unit 1 connecting a unix server to network and enabling services

8/4/2019 Unit 1 Connecting a Unix Server to Network and Enabling Services

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Unit 1 Connecting a Unix server to network and enabling services

1.1 Setting the hostname

The host name is set up when the system is installed. But if not, or if you entered an

incorrect name, you can set it with the command

# hostname s gw.example.org

1.2 Assigning addresses to network interfaces

The kernel detects the interfaces automatically when it start

need to tell it what interfaces are connected to which networks and which address

the system has on each network.

if the network is a broadcast network such as an Ethernet, you need to specify a

range of addresses, which can be reached directly on the network. This selection is

performed with the network mask.

1.3 ifconfig utility

The ifconfig utility is used to assign an address to a network interface and/or

configure network interface parameters.

The ifconfig utility must be used at boot time to define the network address of

each interface present on a machine;

Can be used at a later time to redefine an interface's address or other operating

parameters.

The command to configure an Ethernet interface with a network address is

# ifconfig ep0 inet 220.147.37.5 netmask 255.255.255.0 up

This command tells the interface to use the Internet protocols (the default) and

up tells it to bring it up. The address is class C .

Point to point connection


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# ifconfig tun0 139.130.136.133 139.130.136.129 netmask 255.255.255.255

Can be used to set up ppp

Checking configuration

# ifconfig -a

-a flag lists the configuration of all interfaces

1.4 Daemons

These are programs that are not designed to be run with continuous user input

and they disconnect from the terminal at the first opportunity.

There is a convention to name program that normally run as daemons with a

trailing d.

Exception - sendmail

Signals with daemons

SIGTERM is a polite a way of killing a process. It will close all the files it may have

opened and generally finish whatever it is doing at the time before shutting down.

The process cannot ignore SIGKILL

SIGHUP is used to send a hangupsignal to make daemon read a changed file

1.5 inetd The internet daemon

This program should be run at boot time by/etc/rc.

It then listens for connections on certain Internet sockets.

When a connection is found on one of the sockets, it decides what service the

socket corresponds to and invokes a program to service the request.

After the program is finished, inetdcontinues to listen to the socket .

inetdallows running one daemon to invoke several others.

Inetd


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Upon execution, inetdreads its configuration information from a configuration file

which by default is/etc/inetd.conf.

A typical entry in this file will be

#ftp stream tcp nowait root /usr/libexec/ftpd ftpd l

#telnet stram tcp nowait root /usr/libexec/telnetd telnetd

Upon execution, inetdreads its configuration information from a configuration file

which by default is/etc/inetd.conf.

A typical entry in this file will be

#ftp stream tcp nowait root /usr/libexec/ftpd ftpd l

#telnet stram tcp nowait root /usr/libexec/telnetd telnetd

Daemons started by inetd

Telnetd

Rshd

Rlogind

ftpd.

Running daemons directly

To runftpddirectly

Add the following line in/etc/rc.local

echo n starting local daemons:

#put your local stuff here

echo ftpd && ftpd -D

Option D tells ftpd to run as a daemon

However you should comment out the ftp line in/etc/inetd.conf by adding a hash

mark (#) in front of it

Either reboot or cause inetdto reread it in the configuration file

#ps waux | grep inetd ( to determine the pid number for inetd)


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# kill 1 [pid] (pid number ofinetd

sshd The daemon program for ssh

Secure shell is set of network connectivity tools used to access remote machines

securely. ssh encrypts all traffic to effectively eliminate eavesdropping, connection

hijacking and other network level attacks

Enabling daemons

rc command is used for autoreboot and daemon starting.

When the system starts up/etc/rcfirst reads the description files

/etc/defaults/rc.confwhich contains the defaults for a number of configuration

variables and /etc/rc.confwhich contains your modification to the defaults.

Starting daemons

If the configuration file for a daemon process such as inetdor httpd is changed,

the configuration file is to be reread.

Start and restart will create an outage.

Most daemons however are written to respond to SIGHUP signal.

By sending a SIGHUP signal to the daemon processes, the file will be reread and

the changes in the file will be implemented.


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Unit 2 Installation of peripheral devices

2.1 Installation of a printer

2.2 Installation of a terminal

2.3 Exercises

2.4 Summary

Important commands in printing

lpr adds a job to a print queue by copying the file into its spooling directory

lpq Lists jobs that are currently in the printer queues.

lprm Delete jobs. Nobody other than root can delete someone elses job from aprint queue

lpd The printer daemon responsible for sending data from spooling directory to a

printer

lpc Administrative interface to printer subsystem.

