blog of someone who never forgets to forget

Making ubuntu LiveUD

2009-03-12T07:39:00.000-07:00

If U have to install Ubuntu and you do not have CD drive .You can make a LiveUsbDisk to install ubuntu from your usb disk.

UNetbootin is a cool util with which U can do it. They have well documented how to make it in their wiki so there is no point in repeating it here.

I installed ubuntu 8.10 through it haven't really tried other linus distros.But they offer the option of many other distros.

rsync for windows

2009-02-26T07:24:00.000-08:00

Transition from *nix to windows platform is hard..it's high time someone writes "*nix to *dos for dummies". Last day I had to sync two folders and I was searching something similar to rsync in unix.Usually most of my search for tools in windows ends up in a free trail installers.Luckily this time i was blessed. Although not so feature rich as rsync it does the work done .I am talking od xcopy command in windows command promt.

enough of bla bla bla tell how do we sync to folders..

xcopy [source] [destination] \s \d

\s : for subdirectories (similar to -r in *nix)
\d : for copying just the folder differences

onsubmit attribute of the html form

2008-10-29T20:29:00.000-07:00

Last day my friend had a problem...she had to do some extra work with the submit button other than embedding the parameter values to HTTP request. She had to clear the form contents before the form get submitted. So the basic question comes down to...

How do we change the behavior of Submit button of the HTML form ??

HTML form tag contains an intrinsic event attribute called onsubmit. The onsubmit event is fired on the click of the submit button. And the form does not get submitted if the expression returns a false (it gets submitted on all the other cases...whatever scrap U write in the expression)

So the only way to stop the html form from embedding the parameters to the HTTP request is to return a false from onsubmit event function should look something like the code below or something which does the same.

<`form action="were_is_the_next_page.jsp" onsubmit="return false;" />

Remember return false do not assign false ... Advise from someone who already fell into the pit..it was too dark out there.. :)

Typical example of using onsubmit button.

I want to reset the form after submitting the data.

resetFormSubmit = function(formObj)
{
formObj.submit();
formObj.reset();
return false();
}

<`form action="something.jsp" onsubmit="resetFormSubmit(this)" >
<`input type="text" name="myVal" />
<`input type=submit value="Submit"/>
<`/form>

Now what role does the return false do here...Try the code with no return false U will see that the request parameters gets filled with resetted value. Its because the form values get embedded to HTTP request twice..Once on the call of formObj.submit() and the second time after it comes out of the resetFormSubmit button.Thus the next page get called with resetted values.

My understanding of sessions in web application.

2008-09-24T03:10:00.000-07:00

Most often in web applications we might need to manage information over a series of requests or over a user session. Lets take the classic example of adding 2 numbers. In an ordinary scenarios we will use a form to get the 2 numbers ,send it to the server which adds the no and sends it back to the user as response. The above mentioned computation consists of only one request and response. Now we want the interactions in different way. Instead of sending the first number and second number in a single page we want the first page sending the first number and second page sending the second number and a third page showing the results. These series of request can be called as user session. Now the issue with this design is that the computation spans over 3 request-response round trip. As HTTP is a stateless protocol, second request does not have any relation to the first request for the server. So now the question is how do we build this relation or how do we make the server aware that a series of request comes under one user session.

There are many ways through which the state information over a series of requests can be stored. It can be done by storing the information in the client side(cookies),in the server side(sessions,continuations) carry it with the request and response headers(hidden form fields).

Session management in servlet container

The container maintains memory space for each session which is identified by an id. All the information we need to keep over a session are maintained in this memory space and are maintained as java objects. The unique id known as session id is used to identify which session a particular request belongs to. So obviously session id has to be carried along with the request. The container allots the session object according to session id of the request.

