Ubuntu: Tips and tricks

An excellent and powerful feature about Ubuntu or any other Linux machines is that you can do anything/everything with a single command in a terminal. As with great powers comes great responsibility of understanding(possible) and remembering(absolutely impossible) those powerful commands! So this post would have simple and sometimes complex(which I doubt) commands for working your way around Ubuntu. Please contribute to this list, if you wish. Thanks in advance.

commands:
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Install eclipse	sudo apt-get install eclipse
Uninstall eclipse	sudo apt-get autoremove eclipse
Copy entire directory in Ubuntu	cp -r src_path/folder/* dest_path
Install LAMP stack	sudo apt-get install lamp-server^ - ^ cadet symbol is mandatory
Restart/Start/Stop Apache	sudo /etc/init.d/apache2 restart/start/stop
Install phpmyadmin	sudo apt-get install libapache2-mod-auth-mysql phpmyadmin
Cleaning up after any uninstall	sudo apt-get autoremove
Start/Stop MySql	/etc/init.d/mysql start/stop
Create softlink for phpmyadmin in Ubuntu 10.04(access as http://localhost/pma/index.php)	sudo ln -s /usr/share/phpmyadmin /var/www/pma
Open folder as root user - AVOID this and prefer using terminal and sudo for executing commands	gksudo nautilus
Print out processor information	cat/proc/cpuinfo
To find out if 32 bit or 64 bit linux kernel	uname -a - if result has i386/i486/i586/i686 GNU/LINUX, then 32 bit or x86_64 GNU/Linux, then 64 bit
To view .chm files in Ubuntu - chm = Microsoft compiled HTML files format	Just to read use "xchm"(sudo apt-get install xchm). To edit the file and republish in different format, follow the source here.
To find the version of JDK in your linux	"java -version"
To create aliases for often used complex commands	Cmd: alias alias_name="alias_cmd --options".If you got more aliases and you need it for all sessions hereafter, create a file "./alias" (name is for your reference, any name should do) in home directory. Add all the aliases command you need.Add the following lines in file $HOME/.bashrc, # Source the aliases, if a separate file exists if [ -e $HOME/.alias ]; then [ -n "$PS1" ] && . $HOME/.alias fi and execute "source $HOME/.bashrc". Source here.
To change password for a user account	cmd: "passwd". You will be prompted to enter current, new and retype new password. It should be more than 6 char long.
To change login shell	Find the current shell, "echo $SHELL", the env variable has the current shell info. Then find list of valid shell logins for your system by "cat /etc/shells". Finally to change to new shell, "chsh -s newshell" where newshell is the full pathname taken from /etc/shells. You will be prompted for password. It will take effect from next login.
To clear the contents of a file without deleting it	We know "cat filename" - print the file on terminal, "nano filename" - to edit line by line, "> filename" - clear the contents of the file without destroying the file.
To use gcc-3.x version instead of default gcc in Ubuntu in 10.04	You need to download required 3.x version, I need 3.4 so I downloaded gcc-3.4-base, cpp-3.4, and gcc-3.4 Then "sudo dpkg -i *.deb", this would select the removed databases and install the gcc. Your default gcc would still be the same and both version co-exist. While compiling just use "gcc-3.4". Note: "gcc -v" displays the version of gcc.
To execute user defined script from any directory in any session	Lets say you got a script named "first" and you want to execute the script just like other linux commands like "echo" from anywhere at shell prompt. In short, you need to add your bin directory to the PATH env variable. Thats what exactly we do here: 1. "mkdir bin" --> in your home directory create a directory called bin . Its a good practice to use "bin" folder to store your scripts. 2. "cp first ~/bin" --> copy all your tested script files to the bin directory. 3. "cd" --> go to root directory, thats where you can find .bash_profile. 4. "vi .bash_profile" --> open the profile in vi editor. This file has all startup settings. 5. "export PATH=$PATH:~/bin" -->append the path of bin directory to env variable PATH so system can find the executable. Don't forget the ":" and replace "~" with full path to your directory if you are using other directory than your home. Save the vi file using ":wq". 4. "first" --> it should work from any directory of the user.
How to install apache server	In terminal: "sudo apt-get install apache2". This would install,configure the server for you. To check in browser, "http://localhost"
How to open "rar"/"zip" files	1. First install "sudo apt-get install unrar". Then "unrar x filename.rar" where filename.rar = your file2. To unzip files, in terminal "unzip filename". -d option specifies the target directory for the files.
Create alias ip address in Linux	"ifconfig INTERFACE:ctr IP_ADDR netmask 255.255.255.0 up"Exp: INTERFACE - the interface in which you want to add the new ip address. ctr - start from 0 and increment for every new alias ip. IP_ADDR - the new alias ip address you want to add. netmask, up - keywords mandatory. Calculate the netmask and update it. If i want to add 192.168.30.10 to eth1 the command would be "ifconfig eth1:0 192.168.30.10 netmask 255.255.255.0 up".
To display unix timestamp	"date" displays the date and time. If you want the unix timestamp use "date +%s". Eg: For Mon Jan 24 21:42:57 PST 2011, the timestamp is 1295934173. Use this site to find out timestamp given date and time.
How to find version of Tomcat	1. "cd $TOMCAT_HOME/bin" -->Go to TOMCAT_HOME - root folder of tomcat installation 2. "sh version.sh" -->will print out the version details of tomcat, jdk
How to open new terminal and execute commands from a terminal	The command is "screen". In the old terminal, "screen command_to_execute" will open up terminal inside this one. Look here for options to detach the new terminal.

