COMP7004 - Systems Scripting

Lecture 4: Service Scheduling & Managing Processes

Dr. Vincent Emeakaroha

vincent.emeakaroha@mtu.ie

Semester 2, 2026

Service Scheduling

• Enables flexibility in system administration

• It might be useful to do a job at a particular time

• Delay a job if system is busy

• Set the frequency of doing particular jobs

• Job scheduling services include:

• cron

• at

• sleep

• Automating task scheduling in UNIX uses “at” and “cron”

• Sleep is used mainly to introduce delays in program

execution

at Scheduler

• Can be used to execute command once in the future

• Example

• To view the list of jobs

• Use atq command

• To remove a job from the list

• Use atrm 3 (removes job listed as number 3)

at 11:45 (press ENTER)

cp /etc/passwd $HOME/tmpFile1 (to exit CTRL + D)

cron Scheduler

• Executes jobs at a preset time

• Allows jobs to run automatically in the background

• Text-based configuration

• Uses crontab –e command for editing

• crontab -l will list the contents to stdout

• /etc/cron.allow and /etc/cron.deny can control access

to cron facilities

• cron job can be schedules with the following format:

[min] [hour] [day of month] [month] [day of week] [program to be run]

cron Job Formatting Example

• [min] 0 – 59, * runs once a minute

• [hour] 0 – 23, * for every hour

• [day of month], 1 – 31, * for every day

• [month], 1 -12, * for every month

• [day of week] 0 – 6, * for every day

• Sunday = 0 and Saturday = 6

• [program] full path to program to be executed

• Example: 15 16 5 * * cp /etc/passwd $HOME/tmpFile

Process Overview

• UNIX Processes

• Running programs

• Monitoring processes

• Terminating processes

Program vs Process

• Program – static file which can be run

• Binary executables

• Bash scripts

• Process – A running program

• Loaded in physical memory

• Stored in virtual memory (swap)

• Textbooks use the terms job and process almost

interchangeably

Process Components

• Address space map

• Status

• Owner

• Signals

• Priority

• Resources usage

• Sleeping

• Stopped

• Running

• Runable

• Zombie

Process Attributes

• Process ID – PID

• Unique identifier for every process

• Parent PID – PPID

• There is mostly a process that starts other child processes

• init is the starter process

• But in Ubuntu 15.04 and above, it is Systemd

• Real and Effective User ID – UID and EUID

• EUID determines what permission the process has

• Real and Effective Group ID – GID and EGID

• Niceness

• Specifies scheduling priority that determines the available CPU

• Users can make their process “nicer” to the rest of the system

• Control terminal – where Stdin, Stdout and Stderr are attached

Process Starting and Ending

• Processes are created …

• When the system boots

• By the actions of another process (more later)

• By the actions of a user

• By the actions of a batch manager

• Processes terminate …

• Normally – exit

• Voluntarily on an error

• Involuntarily on an error

• Terminated (killed) by the actions a user or a process

Process Lifecycle

Wait

Done

New Ready

Admitted

Scheduler

Dispatches

Interrupt

Run

Exit

I/O

Request

I/O Completed

Process Creation

• Unix/Linux

• Create a new (child) process with a function call –

fork();

• Allocates new Process Control Block (PCB)

• Clones the calling process (almost)

• Copy of parent process address space

• Copies resources in kernel (e.g. files)

• Places new PCB on

Ready queue

• Return value from fork() call produces

• 0 for child

• child PID for parent

• exec system call used after a fork() to replace the

process’ memory space with a new program

Process Creation 2

• Whenever a (parent) process calls a fork(), a child process

is created with its own descriptor, including its own copies

of the parent’s program text, data, and stack segments

• The child and parent processes execute in their own

separate address spaces

• This means that even though they have access to the same

information, when the child is created, both the child and

its parent each reference its own copy of the information

Process Creation 3

• Child process can “fork” (create) another child

• Becomes its parent

• When child process finishes

• Sends status message to parent

• Zombie – waits for parent acknowledgment

• Process in terminated state

• If a parent process dies unexpectedly, its child

processes become

• Orphan – init (PID 1) becomes parent

Linux Process Tree

• Command pstree prints this tree in different Linux distributions

• It is known as Process Group in UNIX

Process Control Block (PCB)

• Data is stored in a process control block

• Information associated with each process

• Process state

• Program counter

• CPU registers

• CPU scheduling information

• Memory-management information

• Accounting information

• Operating System maintains this data structure

Running Commands

• Input command/script name in a terminal

• Shell searches for the command or script

• If found, shell “forks” the command to execute it

• How is the command found?