/etc/printcap file

This file lists all devices serviced by the spooling system.

Usually describes many common printers all commented out.

Colons separate all individual items in a printcap entry

/dev/lpt0 indicates device special file

mx maximum file size

mx # 10000

pl page length in lines

pw page width in characters

Spooling directory

This holds files destined for a particular printer until the daemon lpdcan print

them.


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Spooling directories are conveniently located in/var/spool

Each printer should have its own spooling directory

Terminals

Terminals provide a convenient and low-cost way of accessing the UNIX system

when you are not at the computer console or a connected network.

Using a terminal attached to an unused serial port, you can log in and run any text

program that you would normally run on the console.

Dumb terminals

Are specialised pieces of hardware that let you connect to computers over serial

lines.

Have the computational power to display send and receive text

Examples : Digital Equipment Corporation VT 100 and Wyses WY-75.

Init process

Responsible for all process control and initialisation at system start up

Reads the /etc/ttys file and start a gettyprocess on the available terminals.

Gettyprocess is responsible for reading a login name and starting the login

program.

/etc/ttys file

The first field normally specifies the name of the terminal special file as found in

/dev

The second field is the command to execute for this line, which is usually getty.

The third field is the type of terminal usually connected to a ttyline

The fourth field specifies if the port should be enabled

The final field is used to specify whether the port is secure. If it is secure, rootlogin

are allowed from this port.


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Virtual terminals

UNIX is a multitasking system.

PC has only one screen.

In Free BSD, virtual terminals solve this problem.

In text mode you can change up to 12 different screens with the combinations of

Alt key and a function key

UseAlt-F1 toAlt-F12

By default 8 terminals are active./dev/ttyv0to /dev/ttyv7.

The system console is/dev/ttyv0.

Pseudoterminals

FreeBSD offers an additional class of terminals called pseudo terminals.

They come in pairs

a master device also called aptyis used by processors which use the interface and

has a name like/dev/ptyp0or /dev/pty/0.

The slave device looks like a terminal and has name like /dev/ttyp0.

Major and minor device numbers

Major device number is an index into a list of drivers and the minor number is a

number, which the driver uses to distinguish the individual device

Devices can be either block or character type

On block devices , you can store a file system Eg: disk drive, floppy drive

On character devices you cannot store a file system. Printer, terminal


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Unit 3 Monitoring Security

examine the different methods of improving the security of UNIX system.

Physical security

Search path issues

Plugging security holes due small mistakes

Using groups more effectively

Minimising the risks from freely available software

Preventing theft and vandalism

Limiting access to the console and cpu unit to prevent someone form crashing thesystem and rebooting to single user mode

Controlling environment factors such as UPS, line conditioners, surge suppressors

Restricting or monitoring other parts of the system like terminals, workstation

cable

Limiting access to backup tapes

Search path issues

Place the current directory and bin sub directory of user at the end of the path list

after the standard location of the commands

Plugging security holes

Do not use/tmp as any users home directory

Think carefully about which users can be supplementary members of the group 0

and any other systems groups. Implications should be correctly understood.

Roots umask should be 057 or more restrictive setting

Using groups more effectively

Creating new groups

SGID access

User private groups


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Minimising the risks from freely available software

freely available software should be checked for potential problems

Detecting problems

Checking Password files

Monitoring the file system

Looking for SUID andInvestigate system activity

Monitoring unsuccessful login attempts

History on the root account.

Configuring error logging

SGID files

Checking modification dates and inode numbers

Checking password files

The password file should be examined regularly for potential account-level security

problems.

The master password file or shadow password file should also be checked

Accounts without passwords

# grep ^[^:]*:: /etc/passwd or

# grep ^[^:]*:: /etc/master.passwd

Detecting accounts without password

Accounts with UID or GID of 0 can be located with grep

#grep :00*: /etc/passwd


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Checking new accounts

New accounts created can be checked using a diffcommand

# diff /etc/passwd /usr/local/bin/old/wps

Where /usr/local/bin/old/wps is a copy of the password file stored in an obscure

location

Checking ownership of password files

#cd /etc ; ls l passwd master.passwd

Thepasswdfile should be readable and writeable by root and readable by group

and others.

The master.passwdfile or the shadow password file should be readable and

writeable by root only.

Monitoring file systems

Checking file ownership and protection

This will involve

Checking the ownership and protection of important system configuration files

Checking the ownership and protection of important directories

Verifying the integrity of important binary files

Checking modification dates and inode numbers

It is important to check modification times and inode numbers to find if any system

files have been modified

The following command checks inode numbers and modification times for files and

directories starting with rc.

