Linux+ Unit 11

Course ILT

Managing system processes Unit objectives  Categorize the different types of processes, view processes using utilities, and illustrate the difference between kill signals  Describe how binary programs and shell scripts are executed, create and manipulate background processes  Use utilities to modify the priority of a process, schedule commands by using the at daemon, and schedule repetitive commands by using the cron daemon

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Topic A  Topic A: Starting, viewing, and killing processes  Topic B: Foreground and background processes  Topic C: Process priorities and scheduling

Programs and processes Course ILT

 Program – An executable file – May be executed to create a process

 Process – An instance of a executing program

Linux processes Course ILT

 User process – Process begun by a user that runs on a terminal

 Daemon process – System process that is not associated with a terminal

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Linux processes, continued Course ILT

 Process ID (PID) – Unique identifier assigned to every process

 Child processes – Refers to a process that was started by another process (parent process)

 Parent processes – Process that has started other processes (child processes)

 Parent Process ID (PPID) – The PID of the parent process that created the current process

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Parent and child processes

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Process genealogy

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Activity A-1

Discussing processes

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The ps command  Most versatile and common utility that can view processes  Without arguments, displays processes running in the current shell

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Process states Process state

Description

S

If a process is not being run on the processor at the current time, then you’ll see an S (sleeping) in the process state column.

R

Process is currently running on the processor.

T

Process has stopped or is being traced by another process.

Z

This indicates a zombie process.

Process states Course ILT

 Zombie process – Process that has finished executing, but whose parent has not yet released its PID – Also known as defunct processes

Process priority and nice value Course ILT

 Process priority (PRI) – Number assigned to a process, used to determine how many time slices on the processor it will receive

 Nice value (NI) – Value that indirectly represents the priority of a process  The higher the value, the lower the priority

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Options of the ps command

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top command  Most common command used to display processes aside from ps  Display organized by processor time – Processes that use the most processor time are listed at the top of the screen

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top command, continued Course ILT

 Rogue processes – Process that has become faulty and consumes more system resources than it should

 Can be used to change the priority of processes or kill them

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Activity A-2

Using the ps command to view processes

Sending signals to processes Course ILT

 Kill signal – Type of signal sent to a process by the kill command – Different kill signals affect processes in different ways

 kill command – Command that kills all instances of a process by command name

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Common administrative kill signals

Trapping and killall Course ILT

 Trapping – The process of ignoring a kill signal

 killall command – The command that kills all instances of a process by command name

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Activity A-3

Using the kill command to terminate processes

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Topic B  Topic A: Starting, viewing, and killing processes  Topic B: Foreground and background processes  Topic C: Process priorities and scheduling

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Process execution  The three main types of Linux commands that you may execute – Binary programs – Shell functions – Shell scripts

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Forking  The act of creating a new BASH shell child process from a parent  Carried out by the fork function in the Linux kernel

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Process forking

Background and foreground processes

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 Foreground processes – Process for which the BASH shell that executed it must wait for its termination

 Background processes – Process that does not require the BASH shell to wait for its termination – Upon execution, the user receives the BASH shell prompt immediately

Background processes Course ILT

 jobs command – Command used to see the list of background processes running in the current shell

Background processes Course ILT

 foreground (fg) command – Command used to run a background process in the foreground

 background (bg) command – Command used to run a foreground process in the background

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Activity B-1

Running a process in the background

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Topic C  Topic A: Starting, viewing, and killing processes  Topic B: Foreground and background processes  Topic C: Process priorities and scheduling

Process priorities Course ILT

 Time slice – Amount of time a process is given on a CPU in a multiprocessing operating system – The more time slices a process has, the more time it has to execute on the CPU and the faster it will execute – Time slices are usually measured in milliseconds

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The nice value scale

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The nice value and command  Processes are started with a nice value of 0 by default – On some systems, background processes are given a nice value of 4 by default to lower the chance they will receive time slices

 nice command – The command used to change the priority of a process as it is started

The renice command Course ILT

 renice command – Used to alter the nice value of a process currently running

 As with the nice command, only the root user may change the nice value to a negative value by using renice

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Activity C-1

Experimenting with scheduling priority

Scheduling commands Course ILT

 The at daemon (atd) – System daemon that executes tasks at a future time – Configured with the at command

 The cron daemon (crond) – System daemon that executes tasks repetitively in the future – Configured using cron tables

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Common at commands

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Scheduling commands with atd  /etc/at.allow – A file listing all users who can use the at command

 /etc/at.deny – A file listing all the users who cannot access the at command

crond scheduling commands Course ILT

 cron tables – A file specifying tasks to be run by the cron daemon – User cron tables and system cron tables – Six fields separated by space or tab characters

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User cron table format

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User cron table entry

Location of cron tables Course ILT

 /var/spool/cron – Stores user cron tables

 /etc/cron.d – Contains additional system cron tables

 /etc/crontab – Default system cron table

User cron tables Course ILT

 /etc/cron.allow – File listing all users who can use the cron command

 /etc/cron.deny – File listing all users who cannot access the cron command

 crontab command – Command used to view and edit user cron tables

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The system cron table  Systems are scheduled to run commands during non-business hours  Commands may perform system maintenance, back up data, or run CPU-intensive programs  Most commands are scheduled by the cron daemon from entries in the system cron table /etc/crontable

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Activity C-2

Using at and crontab utilities to schedule processes

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Unit summary  Categorized different types of processes, viewed processes by using utilities, and learned the difference between kill signals  Described how binary programs and shell scripts are executed and created and manipulated background processes  Used utilities to modify the priority of a process and schedule commands