Thursday, July 23, 2009

Git supports commiting specific parts of a file

While working on my web based learning project, I made a lot of changes to a Django view file without committing them. When I actually decided to commit, I realized that the changes in my view file should go into separate commits. I knew it was possible to commit parts of a file in Git, but I was not sure exactly how it could be done.

I asked the question on StackOverflow and got an answer within minutes :-)

Anyways here's how I did it. I had 3 files in my index:


urls.py
app/courses/templates/course/show.html
app/courses/view.py


The first 2 files were fine, but I did not want to commit the entire view file with the first two, I wanted only one change in that file to go in this commit.

Since I had already added the view file to the index, I first had to unstage it with:


git reset app/courses/view.py


Then I did an interactive add to add only one hunk from the view file:


git add --patch apps/courses/view.py


Running this command showed me the first hunk from the file and an option asking me if I wanted ti stage that hunk. I decided I did not want the first hunk, then it showed me the second hunk. This was the change I wanted, so I entered 'y' at the option and proceeded to enter 'n' for all the remaining hunks.

After this I wanted to ensure that the right hunk from the file was staged to be committed, so I typed:


git diff --color --cached apps/courses/view.py


This showed me all the changed that were staged. After ensuring that the changes I wanted were the ones staged, I commited them with:


git commit

Saturday, July 11, 2009

JQuery selectors and future elements

I am using JQuery for the AJAX and dynamic aspects of my web based learning platform.

I like the fact that JQuery allows us to use CSS type selectors to select elements on the page to either manipulate them, add event handlers, or a host of other things.

So, the code below intercepts clicks on all links and dynamically adds a new link when a link is clicked.


<html>
<head>
<link rel="stylesheet" type="text/css" href="/site-media/al/style.css" />
<script type="text/javascript" src="./jquery-1.3.2.min.js"></script>
</head>

<body>
<script type="text/javascript">
$(document).ready(function() {
$("a").click(function() {
$("ul#questions").append("<li><a href='#'>click me</a></li>");
});
});
</script>
<div>
<ul id="questions">
<li><a href="#">Click Me: 0</a></li>
</ul>
</div>
<textarea style="Width: 400px; Height: 200px" name="textarea" cols=20>
<head>


There is a problem with this code. Everytime you click on the link called "Click Me: 0" a new link is added, but if you click on the new link, then nothing happens. So, why does JQuery not work with dynamically added elements?

The reason is because the following call


$("a").click(function() {
//...
});


only binds to the anchor elements present on the webpage when the $(document).ready() function is called. It will not bind to future elements. To bind to future elements as well, we have to use the live() function.

Try using this code to bind to the "a" elements and it will work.

$("a").live("click", function() {
$("div#questions").append("click me");
});

Thursday, July 02, 2009

Refreshing iptables

The last time I played around with IPTables was about 6 - 7 years back. I have been working with a Linux box again from the last few months and I am absolutely thrilled about it. Nevertheless, I have also been meaning to set up a simple IPTables firewall on my machine.

Since I had forgotten all my IPTables concepts, I decided to hunt the Internet for some good articles. I found some really nice resources to refresh my memory as well as learn new things.

Here is a nice 3 part video series from Linux Journal.
  1. iptables part 1
  2. iptables part 2
  3. iptables part 3
The entire series takes less than a half hour and is a good refresher or introduction to IPTables.

Here is a nice picture which explains how a packet is routed through various chains in the ip tables. If you are looking for a quick refresher, this might help you out.

If you want more details, here is a tutorial, and yet another tutorial, and Netfilter's excellent documentation on IPTables.

So now there is no excuse for not setting up a little firewall on your Linux box.