Install HTTrack On CentOS

Since I could not find the rpm in the repo, here is the quick How To to install HTTrack website copier on CentOS.

$ yum install zlib-devel
$ wget http://download.httrack.com/cserv.php3?File=httrack.tar.gz -O httrack.tar.gz
$ tar xvfz httrack.tar.gz
$ cd httrack-3.47.27
$ ./configure
$ make && sudo make install

This should do all. If you wish not to install zlib compression support, you can skip the first step and run the configure as ./configure --without-zlib. I hope this helps :)


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JPEG To PDF With Imagemagick

ImageMagick is an awesome toolkit with several powerful features for image creation and manipulation. You can use ImageMagick to translate, flip, mirror, rotate, scale, shear and transform images, adjust image colors, apply various special effects, or draw text, lines, polygons, ellipses and Bezier curves. Here, I will show how you can use ImageMagick suite to convert JPEG to PDF quickly.

First make sure imagemagick suite is installed in your system.

Ubuntu/Debian

$ sudo apt-get install imagemagick

CentOS/Fedora

$ sudo yum install imagemagick

Below are some of the examples of using convert which is a part of ImageMagick to convert Jpeg to PDF.

Single Image

$ convert image.jpg image.pdf

Multiple Images

$ convert 1.jpg 2.jpg 3.jpg output.pdf

Resize and Convert

$ convert -resize 80% image.jpg image.pdf

Negate and Convert

$ convert -negate image.jpg image.pdf

You can actually use different available switches to get your output as expected. I usually use PdfTk in conjunction with this technique to work in different scenarios and it really works great. I hope this helps :)


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Fix Your Ubuntu

Recently Ubuntu has been known for turning into an advertising company and has been accused of not protecting user's privacy so just came across this site that fixes your ubuntu by applying some patches to turn off some of the invasive features of Ubuntu.

FixUbuntu.com


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Recursion and Memoization - A Fibonacci Example

In this post, I will try to describe why memoization can be a great optmization technique in the recursive function implementations with an example of fibonacci sequence.

Straight from Wikipedia, memoization is an optimization technique used primarily to speed up computer programs by having function calls avoid repeating the calculation of results for previously processed inputs.

Basically, we maintain a lookup table and store the computer values for particular cases which lets us query and use the corresponding value for particular case present in the lookup tables. This reduces function call overheads. Now in order to understand why this is a great optimization technique in recursion, lets first draw a recursion tree for finding n^th term in fibonacci sequence.

                           fib(5)                          
                             /\                              
                            /  \
                           /    \
                          /      \                      
                     fib(4)       fib(3)                                 
                      /\               /\                          
                     /  \             /  \
                    /    \           /    \
                   /      \         /      \                         
               fib(3)    fib(2)     fib(2) fib(1) -> 1                                    
                  /\         /\          /\                          
                 /  \       /  \        /  \                         
                /    \     /    \      /    \
               /      \   /      \    /      \                        
          fib(2) fib(1) fib(1) fib(0) fib(1) fib(0) -> 0                                        
          /\        |     |      |        |    |               
         /  \       1     1      0        1    0               
    fib(1) fib(0)                                                       
       |      |                                               
       1      0 

We can clearly see the calls to fib() with same arguments several times. For example, fib(1) is called 5 times and fib(2) 3 times. Thus, we are repeating same calculations multiple times and imagine how this would look like for large value of n. If we would have maintained the value of fib(n) in the lookup table when computed the value for the first time.

The python code without memoization looks like below and notice the runtime:

#!/usr/bin/python

def fib(n):
        if n == 0:
                return 0
        if n == 1:
                return 1
        val = fib(n-1) + fib(n-2)
        return val

print fib(50)

And, now with the memoization, you will notice significant improvement in runtime.

#!/usr/bin/python

known = {0:0, 1:1}

def fib(n):
        if n in known:
                return known[n]
        known[n] = fib(n-1) + fib(n-2)
        return known[n]

print fib(50)

If you run and compare above two codes, you will find that the addition of memoization significantly improves the performance of recursive functions. Recursion are generally known to be terribly slow however memoization can make the difference insignificant. Some languages now provide memoization as the language feature natively or via third party APIs such as groovy memoize.


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Pattern Based Database GRANT In MySQL

At our workplace, we need to manage database access for different teams and rather than adding another grant on the addition of new database, I've been following a pattern based database access grants in MySQL.

We let different teams work on replicas of same database and hence append the terms such as _dev and _qa as the database prefix. And, we define GRANTS based on these patterns. An example would be something like below:

GRANT ALL ON `%\_dev`.* TO 'user'@'%' IDENTIFIED BY 'password' WITH GRANT OPTION;

I hope this proves useful for some of you guys :)


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How I am Trying To Keep My Eyes Safe On Computer

Lately I've been on computer a lot and with this, the usual problem with most computer users has started to bother me. Going through some of the blogs online for keeping eyes safe while using computer, I came through few suggestions and in this post, I'm writing how I'm trying to keep my eyes safe. Though not tremendously helpful for everybody, I thought I would share this and you could also use my technique.

The problem with computer addicts is not getting their eyes off the computer for much longer period and though I've been trying to remember to keep my eyes off the computer in regular interval, I usually never implement this.

My two principles based on my readings on different websites are:

• 20-20-20: In the 20 minutes interval, keep your eyes away for 20 seconds (& view other objects which are around 20 feet away)
• 2 hrs rule: In the 2 hours interval, stay away from computers for at least 2 minutes.

But, you can not really follow the rules so easily and I had to find some other alternative to do so. This is how I am doing it now.

Create two cron jobs for each of the above mentioned methods such that notify-send is triggered in each 20 minutes and each 2 hours informing you to keep yourself safe from computers. So my /etc/crontab looked like this:

*/20 * * * * techgaun export DISPLAY=:0.0 && /usr/bin/notify-send -i /home/techgaun/Samar/scripts/eye_inv.ico "20 - 20 - 20" "Time to take rest. Keep your eye safe :)"
01 */2 * * * techgaun export DISPLAY=:0.0 && /usr/bin/notify-send -i /home/techgaun/Samar/scripts/eye_inv.ico "2 hrs eye rest" "Time to take rest for 2 minutes. Keep your eye safe :)"

You need to replace techgaun with your username and need to give correct path to the ico file if you like to use icon like me. Otherwise, you could just omit the icon in notify-send command. I hope this proves useful for some of you :)


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Two Ways To Print Lines From File Reversely

Ever tried to print lines in files in the reverse order? You will know two simple methods to print lines from file in the reverse order.

Imagine a file somefile.txt with content something like this:

a
b
c
d
e

Method 1:

$ tac somefile.txt
e
d
c
b
a

Method 2:

$ sort -r somefile.txt
e
d
c
b
a

You can achieve the same effect through other techniques as well but I'll stick to these simple ones :)


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Compile 32 Bit Binaries On 64 Bit Machine

Well I had this special need if you recall my previous blog post since my friend had 64 bit machine. Sometimes, there might be this necessity to compile 32 bit binaries on your 64 bit machine. This post describes how to do so.

First make sure the necessary x86 libraries are installed. We require 32-bit shared libraries for AMD64 to compile binaries in 32 bit format. The command below installs the i386 version of libc6-dev:

$ sudo apt-get install libc6-dev-i386

Now you can compile your code in 32 bit binary format using the -m32 flag where 32 represents the x86 processor (-m64 would mean x64 processor).

$ gcc -m32 -o test test.c

I hope this helps :)


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