P.S: Usually blogger has a bugging problem of inserting unwanted spaces if you insert a table in Edit Html section. To avoid the pitfall, after pasting the table code, remove all the spaces. Hence the code like this (avoiding printing it as table)

table
tbody
tr
td /td
tr
/tbody
table

After removing the extra spaces would look like this,

table tbody tr td /td /tr /tbody /table

Now the code looks confusing, so better edit the table with spaces and finally before publishing it remove the spaces. Hope this helps someone!

To-Do: Run a marathon

Just like every other Sunday morning, I started the hike in Mission peak. The trial never gets easy even if you hike them every week. The climate was awesome as usual. While we were climbing down the trail we saw the runners of 23rd Annual Ohlone 50K Wilderness Run

Its a 31 mile run for some good cause. I saw people from all age groups run for the cause. When I saw them, I added a new item to my bucket list[list of things I plan to do before life ends] and that is to run a marathon for a cause. And I really do hope I am able to do it not just once but almost every year. I will make it happen soon!! For now I will just add my small achievement of touching the mission peak here.

View from Mission peak

Looking back at the trail

PHP MySql POST GET Cookies Session - Bits and information

I always feel its easier to remember details if we learn it as differences between two objects. So I want to blog the basic differences between cookies and session, POST and GET method. And bits and pieces of information about UNION,UNION ALL and some more. This post has so many random information. Let me know if you find something wrong or different.

Three ways to move information across web pages:

1. links: a link with href set to destination page.
   
2. Forms: set action to point to target, method of how server should process information sent, encoding type optional. Both 1 and 2 requires user action like a click.
   
3. Without any user action, user header in PHP to move to another page. header("Location: page.php");
   

Cookies:

• Cookies are stored at the client's computer.
• Once stored at client comp by an application, it can be accessed by any web page of that application.
• It contains variable = value pairs of information.
• Cookies are more useful for applications that has no database.
• Cookies will be available from the next page and not in the current page where it was set.
• In PHP, setcookie('variable','value'); sets the cookie and if you don't mention time, this cookie will expire when user exits the application.
• On setting the cookie, it gets stored in one of many built in arrays of PHP called $_COOKIE. For accessing your cookie variable just refer the array with the variable name like $_COOKIE['variable']
• If we want our cookie to be present even after user exits application then we have to mention the expiration time while we set the cookie. We can use two functions along with setcookie() - time and mktime. time() - returns the current time, we have to add the seconds for which the cookie needs to be alive. eg. setcookie('var','val',time()+3600) - to set cookie alive for 1 hour(60*60 seconds). mktime() - returns date and time and the order of arguments passed is hr,min,sec,mon,day, year. eg. setcookie('var','val',mktime(3,0,0,5,6,2010)); - to make it expire by 3am of May 6th 2010.
  