Now its time to get our hands wet...We shall create 3 JSP pages

firstpage.jsp

<.html>
<.head>
<.title> The first page< /title >
< /head >
<.body>
< .form action="secondpage.jsp" method="get">
First no :
<.input type="text" name="firstno" value="0"> <>
<.input type="submit" value="Next">
< /form >
< /body >
< /html >

This page serves as interface to get the 1st no from the user. The page gets the no and puts it in the request object.

secondpage.jsp

<.html>
<.head>
<.title> The Second page< /title >
< /head >
<.body>
< % String a = request.getParameter("firstno"); session.setAttribute("firstno",a); % >
<.form action="resultpage.jsp" method="get">
Second no :
<.input type="text" name="secondno" value="0"> <>
<.input type="submit" value="Next">
< /form >
< /body >
< /html >

Similar to the first page this page receives the 2nd no puts it in the request object. In addition, it also adds the value of the 1st no to the session object so that it can be retrieved later.

resultpage.jsp

<.html>
<.head>
<.title> Result Page< /title >
< /head >
<.body>
< % int b=Integer.parseInt(request.getParameter("secondno")); int a=Integer.parseInt(session.getAttribute("firstno").toString()); out.println(a+b); % >
< /body >
< /html >

This page processes the input values and gives the result. It retrieves the second number from the previous request object and the first number from the session object.

The above web application should run without any problem in a simple jsp/Tomcat setup. OK if you were success full in running the above web application we can continue with the
discussion. Now we have used session object to store our values in it.

One question which used to bother me when I did session programming is how does the request carry the session id. HTTP spec does not contain any information on session, so HTTP has no clue what a session is. So the container have to implement some way to pass the session id from one page to the next. This is what Apache Tomcat a jsp/servlet container
does. It checks a parameter with key JSESSIONID in cookie values as well as request URL ,if it finds one it picks up the session object corresbonding to that particular session-id.

My professor use to teach me web technology by doing postmortem on HTTP request/response packets. I have found itthe best way to learn web technology of any level.So lets see the HTTP request headers of our small web application.

HTTP request headers of the first page

Host localhost:8080
User-Agent Mozilla/5.0 Gecko/2008070208 Firefox/3.0.1
Accept text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language en-us,en;q=0.5
Accept-Encoding gzip,deflate
Accept-Charset ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive 300
Connection keep-alive
Cookie JSESSIONID=DF88F04F5D2574711121214DD4AC9FA7

Nothing intresting in HTTP response. But if you see the HTTP request you can see teh cookie parameters send to the server with t he first request

Cookie JSESSIONID=DF88F04F5D2574711121214DD4AC9FA7

This is how the session managment is done is using cookies. But we are not using sessions in our first page then why do it pass the session id. Its because JSP by default checks weather there is a session existing for that request if it does not exit it creates a session and sets its id in the cookie parameter. That's why we see session id from the first page request. Now lets scan the second pages request and response headers.

HTTP request headers of the second page

Host localhost:8080
User-Agent Mozilla/5.0 Gecko/2008070208 Firefox/3.0.1
Accept text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language en-us,en;q=0.5
Accept-Encoding gzip,deflate
Accept-Charset ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive 300
Connection keep-alive
Referer http://localhost:8080/httpsession/firstpage.jsp
Cookie JSESSIONID=DF88F04F5D2574711121214DD4AC9FA7

If you look at the JSESSIONID value in the cookie header of the request you will find the the same session id value. So the container keeps the session id in the cookie till the session gets destroyed. Now the third request header will also have a similar cookie header parameter. This is how sessions are implemented over cookies. Now try to disable cookies in
your browser, you will find that your web application does not work. So how do we do session management when cookies are disabled we can see in the later discussion.

Ohhh...Cookies are disabled..! URL rewritting for yor rescue :)

So what you do when cookies are disabled.You have to find some other way of transferring the session id. I have mentioned in the earlier discussion that Tomcat looks in the cookie as well as request parameters for JSESSIONID parameter. So we have to embed session id in the request URL. Lets try it out.

