MANAGING PROCESSES IN LINUX/UNIX : TOP, PS, KILL, DF, FREE, NICE

Managing Processes in Linux/Unix : top, ps, kill, df, free, nice

What is a process? An instance of a program is called a Process. In simple terms, any command that you give to your Linux machine starts a new process.

It’s possible to have multiple processes for the same program.

Types of Processes:

• Foreground Processes: They run on the screen and need input from the user. For example : Office Programs
• Background Processes: They run in the background and usually do not need user input. For example Antivirus.

Running a foreground Process

To start a foreground process you can either run it from the dash board or you can run it from the terminal.

When using the Terminal, you will have to wait, until the foreground process runs.

Running a Background process

If you start a foreground program/process from the terminal, then you cannot work on the terminal, till the program is up and running.

Certain, data intensive tasks take lots of processing power and may even take hours to complete. You do not want your terminal to be held up for such a long time.

To avoid such a situation, you can run the program and send it to the background so that terminal remains available to you.  Let’s learn how to do this -

Fg

You can use the command “fg” to continue a program which was stopped and bring it to the foreground. 

The simple syntax for this utility is:

fg

Example

1. Launch  ‘banshee’ music player
2. Stop it with the ‘ctrl +z’ command
3. Continue it with the ‘fg’ utility.

Let’s look at other important commands to manage processes -

Top

This utility tells the user about all the running processes on the Linux machine.

Press ‘q’ on the keyboard to move out of the process display.

The terminology follows:

Field	Description	Example 1	Example 2
PID	The process ID of each task	1525	961
User	The username of task owner	Home	Root
PR	Priority Can be 20(highest) or -20(lowest)	20	20
NI	The nice value of a task	0	0
VIRT	Virtual memory used (kb)	1775	75972
RES	Physical memory used (kb)	100	51
SHR	Shared memory used (kb)	28	7952
S	Status There are five types:           'D' = uninterruptible sleep           'R' = running           'S' = sleeping           'T' = traced or stopped           'Z' = zombie	S	R
%CPU	% of CPU time	1.7	1.0
%MEM	Physical memory used	10	5.1
TIME+	Total CPU time	5:05.34	2:23.42
Command	Command name	Photoshop.exe	Xorg

PS

This command stands for ‘Process Status’.  It is similar to the “Task Manager” that pop-ups in a Windows Machine when we use Cntrl+Alt+Del . This command is similar to ‘top’ command but the information displayed is different.

To check all the processes running under a user, use the command -

ps ux

You can also check the process status of a single process , use the syntax -

ps PID

Kill

This command terminates a running processes on a Linux machine.

In order to use  this utility you need to know the PID (process id) of the process you want to kill

Syntax –

kill PID

To find the PID of a process simply type

pidof  Processname

Let us try it with an example

NICE

Linux can run  a lot of processes at a time ,which can slow down the speed of some high priority processes and result in poor performance.

To avoid this, you can tell your machine to prioritize processes as per your requirements

This priority is called Niceness in Linux and it has a value between -20 to 19. The lower the Niceness index the higher would be priority given to that task.

The default value of all the processes is 0.

To start a process with a niceness value other  than the default value use the following syntax

nice –n ‘Nice value’ process name

If there is some process already running on the system then you can ‘Renice’ its value using syntax.

renice ‘nice value’ –p ‘PID’

To change Niceness you can use the ‘top’ command to determine the PID (process id) and its Nice value.  Later use the renice command to change the value.

Let us understand this by an example.

DF

This utility reports the free disk space(Hard Disk) on all the file systems.

If you want the above information in a readable format , then use the command

‘df –h’ 

Free

This command shows the free and used memory (RAM) on the Linux system.

You can use the arguments

free -m  to display output in MB

free -g  to display output in GB

Summary:

• Any running program or a command given to a Linux system is called a process
• A process could run in foreground or background
• The priority index of a process is called Nice in Linux. Its default value is 0 and it can vary between 20 to -19
• The lower the Niceness index the higher would be priority given to that task

Command	Description
bg	To send a process to background
fg	To run a stopped process in foreground
top	Details on all Active Processes
ps	Give the status of processes running for a user
ps PID	Gives the status of a particular process
pidof	Gives the Process ID (PID) of a process
kill PID	Kills a process
nice	Starts a process with a given priority
renice	Changes priority of an already running process
df	Gives free  hard disk space on your system
free	Gives free RAM on your system