# ls lsid /etc/rc*


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Investigating system activity

If you regularly monitor the system you will be able to check for security breaches

It will be worthwhile checking for processes that remain idle for long periods of

time, users logged in at unusual times or from unusual locations and the like.

# ps auxww | head 3

Monitoring unsuccessful login attempts

Repeated unsuccessful attempts for any user accounts can indicate someone trying

to break into the system.

In Free BSD you can see a list of events in

/var/log/auth.log

Some versions of UNIX provide some mechanism for logging all attempts to

become superuser. Such logs can be very useful when trying to track down who did

something untoward

History on the root account.

Give root a shell that can support a history mechanism (the C shell or Korn shell )

Set the number of commands saved across login sessions to a large number in the

roots initialisation file

The different commands executed as root can be traced. C

Copy the history file to some secure location periodically using the cron facility

Configuring error logging

The syslog message logging facility provides a general way to specify where and

how some types of system messages are saved

syslogd collects messages sent by the various system processes and routes them to

the final destination based on instructions given in its configuration file.

/etc/syslog.conf


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/etc/syslog

Structure of the file

Facility.level destination

Facilities- kern, mail, lpr, daemon

Severity level emerg, alert, crit, err, notice, info debug, none

Tracking user activities

Use utilities that can determine what users have been doing on the system

Will enable you to track down the cause of a security problem

The lastcommand displays data for each time a user is logged into the system. Last

may also be followed by a list of usernames and/or terminal names


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Unit 4 Monitoring system performance

Monitoring the system parameters

Monitoring the system parameters

Monitoring CPU activity

Monitoring Memory use and paging activity

Monitoring CPU activity

UNIX provides the ability to monitor process execution

Allows to a limited extent, specify their execution priorities

Monitoring Memory

A system should have adequate memory not just for the largest jobs but also for

the overall mix of jobs run every day

Paging and swapping are the means by which UNIX distributes available memory

among current processes when their total memory needs exceed the amount of

physical memory

Swapping

Swapping refers to writing an entire process to disk thereby freeing all its memory.

A swapped out process should then be reread into memory from the disk when

execution resumes

Paging

Paging involves moving sections of a processs memory in units called pages to disk

in order to free up memory needed by that process or another one.

Apage faultoccurs when a process needs a page of memory that is not resident

and must be reread it from the disk

Problem with paging

The main problem occurs when there is not enough memory on the system for all

the processes currently running

An appreciable CPU time is spent for page faulting and the whole process is very

inefficient.


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Uptime command

The uptime command will give a rough estimate of the system load.

#uptime

3:25PM up 2 days, 2:41, 16 users , load average: 1.90, 1.43, 1.33

ps command

The ps command gives a more complete picture of system activity. This utility

gives a report summarising execution statistics for current processes.

ps aux produces a table of all processes, arranged in the order of cpu usage.

ps aux command

USER username of process owner

PID Process ID

% CPU Estimated fraction of CPU consumed.

% MEM Estimated fraction of system memory consumed.

VSZ Virtual memory used in K

RSS Real memory used in K

TT, TTY Terminal port associate with process

STAT Current process state

Top utility

This provides a continuous display of the system status and most active processes

which it automatically updates every few seconds.

-dcount show only countdisplays and then exit. A display is considered one

update of the screen

-stime set the delay between screen update in seconds. Default is 2 secs

-Uusername Show only those processes owned by username.


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Typical display

Mem: 9220K Active, 1032K Inact, 3284K Wired, 1MB Cache, 2M Buf, 1320K

Free Swap: 91M Total, 79M Free, 13% Inuse, 80K In, 104 K Out

Priority for processes

Unix has two priority levels

Its nice number, which is the requested execution priority with respect to other

processes. Rootor the processors owner sets this.

Its current execution priority, which is computed and dynamically updated by

the operating system ( shown as PRI)

Nice command

Nice execute a utility at an altered scheduling

Syntax

nice [ -n increment ] utility [argument]

# nice n 5 date

renice alter priority of running processes

# renice 10 -u tjones

Sets the nice value of all processes owned by tjones to 10

# renice n +10 -g users1

Increments the nice number of all processes of the group users1 by 10.

# renice -20 567

Sets the nice number of process ID 567 to -20

Vmstat

vmstat reports certain kernel statistics kept about process, virtual memory, disk,

trap and cpu activity.


