Technical Foundation of Computer Science 5

Network Programming I

Maria Sawaby

Department of Communication and Operating System

May 4, 2015

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Contents

1 File In, File Out

2 Making Files Immutable

3 Loopback Files and Mounting

4 Mounting ISO �les as loopback

5 Finding di�erence between �les, patching

6 wc

7 Printing directory tree

8 Summary

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File In, File OutGenerating �les of any size

The easiest way to create a large sized �le with a given size is to usethe dd command dd if=/dev/zero of=new�le bs=1M count=1

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Intersection and set di�erence on text �les

intersection and set di�erence operations are used in mathematicalclasses on set theory

we can have similar operations on text �les

comm command is used to perform comparisons between two �lesI Intersection: will print lines that are common between two �lesI Di�erence: will print lines which are not the same in all �les

Note that comm takes sorted �les as input

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Intersection and set di�erence on text �les

$ sort �le1 -o A.txt ; sort �le2 -o B.txt

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Intersection and set di�erence on text �les

Some options are available to format the output as per ourrequirement

I -1 removes �rst column from outputI -2 removes the second columnI -3 removes the third column

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Intersection and set di�erence on text �les

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Making Files Immutable

When a �le is made immutable, any user or super user cannot removethe �le until the immutable attribute is removed from the �le

chattr can be used to make �les immutable

$sudo chattr +i �le1

$rm �le1rm: remove write-protected regular �le '�le1'? yrm: cannot remove '�le1': Operation not permitted

sudo chattr -i �le1

make /etc/shadow �le immutable to make it secure from modi�cation

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Loopback Files and Mounting

We normally create partitions on a physical hard disk

Using Loopback �lesystems we can create in �les rather than aphysical device!

$dd if=/dev/zero of=loopback�le.img bs=1GB count=1

$mkfs.ext4 loopback�le.img

$sudo �le loopback�le.img (or: �le -b lookback�le.img)

$sudo mkdir /mnt/loopback

$mount -o loop loopback�le.img /mnt/loopback

This is the shortcut method. We do not attach it to any devices. Butinternally it attaches to a device called /dev/loop1 or loop2

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Loopback Files and Mounting

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Loopback Files and Mounting

We can do it manually as follows:

# losetup /dev/loop1 loopback.img

# mount /dev/loop1 /mnt/loopback

# umount /mnt/loopback

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Mounting ISO �les as loopback

An ISO �le is an archive of any optical media. We can mount ISO �lesin the same way that we mount physical discs by using loopbackmounting.

A mount point is just a directory, which is used as access path tocontents of a device through a �lesystem

# mkdir /mnt/iso

# mount -o loop /media/maria/30ECFD7EECFD3EA2/ubuntu-10.04-desktop- i386.iso /mnt/iso/

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Mounting ISO �les as loopback

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Finding di�erence between �les, patching

If the �les are of 1000s of lines, they are practically very di�cult andtime consuming to compare!

The di� command utility is used to generate di�erence �les

$ di� -u version1.txt version2.txt

-n option is used to show line number

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Finding di�erence between �les, patching

A patch �le can be generated by redirecting the di� output to a �le,as follows:

$ di� -u �lev1.txt �lev2.txt > version.patch

Now using the patch command we can apply changes to any of thetwo �les

To patch a �le:

$patch -p1 �lev1.txt < version.patch

To revert the changes:

$patch -p1 �lev1.txt < version.patch

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patching

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Counting number of lines, words, and characters in a �le

wc is the utility used for countingcount number of lines:

$wc -l �lein order to use stdin as input:

$cat �le | wc -l

count number of words:

$ wc -w �le$cat �le | wc -w

count number of characters:

$ wc -c �le$cat �le | wc -c

count number of characters in a text:

$echo -n this is a text | wc -c

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wc

when wc is used with no option:

$wc �le

it will print number of lines, words and characters

wc can also be used to print the length of longest line using -L option

$wc �le -L

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Printing directory treetree command

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Printing directory treeHTML output for tree

It is possible to generate HTML output from the tree command

$ tree . -H http://localhost -o out.html

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Summary

Generating Files of any size by dd command

�nding di�erences and intersections in �les using comm command.

making �les immutable using chattr

mounting loopback �les using dd, mount, losetup

patching using di� and patch commands

counting number of words, lines and characters using wc command

printing directory tree by tree command

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