• PATH environment variable

• which – Displays full path to a command

• Beware of “.” in your PATH

How Shell Execute Command

fork

wait

exit

exec

Required job

Parent shell

Child

• When you type a command, the shell forks a clone of itself

• The child process makes an exec call, which causes it to stop

executing the shell and start executing your command

• The parent process, still running the shell, waits for the child

to terminate

Process Running States

• Possible process states

• Running: executing

• Blocked: waiting for I/O

• Ready: waiting to be scheduled

Running Processes

• Foreground process

• Returns control to shell after it finishes

• Most common usage

• Background process

• Returns control to shell immediately

• Use “&” typed after command name to run process

in background mode

• Runs concurrently

Process Priority

• How much CPU time granted relative to other

processes

• nice – Sets the priority value at start

• renice – Changes priority

• Ranges

System Range

Solaris 0 to 39

RedHat -20 to 20

Ubuntu -20 to 19

Where

• Minus values are highest

• Plus values lowest

Signals

• Signals are process-level interrupt request

• Importance

• Means to communicate with processes

• Terminal signals

• <Ctrl><c> - Terminate process

• <Ctrl><z> - Stop (suspend) process

• Default is to terminate the process

• Can be sent by admin for various purposes

• Can be sent by kernel when a process breaks a rule

• E.g., division by zero

Common Signals

# Name Description

1 HUP Hangup

2 INT Interrupt

3 QUIT Quit

9 KILL KILL

15 TERM Software Termination

STOP Stop

USR1 User defined Signal

Handling Signals

• Process can designate a signal handler for a particular

signal

• If no handler, kernel takes some default action

• When handler is finished catching signal, execution

continues where the signal was received

• Process can request that particular signals be ignored, or

blocked

• If signal is received while blocked, one instance of that

signal is buffered until it is unblocked

Sending Signal

• kill [-signal] pid

• kill sends TERM signal by default

• kill [-s ignal] 910 3044

• kill -9 pid === kill -KILL pid

• “Guarantees” that the process will die

• sudo killall –USR1 httpd

• killall removes need for pid

Process Termination

• Besides being able to terminate itself with exit, a process can

be killed by another process using kill command:

• kill( pid, sig ) - sends signal sig to process with process-id pid.

One signal is SIGKILL (terminate the target process immediately)

• When a process terminates, all the resources it owns are

reclaimed by the system:

• “process control block” reclaimed

• its memory is deallocated

• all open files closed and Open File Table reclaimed

• Note: a process can kill another process only if:

• it belongs to the same user

• super user

Monitoring System Load

• Average number of “Runnable” processes

• Measure of how busy the system is

• Performance deteriorates at high loads

• Excess number of running processes

• Good for creating a baseline

• uptime command shows Load average

• (5, 10, 15 minutes)

Monitoring Linux Process

• ps command

• Stands for

Process State

• Berkeley Software Distribution (BSD) – Sorts by %CPU

Usage

• System V Release 4 (SVR4) – Sorts by PID

• Example: ps aux

• top command

• Full terminal screen display

• Sortable

• Can kill and renice processes

“ps -ef” Information

• UID – Username

• PID – Process ID

• %CPU/%MEM - % of System CPU/Memory

• PRI/NI - Priority/Nice level

• RSS/SIZE - Resident/Total Memory used

• STAT - Process State

• TIME - CPU time used

• CMD – Command issued

ps Examples

• To list all the processes running in the operating

system, including system processes, enter the

command:

• ps –ef

• To list processes with the parents process ID along

with the process state, enter the command:

• ps –lf

• To find a particular process, enter the command

• ps –ef | grep “processName”

Runaway Processes

• Uncontrolled processes running on a system (Zombie)

• Can eat up your resources

• Lists users and PIDs accessing a resource

• fuser: Command for identifying process

using particular files

• lsof: command for showing a list of open

files and their PID

• Kill the process and clean up

Killing Process

• If you want to terminate a running process, enter the

command:

• kill pid_of_process_to_be_killed

• Many a time, if the process is not killed by the “kill”

command, you may need to pass additional option to

ensure that the required process is killed. Use the

following command:

• Kill -9 pid_of_process_to_be_killed

Monitoring Window Process

• Task Manager

• Graphical viewer

• Can also sort or kill processes

• tasklist

• Command line Viewer

• Can view associated dll files

• wmic utility tool

• Can view, start, and kill processes

• Remote connection capabilities

Thank You!