priority.

3. Command‑Line Options

All interactive commands have corresponding command‑line flags, allowing you to start top with a predefined configuration: -b – batch mode (useful for redirecting output to a file). -c – remember the last state of the command‑line display toggle. -d <seconds> – set the refresh delay (e.g., top -d 1 for a 1‑second interval). -i – invert the previous idle‑process setting. -n <iterations> – exit after the specified number of updates (e.g., top -n 3). -p <PID> – monitor a specific PID (PID 0 monitors the top process itself). -u <user> or -U <UID> – show only processes belonging to the given user; these options are mutually exclusive with -p.

Attempting to combine mutually exclusive options results in an error such as “conflicting process selections”.

By mastering both the interactive keys and the command‑line flags, you can tailor top to a wide range of monitoring scenarios, from quick ad‑hoc checks to scripted batch reports.

$ top -d 1

$ top -n 3