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Unit 5 Downloading and installing programs in UNIX

Learn how to archive and compress files

Use ftp to download files

Set up the UNIX machine on the network

Download a program and install it

Archiving and Compressing files

Tar utility

gzip and gunzip

Tar Utility

stores a number of files and directories into a single file.

designed to read and write data on tape and the name is derived from tape

archive.

can be used for packaging directories and unpackaging them

Options with tar command

a Adds files to an existing tarfile

c Creates a new tarfile

-- delete Deletes a file from tar file

f Name of output tar file

t Lists contents of file

v Produce verbose listing of file processed

x Extracts files from tarfile

z applies compression to tar file using gzip and gunzip


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gzip and gunzip

gzip ( gnu zip) command is generally considered a good option for compressing files

gunzip decompresses files compressed by gzip.

Downloading files using ftp

Used to download files directly to your computer from another computer on the

Internet

can be used on the command line of the console or a terminal window

ftp name of a remote site.

FTP download

% ftp ftp.tenon.com

% ftp 206.246.150.88

FTP commands

! Run a shell command

ascii Specify text file downloads

binary Specify binary file downloads

bye Close the open connection and exit ftp

cd nnn Change to directory nnn

close Close the open connection and quit

get file Downloadfile from the current directory of the remote computer

help List help topics forftps commands

? gives a list of commands

open Open connection on a remote computer

pwd Print current working directory on the remote computer

put Send (upload) a file from your computer to the remote computer


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Using ftp to download files

Use theftp command from the command line followed by the name of a remote

system.

After ftp connects to the remote computer, the remote computer usually presentsa login prompt.

If you do not have an account, on the remote computer system, type anonymous

at the Name prompt and press Enter. At the password prompt, type your e-mail

address.

Installation of a program

Building programs with make command

The make utility provides a scripting capability to automate the compilation and

linking of files

If the program source comes with a makefile and the documentation says the file

supports Linux, making the program work on your system will be quite easy.

Make command

make < make target >

Make runs only on dependencies which means that certain action must be

completed successfully before the system can carry out another action.

This process is usually implemented to force make to build the program before

initiating the install script

It has built in defaults

Configure

usually walks you through a script to establish the programs configuration and to

configure other files before you can run make and make install.

If the documentation says the program supports configure option, you can run it

by typing make configure or make config depending upon your program


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install

To build and install the program, use the make installcommand.

This tells make to build the program and if everything is successful to install its

executable and configuration files in the appropriate directories with the correctpermissions ready for users on your system to use.

Clean

Large program often leave a substantial number of intermediate files lying about in

their source directories, which can consume a significant amount of space.

To make sure that these are cleaned up or to clean up after a failed build of the

program use the command make clean.


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Unit 6 Building a custom Unix Kernel

Overview of Kernel

There are three layers of the UNIX system.

Hardware

Operating System Kernel

User level program

Functions of Kernel

Kernel manages resources in the UNIX system.

It manages the processes, signals, virtual memory, file systems and network

connections.

All device drivers are linked into the kernel. It translates the request from

processes to device driver instructions.

Advantages of Custom Kernel

Faster boot time.

The kernel will probe only the hardware on your system.

Less Memory usage

Allows operating system upgrade and addition of new device drivers

Configuration file

The configuration file is contained in the directory /usr/src/sys/i386/confand is

named GENERIC

Each line contains a keyword and one or more arguments

An exhaustive list of options and more detailed explanation of the device lines is

present in the LINT configuration file, located in the same directory as GENERIC.

Mandatory fields

machine i386

Type of cpu


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ident GENERIC

maxusers n

# Floating point support do not disable

# Pseudo devices the number indicates how many units to allocate

Configuration of custom kernel

Make a list of your system hardware

Get the components required to make a kernel for your system. If it is not already

available, install it from the CDROM.

Make a duplicate of the GENERIC kernel. You may call it MYKERNEL. The generic

kernel will be available in/usr/src/sys/i386/confdirectory

Store a copy of the file MYKERNEL in another directory.

Configuration of Kernel

Build up a configuration file by making necessary changes to MYKERNEL.

Run configuration of the files while you are in /usr/src/sys/i386 directory

Run make depend

Type

# make on the command prompt

Install the kernel.

# make install

The new kernel is installed in the boot directory and the old kernel is saved as

kernel.old.

Test, debug and reboot from the new kernel.

Document the changes


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Unit 7

System Initialisation in Linux

Setting initial run-level

The initial run level in Centos Linux can be set by modifying the/etc/inittab file.

