top command

 

Command: top (Display Linux Processes in Real Time)

---

🔍 Purpose

The top command displays dynamic, real-time information about system processes, including:

• Process IDs (PIDs)

• CPU and memory usage

• User ownership

• System uptime

• Load average
  and much more.

It’s essentially a live dashboard for your system.

---

🧩 Basic Syntax

top [options]

or simply:

top

---

📘 Example:

top

Typical Output:

top - 10:41:07 up 2 days,  2:21,  2 users,  load average: 0.23, 0.14, 0.09
Tasks: 198 total,   1 running, 197 sleeping,   0 stopped,   0 zombie
%Cpu(s):  3.2 us,  1.0 sy,  0.0 ni, 95.6 id,  0.0 wa,  0.0 hi,  0.2 si,  0.0 st
MiB Mem :   7989.7 total,   2567.4 free,   1834.1 used,   3588.2 buff/cache
MiB Swap:   2048.0 total,   2048.0 free,      0.0 used.   5332.1 avail Mem 

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND     
 1254 root      20   0  700484  8236   5852 S   1.7   0.1   0:04.76 systemd      
 2305 user1     20   0  159720   6508   5152 S   0.3   0.1   0:00.92 bash         
 2410 user1     20   0  242348   8652   7068 R   0.3   0.1   0:00.45 top          

---

🧩 Explanation of Each Section

🟢 1. System Summary (Header)

top - 10:41:07 up 2 days,  2:21,  2 users,  load average: 0.23, 0.14, 0.09

Field	Meaning
10:41:07	Current system time
up 2 days, 2:21	System uptime
2 users	Number of logged-in users
load average	Average number of processes waiting for CPU over 1, 5, and 15 minutes

🧠 Load average of 1.00 on a single-core CPU means fully utilized CPU.
Values above 1.00 indicate CPU congestion.

---

🟢 2. Task Summary

Tasks: 198 total, 1 running, 197 sleeping, 0 stopped, 0 zombie

Field	Meaning
total	Total number of processes
running	Currently executing processes
sleeping	Idle processes waiting for an event
stopped	Suspended (SIGSTOP) processes
zombie	Defunct processes waiting to be cleaned up

🧠 A zombie process means the process has finished execution but its parent hasn’t collected its exit status.

---

🟢 3. CPU Usage

%Cpu(s):  3.2 us,  1.0 sy,  0.0 ni, 95.6 id,  0.0 wa,  0.0 hi,  0.2 si,  0.0 st

Field	Meaning
us	User mode time
sy	System (kernel) mode time
ni	Time with adjusted nice priority
id	Idle time
wa	I/O wait (time waiting for disk I/O)
hi	Hardware interrupt time
si	Software interrupt time
st	Stolen time (time used by hypervisor for another VM)

🧠 Ideal systems have high id (idle) and low wa (I/O wait).

---

🟢 4. Memory Usage

MiB Mem : 7989.7 total, 2567.4 free, 1834.1 used, 3588.2 buff/cache
MiB Swap: 2048.0 total, 2048.0 free, 0.0 used. 5332.1 avail Mem

Field	Meaning
total	Total physical memory (RAM)
free	Currently unused memory
used	Memory actively used by processes
buff/cache	Memory used for buffers and cache (can be freed if needed)
swap	Disk space used as virtual memory when RAM is full

🧠 A common misconception is that high memory usage is bad — Linux uses free memory efficiently for caching.

---

🟢 5. Process List (Main Table)

Each line represents one process.

Column	Description
PID	Process ID
USER	Process owner
PR	Priority
NI	Nice value (used to adjust process priority)
VIRT	Virtual memory used (includes code + data + shared libraries)
RES	Resident memory (actual RAM used)
SHR	Shared memory with other processes
S	Process state (R=Running, S=Sleeping, T=Stopped, Z=Zombie)
%CPU	CPU usage percentage
%MEM	Memory usage percentage
TIME+	Total CPU time used by the process
COMMAND	Name of the command or process

---

⚙️ Interactive Keyboard Shortcuts in top

When top is running, you can interact with it using single-letter commands:

Key	Action
q	Quit
h	Help screen
k	Kill a process (enter PID)
r	Renice a process (change its priority)
P	Sort by CPU usage (default)
M	Sort by memory usage
T	Sort by time
1	Show CPU usage for each core separately
u	Filter processes by username
Shift + W	Save current configuration
Shift + H	Show threads as separate processes
Shift + F	Customize columns displayed

---

📘 Example 2: Sort by Memory Usage

While in top, press:

M

Processes will be sorted by memory consumption, highest first.

---

📘 Example 3: Filter by User

To see only your processes:

u

Then type your username (e.g., student).

---

📘 Example 4: Kill a Process

In top:

k

Enter the PID and press Enter.
You can specify a signal number (e.g., 9 for SIGKILL).

---

📘 Example 5: Show All CPU Cores

Press:

1

This displays CPU usage for each core — useful on multi-core systems.

---

🧾 Common Options (Command-Line)

Option	Description
-u <user>	Show only processes for a specific user
-n <num>	Display for <num> iterations and exit
-b	Batch mode (non-interactive, for logging)
-d <delay>	Delay (in seconds) between refreshes
-p <PID>	Monitor only specific PID
-i	Ignore idle and zombie processes

---

Example:

top -u student -n 5 -d 2

→ Shows processes owned by student, updates every 2 seconds, and runs 5 cycles before exiting.

---

🧪 Simple Lab Exercise

Aim:

To study and analyze system process information using the top command.

---

Procedure:

1. Run the top command.

   top
   

2. Observe:

   • CPU usage

   • Memory usage

   • Load average

3. Press:

   • M → to sort by memory

   • P → to sort by CPU usage

   • 1 → to show all CPU cores

   • u → to display only your processes

4. Kill a process:

   • Press k → enter PID

5. Exit using q.

---

Sample Observation Table

Parameter	Observation
Load Average	0.15, 0.10, 0.05
Running Tasks	1
Sleeping Tasks	190
CPU Usage (User/System)	3.2% / 1.0%
Memory Usage	2.0 GB used, 5.9 GB free

---

🧠 Learning Outcomes

Students will:

• Understand real-time process monitoring

• Identify CPU-bound and memory-bound processes

• Learn how to sort, filter, and kill processes

• Understand system load, memory usage, and process states

• Relate the output to process scheduling and resource allocation concepts in OS theory

---

🧾 Quick Reference

Command	Purpose
top	Start real-time monitoring
top -u user	Show processes of a user
top -n 5 -d 1	Run 5 updates, 1s apart
M	Sort by memory usage
P	Sort by CPU usage
k	Kill a process
1	Show all CPU cores
q	Quit top