• To remove cookies, use the same setcookie without any values passed. eg. setcookie('var'); setcookie('var','');
  
• setcookie limitation: It has to be before any output is sent to the client browser. This limitation is with both session and cookie.
  

Session:

• Session details are stored in a file at the server side. In Unix and Linux systems, in \tmp folder and in Windows folder called sessiondata. We can change the location of where the file is stored by chaning session.save_path value in php.ini.
  
• Session ID - long nonsense number for every client which cannot be guessed or forged. In PHP, system variable is used for session ID - PHPSESSID.
• Session ID is passed to every page to access the session details. There are three ways to pass them. 1. If cookies turned on, use the cookies. 2. For links, use the URL. 3. Use hidden variables for form with POST method.
• Session variable is got and stored in $_SESSION built in array(similar to $_COOKIE array in cookies). They are accessed same way as in cookies.
• For session to work we need to enable, track_vars while installing PHP.(default its turned on from PHP 4.0).
• If cookie is turned off at the client side, trans_sid should be enabled to transfer session id. To enable that use session.use_trans_sid = 1 in php.ini.
  
• Start a session: session_start() - If sessionID is found, then load $_SESSION with variables and their values. If no ID is found, then it is first time so create a new session and set PHPSESSID.
• Save a session variable: $_SESSION['var'] = 'val';
• Close a session: session_destroy() - destroys the session details.
• session_id() - returns the PHPSESSID value - current session id.
• unset($_SESSION) - unset the session details in the current page.
• Limitation: session has to be set before any output is sent to the client browser.
  

We know that POST and GET are ways of indicating how the server should process the information sent by the form.

POST:

• Sends information in 2 steps. 1. browser contacts form processing server specified in action. 2. Once contact has been established, send data to server in separate transmission.
• On server side: 1. read parameters from a standard location. 2. After read, decode parameters before application use form variables.
• To get post variables from earlier form use built in array $_POST.
  

GET:

• In a single transmission, data is sent to the server. Data is appended by the browser to action URL.
• This is the default method if not specified otherwise.
  
• On server side, gets information passed at the end of the URL.

POST or GET what to use?

• GET: best transmission performance(single transmission) and apt for small forms with short/few fields.
• POST: Apt for forms with many/long text fields.
• If inexperienced with server programming use GET to avoid extra steps of processing- read/decode as in POST.
• For security purpose, use POST to avoid the information you transmit to be available in open for hackers to track. POST has security holes too but atleast it has encoding when transmitting.
• To invoke server processing outside form tag, eg in a tag, use GET because it lets us use form-like parameters as part of URL.
  

ENCTYPE field in FORM tag:

• Two types of enctype options available. 1. Multipart and 2. Text/plain
• Multipart: forms with file selection fields for upload by user.
• text/plain: used along with mailto in action attr of form tag. While sending forms to email server rather than a server.
• The default encoding type: Internet media type Application/x-www-form-urlencode.

UNION and UNION ALL:

• Union all - combines rows from multiple row sources into one result set. It includes duplicates.
• Union - does the same thing as union all but excludes duplicates, result would be sorted here without duplicates.

Polymorphism in OO langauges:

• Ability of 2 or more objects belonging to different classes to respond to exactly same message in different class specific ways.
  

Thread programming in Java

I spent a day in library refreshing stuffs I learned long back(not so long!) while I was in undergrad. Since my memory is fresh with these topics, I thought I should blog them so next time I can find it all in one place. On a different note, a day spent studying/reading in library is well spent! Okay back to Threads in Java.

Thread - class, Runnable - interface which has the thread implementations. For our application class to use thread we need to either implement or subclass or make the class a member class of our application. Thus three ways to create threads:

1. Extend Thread class: For standalone applications that don't need to extend another class.
2. Implement Runnable interface: For classes that need to extend other classes. Due to single inheritance, we can achieve threads by implementing the interface. Eg: Applets that extend JApplet and can't extend Thread too.
   
3. Construct a thread passing an inner class that is Runnable: For threads with tiny run methods and little outside interaction.