The default setting for this Linux is level 5 which starts up the system in multi-user

mode with full networking support with GUI.

Common runlevels and what they mean

0 halt the systems

1- single user mode

2- multi-user mode but with no networking support

3- multi-user mode with full networking support

5 multi-user mode with full networking support and start the graphical user

interface.

id:5:initdefault:

Entry in/etc/inittab shows the default setting in Centos.

Respawning process in Linux

you will learn how to configure a process so that it is respawned or restarted, if it is

terminated for any reason.

The most obvious practical application is to restart virtual consoles when users log

out of them.

The init process may be configured via the/etc/inittab file to run a processforever.

6:2345:respawn:/sbin/mingetty tty6

This line in /etc/inittab tells the init program to start the sixth virtual console

(tty6) whenever Linux is running in run levels 2,3,4 and 5 and to start the program

anytime it stops.

6:2345:once:/sbin/mingetty tty6

The login will not be spawned when a user logs out. It will be run only once


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Once changes are made to/etc/inittab files, you need to type

kill HUP 1 or

kill -1 1

showkeys comand

Shows the key code for any keys on the key board.

Type showkeys on the command line and press the key to find its key code

Page up ------ 104

Page down --- 109

Upper arrow --- 103


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Unit 8

Managing Linux processes

Displaying parent/child relationships

ps f

you can see PID and PPID values. The PPID is the PID of the parent process

Using the PPID values, you can see the parents of all processes

ps eH

e option selects all processes and the H option produces a process hierarchydisplay. Child process appears below their parents and is indented by two spaces.

ps axjf

The f option causes ps to use graphical characters ( \ and _) rather than spaces to

show the parent/child relationships.

pstree

all the processes on the system arranged in a parent/child hierarchy.

Customising ps output

ps e

-e option tells ps to display all processes. There are four columns: PID, TTY, TIME

and CMD

Other options

-f full listing

-j jobs formt

j job control format

l long listing

s signal format

v virtual memory format

X i386 register format ( only with Intel processor-based system)


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Selective listing of outputs

ps eo pid,cmd | more

Displays only PID and CMD columns


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Unit 9

Common Administrative tasks in Linux

Configuring virtual consoles

Edit /etc/inittab file to include new virtual terminals

Use the command kill -1 1 to let init know the changes

Check whether these terminals are active using w command

Testing your logging

In this section you will learn about the setting up logging options in

/etc/syslog.conffile in Centos Linux

You can test the syslog logging ( the file is called syslog) to see if it worked the way

you intended when you created or modified the/etc/syslog.conffile.

Syslog file

Entries in/etc/syslog.confhave the following format.

Facility.level destination

Facility is the name of the subsystem sending the message, level is the severity

level of the message and destination is the file, device computer of user name to

send the message to

facilities

kern The Kernel

mail The mail system

lpr The printer subsystem

daemon System Server processes.

auth Login authentication system


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Levels in the order of decreasing seriousness

Emerg System Panic

Alert Serious error requiring immediate attention

Crit Critical errors like hard device errors

Err Errors

Notice Non-critical messages

Info Informative messages

Debug Extra Information helpful for tracking down problems

None Ignore messages from this facility

Finding broken links and files with no owners

If there are files with no owners it is likely that they will be allotted to new users

when they are created. There can also be symbolic links that no longer point to a

file.

This section discusses techniques to find such files and links. The commands to

delete unowned files and redundant symbolic links are discussed.

To find all dangling links

# symlinks r / | grep dangling

To remove all dangling links

# symlinks r d /


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Understanding date and time stamps of files

Three time stamps for files

ctime

mtime

atime

Helps administrators with housekeeping

How do you determine the time stamp for a file

ls l gives mtime

ls l -time=ctime gives ctime

ls l --time=atime gives atime

How to create an alias to fine all the three time stamps

alias dir=ls l -time=ctime | grep root ;

ls l | grep root ; ls l -time=atime | grep root

When we type dir all the three time stamps are displayed

In Ex 9.4.1 , it should be entered while in /home/betty after creating a file file1

cat file1 changes the atime

Modification of file changes all the three time stamps

Except in cases where the file is opened and read, ctime changes for all instancessuch as the use of chmod, chown, chattr

Study the option +d and +a for chattr

Touch command

By default changes all the three time stamps

Study the a and m options of touch


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Unit 10

System Initialisation - Redefining Keyboard Lock/LED Operation

What will you learn in this unit?

