working with processes linux

7/30/2019 Working With Processes Linux

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Working with Processes


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Objectives

After completing this unit, you should be able to:Define a Linux process

Describe the relationship between parent and child

processes

Explain the purpose of a shell

Start foreground and background processesExplain the concept of signals and use them to terminateprocesses

Explain the concept of priorities and manage them


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What is a Process?

A program is an executable fileA process is a program which is being executed

Each process has its own environment:

To see the PID of your current shell process type:$ echo $$

Program name User and Group ID

Internal data Process ID (PID)Open Files Parent PID (PPID)

Current Directory Program variables

additional parameters

process environment


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Starting and Stopping a Process

All processes are started by other processesParent/Child relationship

$ ls -l

bash fork()

exec() ls -l exit()

A process can be terminated because of tworeasons:

The process terminates itself when done

The process is terminated by a signal from anotherprocess


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Environment

Login:

Linux System

Login Process Environment

login: /sbin/mingetty

password: ...forks /bin/bash

$ -bash (login shell)

Program -bashUID 503 (john)

GID 100 (users)

open files /dev/tty1

PID 201


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Parents and Children

-bash

bash

date

PID PPID

561 320

675 561777 675

$ echo $$

561

$bash

$ echo $$

675

$ date

Thu Mar 25 22:28:21 CET 1999

$

$ echo $$

561


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Monitoring Processes

The ps command displays process status information

$ ps aux

USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND

root 1 0.0 0.0 1336 436 ? S Mar16 0:05 init

root 2 0.0 0.0 0 0 ? SW Mar16 0:00 [keventd]

root 3 0.0 0.0 0 0 ? SW Mar16 0:05 [kapmd]

root 4 0.0 0.0 0 0 ? SW Mar16 0:05 [kswapd]

...

root 10248 0.0 0.1 2852 884 pts/2 R 13:47 0:00 ps aux

ps supports a large number of options - you typically use ps aux:

a all processes attached to a terminal

x all other processes

u provides more columns

options are typed without a leading "-" !


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Controlling Processes

Processes can be controlled in two ways:From the shell that started it, using itsjob numberFrom anywhere else on the system, using itsPID

Actions that can be performed on a running process:TerminateKillStop/Continue

These actions are performed by sending signals


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Starting Processes

Foreground Processes

Foreground processes are invoked by simply typing acommand at the command line.

Background processes are invoked by putting an "&" atthe end of the command line

bash wait()

bash

$ find / -name README

$ find / -name README &

Background Processes

J b C t l i th B h Sh ll


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Job Control in the Bash Shell

suspends foreground task

jobs lists background or suspended jobs

fg resume suspended task in the

foreground

bg resume suspended task in thebackground

Specify a job number forbg, fg and kill using %job

J b C t l E l


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Job Control Example

$ find / -name README &

[1] 2863

$ jobs[1]+ running find / -name README &

$ fg %1

/usr/share/doc/packages/grub/README

....

[1]+ stopped find / -name README

$ bg 2863

$ jobs[1]+ running find / -name README &

$ kill %find

Kill Si l


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Signal Keyboard Meaning Default action

01 hangup end process

02 Ctrl-C interrupt end process03 Ctrl-\ quit end process & core dump09 kill end process - cannot be

redefined - handled by kernel15 terminate end process

Kill Signals

Several signals can be sent to a processUsing keyboard interrupts (if foreground process)Using the kill commandSynopsis: kill -signal PID

Using the killallcommand to kill all named appsSynopsis: killall -signal application

Most important signals:

R i L P


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Running Long Processes

The nohupcommand will stop a process from being killed ifyou log off the system before it completes, by intercepting andignoring the SIGHUP and SIGQUIT (hangup and quit) signals

$ nohup find / -name README &

nohup: appending output to `nohup.out

$ logout

Managing Process Priorities


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Managing Process Priorities

Processes are scheduled according to priority

Base priority (0)

user defined priority

(negative values areallowed only to root!)

+

-

+ 19

- 20

dynamic part of priority

(calculated by scheduler)

reserved for kernel

threads... -60

... +39

The nice Command


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The nice Command

The nicecommand is used to start a process with a userdefined priority

nice [-n ]

$ nice -n 10 my_program &

[1] 4862

$ ps l

F UID PID PRI NI VSZ ... COMMAND

0 500 4372 9 0 4860 ... -bash

0 500 4862 15 10 3612 ... my_program

0 500 4863 21 0 1556 ... ps l

The renice Command


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The renice Command

The renicecommand is used to change the priority of acurrently running process

renice

$ renice 15 4862

4862: old priority 10, new priority 15

$ ps lF UID PID PRI NI VSZ ... COMMAND

0 500 4372 9 0 4860 ... -bash

0 500 4862 22 15 3612 ... my_program

0 500 4868 21 0 1556 ... ps l

Integrated Process Management


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Integrated Process Management

Various integrated tools exist for process managementtop, gpm, kpm

Availability depends on distribution

Daemons


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Daemons

The word "Daemon" refers to a never-ending process, usually asystem process that controls a system resource such as theprinter queue or performs a network service

lpd

/dev/lp0

print job 1

print job 2

print job 3

Print Queue

Unit Summary


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Unit Summary

All processes are started by a "parent" process (exceptfor init, which is started by the kernel)

Every process is identified with a Process Identifier (PID)

A special process is the shell, which can interpret usercommands

Processes can terminate by themselves, or upon

reception of a signal

Signals can be sent by the shell, using a keyboardsequence, or by the kill and killall commands

Processes are started with equal priority, but this can bechanged using the nice and renice commands

A daemon is a background process that typically controlsa system resource or offers a network service

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