There won't be any change to the first and the result page they will the same. Remember we do not have problem in adding and removing values to and from the session object but our
problem is how do we pass on the session id between two requests. That's why there is no change in the result page.

<.head>
<.html>
<.title> The Second page< /title >
< /head >
<.body>
< % String a = request.getParameter("firstno"); session.setAttribute("firstno",a); String url =response.encodeURL("resultpage.jsp"); % >
<.form action="<"> method="get" >
Second no : < type="text" name="secondno" vlaue="0"> <>
<.form type="submit" value="Next">
< /form >
< /body >
< /html >

Here we generate the form action URL from the through encodeURL method. encodeURL first checks whether it could add the session-id in the cookies ,if it cannot add it to the
cookies it rewrites the URL adding the session id to the request URL. So if the cookies are disabled in the browser the URL string which is replaced at the form. Lets us examine
the request headers of resultpage.

Host localhost:8080
User-Agent Mozilla/5.0 Gecko/2008091620 Firefox/3.0.2
Accept text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language en-us,en;q=0.5
Accept-Encoding gzip,deflate
Accept-Charset ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive 300
Connection keep-alive
Referer http://localhost:8080/httpsession/secondpage.jsp?firstno=34

You can notice that there is no cookie header in this particular request.So it is obvious that session id is not passed through cookies. Now if we see the URL at the address bar of the page it would be something like this.

http://localhost:8080/httpsession/resultpage.jsp;jsessionid=B5EB1435EDF939B65A20148187884DE8?secondno=45

You can do the same thing by using this code instead of using encodeURL

String url="resultpage.jsp;jsessionid="+request.getSession().getId();

But it won't check whether cookies are enabled or not.

So encodeURL rewrites the URL to embed the session-id parameter to it. The container identifies which session a particular request belongs to by looking at the jsessionid value in the request URL. You might have noticed that unlike the request parameter the jsessionid is embedded in to the URL using a ; delimiter. So jsessionid is not a request parameter in the request.

Enable Smooth fonts on Ubuntu Linux

2008-09-14T21:04:00.000-07:00

How to enable smooth fonts in ubuntu...

http://www.howtogeek.com/howto/ubuntu/enable-smooth-fonts-on-ubuntu-linux

TCP/IP and Indian postel system

2008-08-29T21:15:00.000-07:00

(Disclaimer: Thes are not my thoughts but was told by Amarnath Raja ,InApp CEO to us during network class...I found it so good that I am writing it down)

IP is an unreliable protocol .It takes its best effort to take the packets to wherever its destined to ...that's it..

We could compare IP to Indian postal system.We send a post mail and we hopes it will reach the receiver (let's assume the postal system takes its best efforts to take it to the receiver).And the postal system works because its unreliable.Lets say tomorrow government decides to make the Indian postal system reliable..Then ,if I have to send a postal mail...I won't be putting the mail in the mail box, but I will be handing over to a person and he should give a receipt of receival for it,then the same procedure has to be followed in each point of transfer of the mail..and there would be at least 5 points where the mail information has to be logged.Think of our postal employees doing the same for the huge no: of mails each day. The system will fail miserable. Just the same way if IP protocol had to made sure reliability in each point of its transfer it would have been a failure.

Now we can see how reliability is attained by using TCP over unreliable IP protocol. Consider I am writing a book consisting of 25 chapters (wow! I am writing a book with 25 chapters..really funny) .I find a publisher and he agrees to print it. We agrees that the content will be sent chapter by chapter over post. I wrote the 1st chapter sends it by post writes the second sends it and does the same for 3,4,5,6 and the publisher receives it (put he doesn't tell me he have got it..why to waste money ). Now something mysterious happens to the 7th packet. I completes the 7th chapter and send to the publisher, but he doesn't receives it.The publisher who is well aware of Indian postal system ignores it and doesn't bother to inform me. While I kept on sending 8th,9th,10th chapters.The publisher receives all these chapters and also receives the missing 7th chapter which he happily keeps it in the correct position .If he had called me for informing the missing of 7th packet he would have wasted a phone call and extra effort of resending the chapter. Now the procedure continues in which 14th chapters gets missed again. The publisher ignores it as usual and continues to recieve other chapters till 20th, now the publisher feels that chapter 14 might have vanished to the black hole of Indian postal system. So he calls me up and tells to send chapter 14 again and sends it. During these time he receives rest of the chapters .When he recieves chapter 14 he puts it in his position . Thus a reliable system of transfer is attained over unreliable postal system. TCP does the same thing. It makes sure the transfer of IP packets in an end to end basis.Inshort it does'nt care about the whareabout of each packets when they are send, but at the end of the session (and even inbetween) checks for missing packets and acts according to it.