To control the state of Caps Lock, Num Lock and Scroll Lock function of your

keyboard for your virtual consoles

By default, these three key locks are off for all virtual consoles.

You must press the lock keys to enable lock.

For example you must press the Num Lock key to put the keyboard in the Num

Lock state where the numeric keypads used for entering numbers rather than

controlling the cursor.

The state is indicated by a corresponding LED

setleds D +num

The Num Lock light or LED should turn on

Num Lock is set back to its default state by running the reset program .

You can also control the state of the associated lights or LEDs independent of the

associated key lock.

For example, you can turn on the caps Lock light or LED regardless of the state of

the keyboards Caps Lock state.

Eg : setleds L +caps turns on Caps Lock light regardless of the state of Caps Lock

state

Use of setleds command

You can set up the light to indicate that the user has mail, the file server cannot be

reached, the user is reaching the limit of his disk space and so forth.

You can run the program anytime you like from the command prompt or a script.

You can also run the program in the system startup scripts to preset the locks to a

known state.


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Unit 11 File systems in UNIX

File systems

Logical and physical disk drives

File system types

Organisation of directories in UNIX

Mounting a file system

Unmounting a file system

Shutting down a UNIX system

File system

is a collection of files and directories on a disk or tape in standard UNIX file system

format.

Logical drives

A logical drive refers to a subdivision of physical disk, which you can refer to as if itwere a disk by itself

Partitions in UNIX

FreeBSD uses the term partition differently as compared to Microsoft. It refers to

Microsofts partition as slices.

Microsoft designates one slice as the primary Microsoft partition, the C: drive.

Unix partitioning

Unix systems have their own form of partitioning and are not compatible with the

Microsoft methods.

UNIX implements their own partitioning within a single slice (Microsoft partition).

BSD systems define up to 8 partitions per slice


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Naming partitions in Free BSD

The device entries are in the directory/dev

The name of the driver comes next. It also indicates whether it is an IDE or SCSI

drive

The first disk is indicated by /dev/ad0.

Next is the partition information. The strict slice name is specified by adding the

letter s (for slice) and the slice number. /dev/ad0s1 indicates slice 1 in first disk.

BSD systems name partitions by appending the letters a to h to the disk name.

Thus the first Unix partition of the first slice (which is typically a root file system)

would be called /dev/ad0s1a.

Partition c is an exception. It represents the whole BSD disk where the FreeBSD

resides.

File system types

ufs is the UNIX Files System.

cd9660 is ISO 9660 CD-ROM format with the so called Rock Ridge Extensions whichenable UNIX-like file names to be used.

nfs is the Network File System which is a means of sharing file systems across a

network.

Microsoft files can be accessed with msdos and ntfs file systems

Organisation of directories in Free BSD

/ rootdirectory

/sbin System executable programs needed at start time - System administration

files

/bin executable programs of general use which are needed at start up time

(binary files)

/dev directory of device nodes

/etc Contains many of the configuration files


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/root home directory of user root

/usr/bin common user binaries not required at start time

/usr/sbin System administration programs not needed at start time

/usr/share dictionaries, configuration files for user programs

/usr/share/man manual pages

/var a file system for log files , files which grow or change frequently

/var/spool mail and printer queues

/usr traditionally for users home directories but no longer. Second file system

/usr/local locally installed software

/home home directories of the user


Information on a disk slice or partition becomes available for user only after the

kernel has been notified of its existence through the mountcommand

Mounting a file system logically attaches the information stored in that file systemto a specific directory that already exists.


Each file system is itself a tree with a root directory.

It can be attached or mounted on any directory in the UNIX tree.

Mounting it on one directory versus another changes the path name of the files

down to the root of the file system but has no effect on the structure of the files

and directories within the file system.

Unmounting a file system

For repair

To save memory resident information about the file


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If any part of the file system were used, UNIX would not unmount it.

Some useful commands

df

prints a summary of the free space in all the mounted file systems.

du

Shows the usage of the disk ( Disk Usage )

Shutting down a UNIX system

Shutdown provides an automated shutdown procedure for super users to nicely

notify users when the system is shutting down

Use shutdown [+ time in minutes] command

Options

No options goes to single user mode

Shutdown +1 sends to single user mode in 1 minute

-h halt

-r reboot

shutdown -h now

Shutdown and halt the system immediately

Single user mode

This mode has only one user the person at the console

The user must be at the console.

There is no password protection, so physical security is the only security

Not many processes are running generally- only initand shellyou are using areactive.