Thread life cycle functions:

• Thread() - When subclassing Thread class
• Thread(name) - when subclassing + for debugging
• start() - Begins process of assigning CPU time to a thread. Finally it results in calling run() method of the thread object.
• suspend(), resume(),stop() - deprecated. Because these suspend and resume functions can cause deadlocks.
• interrupt() - interrupt whatever the thread is doing
• boolean isAlive() - returns true if thread is neither finished or terminated by call to its stop()
• join() - to wait for a thread to finish
• get/setPriority() - higher priority threads gets first chance of CPU
• wait(), notify(), notifyAll() - semaphore and for thread synchronization.
• synchronised - keyword - can be used for any object in Java. When used over an object or a method, threads will wait if one thread has already started running.

Thread synchronization:
The wait, notify and notifyAll methods allow any java.lang.Object to be used as a synchronization target.

wait():
This causes the current thread to block in given object until awakened by notify or notifyAll.

notify():
Randomly selected thread waiting on this object is awakened. Then that thread tries to regain the monitor lock. If wrong thread is awakened, will result in deadlock.

notifyAll():
All threads waiting on this object is awakened. Then all try for the monitor lock. Hopefully one of them succeed.

Best practices:

• To avoid deadlock, use notifyAll to wake up waiting threads.
• To avoid using deprecated method - stop, use the variable as a flag and use it to find if the thread is done executing.
  

Today's scribbling

I took a tamil book from Sunnyvale Library for my Mom but I ended up drawing the art that was on the cover! And of course I am reading the book too. Here is my reproduction of the art :)

While I was at it, I drew something for my niece Ovia too. Her favorite character of the bunch, Goofy.

There are lot more art on the book's cover, I will post them later!! Ciao.

Who said I can't touch Mission Peak?

Well, I have this habit of proving myself to be what I claim, when somebody challenges me. So thats what happened when my uncle said I can't hike all the way to the Mission peak and I did it. Its this mountain hiking place in Milpitas. My uncle hikes the place with his pals every Saturday. I am an outdoorsy person. But my uncle had hard time believing it and challenged me to touch the peak this sunday(April 11th, 2010). Hence started one of my memorable experience.

I used to go trekking every summer when I visit here while I was in undergrad. But that was just simple not-so-steep mountain climb. This one was longer and steeper than the usual place in Los Altos. We reached the place on time and started climbing. It was steep right from the foot of the mountain. To be honest initially it was tough, but not something I can't handle. We saw a "big" rabbit sleeping on the grass near the trail, light drizzle and so many beautiful scenes as we hiked. The peak of this experience is the peak itself. We couldn't have picked a day with such worse weather. Though it was so mild and fun at the bottom as we reached peak, wind was very strong. It literally pushed us through in some places, I am not exaggerating. The peak was so beautiful and was worth the pain and cold. The fun part was when we started climbing down. It was more difficult than climbing up because it was very difficult to put a break. If you loose a minute of your body control and let your legs loose you will snowball down to the bottom of the mountain, well may be not in good shape because the trail bends in place!!

It took 3 hours for the climb but as I said, it was all worth it. Now, my family agrees I am an outdoorsy person. I am planning to continue this awesome hiking every weekend in this summer!! But I seriously hope the wind becomes weaker and it is not so cold in the coming weeks.

P.S: I touched the Mission Peak proof is the picture above.

POSIX Thread programming in C

I have to work on a project for Network programming subject this semester. The project objective is to develop an application that acts as a TCP server on top of UDP socket. Hence our project should take care of everything a TCP server basically and internally does for us, right from three way handshake till 4 way handshake. While there is that, the server have to be concurrent and it should handle few attacks like xmas tree, syn flooding.

So for the concurrency in server handling, our team decided to go with thread instead of forking new process to handle incoming connections since thread is lighter than forking. But the real reason being is I have worked on process creation in Unix for some projects in Undergrad. But I never experimented with thread creation. So what is learning when there is no experimentation involved.

While I am at it, I thought I should also blog the things I learn from thread programming in C. These things usually tend to slip away with time. Hence the post. Now this post may contain very basic instructions but for me these are baby steps. So if you are looking for complex details, this is not the post you should be reading. Thanks.

Misc:

• Good sources: source1, source2, source3
• All thread programs need "#include " and while compiling use "gcc -pthread filename.c" to include pthread.h. Hence use IDE to write code but terminal to run the files.