emacs:spellcheck

2008-08-29T20:52:00.000-07:00

to check for spelling in emacs.press ctrl-space and drag to select the area to be spell checked then give ispell command(M-x ispell).

emacs: preview html page in browser

2008-08-29T19:48:00.000-07:00

If you want to preview an html page editing in emacs press
ctrl-c ctrl-v

Setting java path in ubuntu

2008-04-02T05:06:00.000-07:00

To set the java path in ubuntu you need to edit the profile file

# vi /etc/profile

add these lines into the file

JAVA_HOME=/usr/lib/jdk1.6.0_03
export JAVA_HOME
PATH=${JAVA_HOME}/bin:${PATH}
export PATH

the java_home path should be the path were your jvm is located
usually it will be in the above location.

(this is only one of the several available solution)

gmail-notifier in ubuntu

2008-03-26T21:41:00.000-07:00

For getting Thunderbird like mail-notification for gmail... in Ubuntu...

U can use checkgmail package for it...

For installation add these repos in the aptitude source list

$ gedit /etc/apt/sources.list

add the lines

deb http://asher256-repository.tuxfamily.org dapper main dupdate french
deb http://asher256-repository.tuxfamily.org ubuntu main dupdate french

now update the aptitude and install checkgmail

$ aptitude update
$ aptitude install checkgmail

we can invoke the application from command prompt..

$ checkgmail

If there is no luck with first command try this

$ checkgmail -no_cookies

(google has disabled cookies for security reasons it seems...I had to give the -no_cookies to work in my system)

Now you can fill in the account details and other option in the interface which comes up...

If there is a proxy you may need to set the https proxy also before running the command

export https_proxy = https://[proxyadress]:[port]

U might get a better explanation at

http://www.debianadmin.com/gmail-notifier-for-ubuntu.html

Setting proxy for shell environment through script

2008-03-05T20:49:00.000-08:00

I was fed up of writing long commands for setting http_proxy.So thought putting it as a script and invoking it whenever needed.

This was the one and only line in the script
export http_proxy=http://[ip address]:[port no]

I tried to invoke the script as normal
sh proxy.sh

The scripts works fine but the proxy settings does'nt change.
After lot of googling I found that it will work if we invoke in some other way.

. proxy.sh
([space] [dot] proxy.sh)

It worked...
I don't know what does dot stands for.I don't know anything of shell scripting...But planning to learn it....Hope to reply this post later..

Ant JVM proxy configuration

2008-02-26T20:04:00.000-08:00

I don't know much about apache ant.Last day I was trying to build a project and the build failed.Because there was an error while downloading a package from the net.After hours of battle it was figured out that there is a proxy configuration setting for Ant's JVM.We can set proxy with

export ANT_OPTS="-Dhttp.proxyHost= -Dhttp.proxyPort="

eg:

export ANT_OPTS="-Dhttp.proxyHost=192.168.0.2 -Dhttp.proxyPort=8080"

To copy,paste between emacs and other applications....

2008-02-09T22:43:00.000-08:00

By default U can't copy from an application say Firefox to emacs...
(a difficult situation for somebody like me who tweak on others code)

Copy pasting from emacs to aother applications are also not supported...