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Generally the prompt is #

Limited set of tools

Getting into single user mode

Set boot flag in the boot program. Typically s represents single user mode.

Boot -s

Once operating system is running , use shutdown command

Getting out of single user mode

CTRL-D will exit and take you into multi-user mode provided there are no

problems with the boot process.

Issue reboot or halt command

Use of a single user mode

There is only one user on the system. There is less activity and less damage if acrash occurs.

It is minimal mode that the system can boot if other file systems are corrupt.

Allows the user to unmount the/usrand other normally busy file systems.


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Unit 12

Unit 12 Compiling and linking with gcc

gcc compiler

gcc is a C compiler developed by Free Software Foundation , which has developed a

number of tools for use on a UNIX system

It compiles the source code and assembles the assembly language code .

The compiler produces and invokes the UNIX loader ldto produce an executable

file.

It also includes a compiler for the C++ language.

Fig 12.1 Stages of compilation of gcc

Stages of compilation

Pre-processing

It is this step which resolves directives like #define #include. A separate utilitycalled cpp is used to do the pre-processing


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Compilation

This produces assembly language files from the input files. The assembler is usually

invoked right away. So output is not normally saved in files.

Assembly

This takes the assembly language as input and produce object files with .o

extensions.

Linking

In this final stage, the various .o modules are placed in their proper places in the

executable files. The library files are also placed in the file. UNIX compilers perform

this phase by internally invoking the linker, which is called Id.

options

c implies Compile but not link

-g Generates an expanded symbol table for debugging. This option enables

debugging with GNU debugger gdb

-v prints the compilers version number and completer details about how each

pass is executed.

-w Suppress all warning messages

-W produce some additional warning messages about legal ( but questionable )

coding practices.

-Wall Produces even more warning messages about questionable coding

practices

-Werror Make all warnings into errors. Do not attempt to produce an object file ifa warning has occurred.

mainprg.c

#include

void printheading (void), printmaxnum(int m);

int funmax (int m, int n);


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int main (void)

{

int a,b,max;

printf("Enter two numbers\n");

scanf("%d%d", &a, &b );

printheading();

max=funmax(a,b);

printmaxnum(max);

return(0);

}

util1.c

#include

int funmax ( int m, int n)

{

if ( m > n)

return (m);

else

return(n);

}

void notused(void)

{

printf ("\n not all functions have to be used \n");

return;

}


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util2.c

#include

void printheading(void)

{

printf("This is a multiple source project \n");

return;

}

void printmaxnum ( int m)

{

printf("The larger number = %d\n", m);

return;

}


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Unit 13

Awk and Sed utilities

Awk utility

Awkutility is a powerful data manipulation programming language that allows the

user to select lines of input based on specified criteria, take action on the data on

these lines.Awkworks on information arranged in a database file.

General structure of an awk command

The general form of the awkcommand is as follows

command pattern {action} input file

awk /Chicken/ {print} groc_base

Selecting lies with specific patterns

The patterns can be matched by operator (~) for a specific field.

awk $1~/C/ groc_base

selects all lines which have C in the first field.

The caret (^) in a regular expression forces a match at the beginning of a line orfield

awk $2~/^M/ groc_base

selects all lines which have M as the first letter of second field.

The dollar ($) forces a match at the end of a line or field.

awk $2~/eg$/ groc_base


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selects all lines which have eg as the last part of a field

Printing a specific field

To print a specific field, specify the first field as $1, second as $2 and so on

awk /Veg/ {print $1 $3 } groc_base

awk /Veg/ {print $1, $3 } groc_base

awk $1~/^[CM]/{print $1, $2, $$3} groc_base

Selecting specific fields

We can select records which satisfy a certain condition using equal to ( = =), greater

than (>), less than ( 1.00 groc_base

Creating a command file

We can create a file which contains the pattern and action. This file can be run with

the awkcommand

Create a file calledprint_dairywhich contains the following line

/Dairy/{print $1,$3}

using catcommand

To run the program enter the following command.

awk -f print_dairy groc_base (-f option tells awkto read a file for its program)


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Improving the readability of files

/Dairy/ {

print $1 , $3

}

/Dairy/ {

name = $1

price = $3

print name, price

}

Mathematical operations

The words within print statement can be quoted and operations can be indicated

in the statement. Operations can be indicated with the variables.

