vim substitution tricks

I wanted to figure out a way to add an incrementing number before a certain character on each line for a program I was writing.
In vim this is fairly simple to achieve.

Here is a sample of some lines in my file

number=1234
number=4543
number=7978
number=3321

Now I want to add an incrementing number before the "=" so I have some unique variables. I can either select the lines I want in vim using the visual selection (shift-v) or I can put an address specification.
For this example I used the visual selection so I get and address specified as '<,'>

Now I supply the ex command

:let i=1 | '<,'>s/=/\=i."="/ | let i+=1

This will now give me

number1=1234
number2=4543
number3=7978
number4=3321

This seemed to work the same as the following command but less typing

:let i=1 | '<,>'g/=/s//\=i."="/ | let i+=1

However if I wish to change the increment I have to use the long command, e.g. to go up in steps of two, change the ex command to

:let i=1 | '<,>'g/=/s//\=i."="/ | let i+=2

Which now gives me

number1=1234
number3=4543
number5=7978
number7=3321

The string to replace in the search and replace pattern above starts with "\=" indicating that this is the start of an expression, subsequent strings are concatenated with ".".

python and numbers 08 and 09

I had noticed that the syntax highlighting in vim did not seem to pick up the numbers 08 and 09 in some code I was writing. However, it was only when running some code containing these numbers that I appreciated what was happening.

If numbers have been represented with a leading 0 then these are interpreted as octal numbers and therefore each digit after can only go up to 7, therefore 08 and 09 are invalid.

This can be seen if you try to run the following code

number = 08
print number

You get an error similar to

  File "number", line 4
    number=08
            ^
SyntaxError: invalid token

Random Programming challenges

I have found some interesting websites that set interesting problems to challenge your programming skills and as I have been messing round more with python I have decided to have a go at some of them using python.

A particularly interesting one is Project Euler where I have just achieved the award Decathlete for solving the first ten problems. I suspect the rest will be much harder.

Another challenging set of problems is at http://escape.alf.nu/ this guides you through some common pitfalls in javascript programming. The aim is to get the sample code to call alert(1).

Rookie mistake python loops and variable scope

While writing some simple python code I stumbled upon a rookie mistake which I appear to still be making so writing it down here to remind me.

Scope of variables is important in loops especially when using them to increment.

I was writing some code to work out if the product of two 3 digit numbers is a palindrome (not very elegant algorithm just brute forces it). Here is my original algorithm

a=100
b=100

numbers=[]

while a < 1000:
  while b < 1000:
    product=a*b
    print a, b, product
    if (product == int(str(product)[::-1])):
        print "palindrome"
        numbers.append(product)
    b+=1
  a+=1

From the output I noticed something strange the outer loop only went round once even though I thought I had incremented a after the inner loop.

Then it struck me I had defined b outside my while loop so it would not get reset upon next iteration of the outer loop and therefore the inner loop would not run again (and print something out). Therefore it looked like I was only looping once on the outer loop.
This could be proven by adding "print a" before incrementing a on the last line.

Moving "b=100" inside the outer loop solves the problem.

Working code looks like so

a=100
numbers=[]
while a < 1000:
  b=100
  while b < 1000:
    product=a*b
    print a, b, product
    if (product == int(str(product)[::-1])):
        print "palindrome"
        numbers.append(product)
    b+=1
  a+=1

Another alternative is to change the while loops to for loops.

Easter egg in gnome-shell

I have just found and easter egg in gnome-shell.

In gnome, move the cursor up to the top left corner as if you were going to open an application, then in the search box type "free the fish"

Wanda

Clicking on the icon or hitting return will bring up a dialog box containing a random fortune.

Drag and Drop in a webpage

I saw a cool feature on a website allowing you to drag and drop a file onto the page and it will be uploaded.

I wondered how this was done and after some experimentation I believe I understand how to do this. I should note that I was working with firefox on Linux and I have not tested this code in other browsers on other platforms.

I created a simple blank html page with a line explaining to drag and drop a file onto the page.

Then a little javascript to handle the dragging and dropping a file.

In my script I need to get the whole document (or the body element) which can be done with
document.documentElement

Then I need to add some event listeners to this to listen for the drag and drop events (in particular the "drop" event). I created some functions to be called when these events are triggered.
To prove a file has been dropped all I did was popup an alert with the name of the file dragged and dropped.
My test code only handled the first file dropped, but it should not be too difficult to add a loop to process the rest.

So my basic javascript would look like this

function noop(e) {
    e.stopPropagation();
    e.preventDefault();
    return false;
}

var drop = document.documentElement;

drop.addEventListener("dragover", noop, false);
drop.addEventListener("dragexit", noop, false);
drop.addEventListener("drop", function(e) {
     e.stopPropagation();
     e.preventDefault();
     //only get the first file, add loop for multiple file handling
     var files = e.dataTransfer.files;
     var file = files[0];
     alert("file '"+file.name+"' dragged and dropped");
     return false;
},false);

This was not successful and kept giving me an error in firefox, this was probably due to trying to do things before the document was ready. Therefore I used jQuery to ensure the document was ready, now my code was wrapped like so

$(document).ready(function() {
...
});

However, this still did not seem to be working. I found some old bugs which suggested that firefox had problems with drag and drop from anything other than nautilus, but I was using nautilus so it should work.
Then it struck me I was dragging items from the recent files window in nautilus, could these be references to the files? I opened the actual directory where the test files were and drag and drop worked perfectly.

I did not do anything with the file dropped, but is should not be difficult to add upload code to pass to a server side script for processing.

Reversing Shellcode

I was looking at some shellcode and there are lots of articles on how to compose shellcode but not that many on how to get it back to assembly for analysis.

The easiest way appears to be to dump the shellcode to a file,  this can be done with a perl one liner e.g.

 perl -e 'print "\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e"' > shellcode

 Then using ndisasm from nasm to convert this shellcode back to assembly mnemonics. The shellcode I was looking at was for 32bit architecture therefore I can use the following

ndisasm -b 32 shellcode

Which gives me
00000000  682F2F7368        push dword 0x68732f2f
00000005  682F62696E        push dword 0x6e69622f

It should be noted that this is a nonsense example as the shellcode only pushes "/bin//sh" onto the stack and does not do anything with it, however it shows how to use the tools.