U can get it done by adding just two lines to your .emacs file..
(U can't copy paste it if U R using emacs for editing .emacs file...)

(setq x-select-enable-clipboard t)
(setq interprogram-paste-function
'x-cut-buffer-or-selection-value)

Done...

Now copy something to your clipboard from an application say firefox and
click C-y in emacs it gets copied....

Now do C-k(kill) a line a try C-v in any other application say gedit..it gets
pasted...

Adding themes to emacs

2008-02-07T00:01:00.000-08:00

U can add cool themes with a single command in ubuntu..

aptitude install emacs-goodies-el

Themes are installed...

Now start emacs and try

M-x color-theme-select

U will get menu list of themes.Select one from the list by pressing RET.

U have a new theme for Ur emacs....

The theme will stay only for that session of emacs...

If U want the theme to stay across sessions...add this code to the .emacs file
in the home directory.U should have .themesettings and .curTheme file in ur home
director(I was not having .themesettings I had to create it for this tweak)

(require ‘color-theme)
(setq theme-load-from-file t)
(defun my-onload()
(setq filename (concat (expand-file-name "~") "/.themesettings"))
(if (file-exists-p filename)
(progn
(load-file filename)
(if (equalp theme-load-from-file t)
(progn
(setq filename
(concat (expand-file-name "~") "/.curTheme"))
(if (file-exists-p filename)
(progn
(message "selecting theme from .curTheme")
(load-file filename)
(color-theme-install (my-color-theme))
)))
(funcall theme-default)
))
(message "selecting theme from color-theme-standard")
(color-theme-install(color-theme-standard)
)))

(defun my-kill-saves()
(if (equalp theme-load-from-file t)
(progn
(color-theme-print)
(write-file (concat (expand-file-name "~") "/.curTheme"))
)))

(add-hook 'after-init-hook 'my-onload)
(add-hook 'kill-emacs-hook 'my-kill-saves)

Now the selected theme will stay if we restart emacs.

Problem with python-mode.el in emacs22.1

2008-01-31T21:26:00.000-08:00

When I tried to get into python-mode in emacs 22.1 I got this error:
Cannot open load file: python-mode

Solution to this problem:
Entering the following command in the command line solved my problem.Don't know
why it came and how the command solved my problem but did a quick fix...

dpkg --purge python-mode

Modifieng partitions through LiveCD

2008-01-13T21:57:00.000-08:00

Sometimes U may need to modify,resize,or move the partions...
U can do it using Ubuntu Live CD

First Boot up the system using Live Cd

Open the application gparted (GUI based partition modifier which comes packed with Live CD)

In the terminal give this command with root

> sudo gparted

now U can use the GUI tool to edit the partitions...

To install nxml-mode in emacs using aptitude

2008-01-09T04:23:00.001-08:00

aptitude install nxml-mode

To install nxml-mode in emacs using aptitude

2008-01-09T04:23:00.000-08:00

aptitude install nxml-mode

Wi-Fi fix for UBUNTU on Braodcom network card

2008-01-08T01:14:00.000-08:00

This link give a 5 minute solution to Broadcom Chipset problem in ubuntu...

Broadcom Chipset fix in ubuntu 5 minute solution

If U really want to know what U R doing...

Broadcom Chipset fix in ubuntu 5 minute solution detailed solution

To install javascript-mode.el on emacs

2008-01-02T20:55:00.000-08:00

C-h v load-path will give path were U need to put the javascript-mode.el file

Include this in the .emacs file
(if there is no .emacs file make one in the home directory)

;; for javascript files
(autoload 'javascript-mode "javascript-mode" "JavaScript mode" t)
(setq auto-mode-alist (append '(("\\.js$" . javascript-mode))
                             auto-mode-alist))

This should work....

Its explained in more detailed way at
http://www.divmod.org/trac/attachment/wiki/DivmodNevow/Athena/Emacs/Files/javascript-mode.el