The operations are addition (+), subtraction (-), multiplication (*), division (/)

Example of program with mathematical operations

groc1

name=$1

type=$2

price=$3

qty=$4

taxable=$5

discount=0.1

saleprice=price - discount * price

print name, type, saleprice, qty, taxable


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To run groc1 use the command

awk -f groc1 groc_base

BEGIN and END patterns

The awk utility performs the action associated with BEGIN before it processes any

of the lines of the data file. The END pattern works in a manner similar to BEGIN

pattern except that awktakes action associated with it after it has processed the

last of its input lines.

Example of begin and end patterns

BEGIN {

print The list of groceries purchased

print --------------------------------------

print Name Type Unit price Quantity Taxable}

{print}

END {

print NR, items were purchased}

Flow control in awk

Awkuses the if, if-else, forand while loops of C language.

We will have a simple example of a if- else loop here

{ if ( $3 < 1.00)

print $1, $4

else

print $3, $4 }


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Sed editor

UNIX filter

Takes input from a file but its output is sent to standard output without altering

the original file

Syntax

sed line specifier, editing instructions filenames

Line specifier

You can give a line number or a range of line numbers..

3 -third line

5,10 - lines 5 through 10

10,$ - lines 10 through the last line

You can specify a line for sedby providing a matching pattern enclosed in slashes.

/large/ lines containing string large

/[Pp]at/ all lines containing Pator pat

You can combine two forms of line identification.

2, /talk/ from line 2 to the first mention oftalk

Editing instructions

s (substitute)

p(print)

d(delete)

q (quit)


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1i\

text inserts the text before the line 1

2a\

text appends the text after the line 2

Options for sed

-n suppresses the automatic printing of each processed line

-f option followed by a space and filename takes editing commands from anamed file

sed -f edit speech

Commands in editfile are applied to speech

-e option allows the mixing of a command line editing command with commands in a

file

Examples

sedscript1

10 a\

January

sed f sedscript1 months

Appends January after the 10th line


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Practice Exercise

Create a file days

Sunday

Monday

-------

Thursday

Add Friday and Saturday after Thursday

sed f addfile1 days

Create the file addfile1

Add a heading for the file using a file addfile2

Days of the week


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Unit 14

Testing sedscripts

The command

% sed f sedscr testfile

does not make the change in testfile.

It sends to the screen the lines that were modified and the lines that were

unchanged.

You have to capture this output in a new file if you want to save it.

% sed f sedscr testfile > newfile

The output from the command should not be redirected to the input file. This

would overwrite the input file.

Use the diff program to point out the differences between the two files.

% diff testfile newfile

When the results are verified, a backup copy of the original input file can be made

. you can use the mv command to overwrite the original with the new version.

for x

do

sed f sedscr $x > tmp.$x

done

The shell script testsedautomates the process of saving the output of sed in a

temporary file. It expects to find the script file sedscrin the current directory and

applies these instructions to the input file named on the command line.

The output is placed in a temporary file

The name of the file must be specified on the command line. The shell script savesthe output in a temporary file with prefix tmp


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% testsed filename

Diff command

You can also incorporate diff command into the shell script.

add diff $x tmp.$x after the sed command

Modify the file to incorporate the diff command.

#!/bin/sh

for x

do

echo "editing $x: \c"

if test "$x" = sedscr

then

echo "not editing sedscript!"

elif test -s $x

then

sed -f sedscr $x > ./tmp/$x

if test -s ./tmp/$x

then

cmp -s $x ./tmp/$x && echo "file not changed: \c";\

cp ./tmp/$x $x ; echo "done"

else

echo "sed produced an empty file - check your sedscript."

fi

else

echo "original file is empty"

fi


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done

echo "all done"

To run runsed, create a sed script named sedscrin the directory where you want to

make the edits. Supply the name and names of the files to edit on the commandline.

%./ runsed chapter1

runsed invokes sed f sedscr on the named files one at a time and redirects the

output to a temporary file. The program then tests this temporary file to make

sure that the output was produced before copying it over the original

The shell script incorporates testsed. The additional lines are intended to test for

unsuccessful runs for instance when no output is produced.

It compares the two files to see if changes were actually made or to see if an

empty output file was produced before overwriting the original.

You should used testsed first to verify your changes before actually making them

permanent with runsed

Create a new program testsedmod, which can do the same task as testsedwhen

you enter the command

% ./testsedmod sedscript1 months

There should be a reference to sedscript1 and months inside the program as $1

and $2

A program prog1

echo $1 $2 $3

echo $1 $2 $3

echo $1 $2

echo $0

After exec permission is given

Run the script ./prog1 one two three


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What is the output?

Answer

one two three

one two

prog1


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unit 1 connecting a unix server to network and enabling services

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