Chapter-8-Process in Linux System

8/16/2019 Chapter-8-Process in Linux System

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Chapter 8

Process in Linux System


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VIEW AND M ANAGE PROCESSES

To manage processes you need to know the following:

Understand Process Definitions

Learn Jobs and Processes

Manage Foreground and Background Processes View and Prioritize Processes

End a Process

Understand Services (Daemons)

Manage a Daemon Process

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UNDERSTAND PROCESS DEFINITIONS

The following terms are used to describe Linux

processes:

Program — A structured set of commands stored in an

executable file on a Linux file system

Process —

A program that is loaded into memory and

executed by the CPU

User process — A process launched by a user that is started

from a terminal or within the graphical environment

Daemon process —

A system process that is not associatedwith a terminal or a graphical environment

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Figure 10-2 Relationship between daemon processes and user processes


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(CONTINUED)

Properties of a process:

Process ID (PID)

Child process

Parent process Parent Process ID (PPID)

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(CONTINUED)

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Figure 10-3 The relationship between parent and child process ID numbers


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LEARN JOBS AND PROCESSES

Job identifier (job ID)

A numeric value that identifies the running program

uniquely within that shell

Each process is identified using a process ID (PID) thatis unique across the entire system

All jobs have a PID, but not all processes have a usable

job ID

PID 1 always belongs to the init process When performing tasks such as changing the priority

level of a running program, use the PID

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M ANAGE FOREGROUND AND B ACKGROUND

PROCESSES

Processes executed in the foreground

Started in a terminal window and run until the process is

completed

Background process execution Occurs when a process is started and the terminal window

returns to a prompt before the process finishes executing

Existing processes can be switched from foreground to

background execution

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M ANAGE FOREGROUND AND B ACKGROUND

PROCESSES (CONTINUED)

Commands in a shell can be started in the foreground

or in the background

Continue running a stopped process in the background

by entering bg Appending an ampersand to a command starts the

process in the background

Each process started from the shell is assigned a job ID

by the job control of the shell Switch a process to the foreground by entering fg

job_ID

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VIEW AND PRIORITIZE PROCESSES

ps

View running processes with the ps (process status)

command

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Table 10-8 Some commonly used options with ps


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(CONTINUED)

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Table 10-9 Some of the fields (columns) in the process list


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(CONTINUED)

12Table 10-10 Values for the STAT process state


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(CONTINUED)

pstree

Displays a list of processes in the form of a tree structure

Gives you an overview of the hierarchy of a process

nice and renice The nice command assigns a process a specific nice value

that affects the calculation of the process priority

The lower the value of the nice level, the higher the priority

of the process

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(CONTINUED)

nice and renice (continued)

The nice level is used by the scheduler to determine how

frequently to service a running process

Use the command renice to change the nice value of a

running process

top

Allows you to watch processes continuously in a list that is

updated in short intervals

Provides a real-time view of a running system

Can also be used to assign a new nice value to running

processes or to end processes

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Figure 10-4 The output of the top command


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END A PROCESS

kill and killall

The killall command kills all processes with an indicated

command name

The kill command kills only the indicated process

GNOME System Monitor

Start the GNOME System Monitor utility (Computer >More

Applications > GNOME System Monitor) to view and kill

processes

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END A PROCESS (CONTINUED)

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Table 10-13 The more commonly used kill signals


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Figure 10-5 The GNOME System Monitor utility


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END A PROCESS (CONTINUED)

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Table 10-14 Information displayed by default on the Process tab


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UNDERSTAND SERVICES (D AEMONS)

A service is also called a daemon

Process or collection of processes that wait for an event to

trigger an action on the part of the program

Network-based services create a listener on a TCP orUDP port when they are started

Listener waits for network traffic to appear on the

designated port

When traffic is detected, the program processes the trafficas input and generates output that is sent back to the

requester

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M ANAGE A D AEMON PROCESS

Daemons run in the background and are usually

started when the system is booted

Daemons make a number of services available

Daemons are terminal-independent processes, and areindicated in the ps x TTY column by a ?

Two types of daemons are available:

Signal-controlled daemons

Interval-controlled daemons

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M ANAGE A D AEMON PROCESS (CONTINUED)

Each daemon has a corresponding script in /etc/init.d/

Many scripts have a symbolic link in either the

/usr/sbin/ directory or the /sbin/ directory

Find configuration files for daemons in the /etc/

directory or in its subdirectories22

Table 10-15 Parameters used to control daemon scripts


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M ANAGE A D AEMON PROCESS (CONTINUED)

Some important daemons:

cron — Starts other processes at specified times

cupsd — The printing daemon

sshd —

Enables secure communication by way of insecurenetworks (secure shell)

syslog ng — Logs system messages in the directory /var/log/

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M ANAGE LINUX PROCESSES

In this exercise, start and stop processes and change

their priorities

First, start and suspend xeyes, move it to the

background and foreground, and stop it

Then, start xeyes and set the priority of the running

program to a nice value of -5

Start a second xeyes with a nice value of 10

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SCHEDULE JOBS

Automate jobs in Linux by doing the following:

Schedule a Job (cron)

Run a Job One Time Only (at)

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SCHEDULE A JOB (CRON)

Schedule jobs to be carried out on a regular basis by

using the cron service (/usr/sbin/cron)

The service runs as a daemon

Checks once a minute to see if jobs have been defined for

the current time

A file that contains the list of jobs is called a crontab

A crontab exists for the entire system as well as for each

user defined on the system The /etc/sysconfig/cron file contains variables for the

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SCHEDULE A JOB (CRON) (CONTINUED)

System jobs

Control system jobs with the /etc/crontab file

Can add lines to /etc/crontab, but do not delete the lines

added at installation

Information on the last time the jobs were run is kept in

the /var/spool/cron/lastrun/ directory 27

Table 10-16 Directories containing system jobs that will be run by cron


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User jobs

The jobs of individual users are stored in the

/var/spool/cron/tabs/ directory

In files matching the usernames Users create their own jobs using crontab

28Table 10-17 Options for the crontab command


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User jobs (continued)

Each line in a file defines a job

There are six fields in a line

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Table 10-18 Fields in a crontab file


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RUN A JOB ONE TIME ONLY ( AT)

If you want to run a job one time only, use the at

command

To use at, make sure the atd service is started

(rcatdstart) Two files determine which users can run this

command:

/etc/at.allow

/etc/at.deny Can modify or create these text files

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RUN A JOB ONE TIME ONLY ( AT)

(CONTINUED)

If the /etc/at.allow file exists, only this file is evaluated

If neither of these files exists, only the user root can define jobs with at

Example:

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SCHEDULE JOBS WITH AT AND CRON

In this exercise, schedule jobs with at and cron

First, redirect the output of finger to /var/log/messages

three minutes from the current time

Then, schedule the same job for tomorrow at noon

Then, schedule a program to run tomorrow at 2:00

p.m., and afterwards remove the job

In the second part of the exercise, create a cron job as

a normal user that logs the output of finger to

~/users.log every minute

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M ANAGE RUNLEVELS

Managing runlevels is an essential part of Linux

system administration

In this objective, you learn what runlevels are, the role

of the program init, and how to configure and changerunlevels:

The init Program and Linux Runlevels

init Scripts and Runlevel Directories

Change the Runlevel

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THE INIT PROGRAM AND LINUX

RUNLEVELS

The init program

The system is initialized by /sbin/init

Started by the kernel as the first process of the system

This process, or one of its child processes, starts alladditional processes

SIGKILL has no effect on init

The configuration file for init is /etc/inittab

Part of the configuration in /etc/inittab is the runlevel the

system uses after booting

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THE INIT PROGRAM AND LINUX RUNLEVELS

(CONTINUED)

The runlevels

Runlevels define the state of the system

35Table 11-1 The available runlevels


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THE INIT PROGRAM AND LINUX

RUNLEVELS (CONTINUED)

Init configuration file (/etc/inittab)

Each line in the /etc/inittab file uses the following syntax:id:rl:action:process

The first entry in the /etc/inittab file contains the following

parameters: id:5:initdefault:

The next entry in /etc/inittab looks like this:si::bootwait:/etc/init.d/boot

The next few entries describe the actions for runlevels 0 to 6

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INIT SCRIPTS AND RUNLEVEL

DIRECTORIES

/etc/inittab defines the runlevel the system uses after

booting is complete

init scripts

The /etc/init.d/ directory contains shell scripts that are used

to perform certain tasks at bootup and start and stop

services in the running system

The shell scripts can be called up in the following ways:

Directly by init when you boot the system

Indirectly by init when you change the runlevel Directly by /etc/init.d/script parameter

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INIT SCRIPTS AND RUNLEVEL DIRECTORIES

(CONTINUED)

38Table 11-2 /etc/init.d/script parameters


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DIRECTORIES (CONTINUED)

init scripts (continued)

Some of the more important scripts stored in /etc/init.d/:

boot

boot.local

halt

rc

service

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Runlevel symbolic links

To enter a certain runlevel, init calls the /etc/init.d/rc script

with the runlevel as a parameter

This script examines the respective runlevel /etc/init.d/rcx.d/

directory and starts and stops services depending on thelinks in this directory

Each runlevel has a corresponding subdirectory in

/etc/init.d/

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Figure 11-6 The YaST Runlevel Editor module


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Figure 11-7 The YaST Runlevel Editor module in expert mode


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Activate and deactivate services for a runlevel

(continued)

Normally, the default runlevel of a SUSE Linux system is

runlevel 5

Changes to the default runlevel take effect the next time you bootyour computer

To configure a service, select a service from the list

Then, from the options below the list, select the runlevels you want

associated with the service

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CHANGE THE RUNLEVEL

Change the runlevel at boot

Possible to boot to another runlevel by specifying the

runlevel on the kernel command line of GRUB

Manage runlevels from the command line

Can change to another runlevel once the system is runningby using the init command

Like most modern operating systems, Linux reacts

sensitively to being switched off without warning The shutdown command shuts down the system after the

specified time

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Chapter-8-Process in Linux System