Misc things I learn

I post here whatever computers related stuff I need/use/learn.

[leecheve]

Ant properties are immutable

It turns out you can't update the value of an already defined ant property in a build file. I'm new to ant and I was trying to debug an issue that took me almost two days before figuring that out :(

"Properties are immutable: whoever sets a property first freezes it for the rest of the build; they are most definitely not variables." [http://ant.apache.org/manual/Tasks/property.html]

However, if you have ant 1.8 or later, and really need to update a property's value you can use the var task. 

<property name="x" value="6"/> <echo>${x}</echo> <!-- will print 6 --> <var name="x" unset="true"/> <property name="x" value="12"/> <echo>${x}</echo> <!-- will print 12 --> [http://ant-contrib.sourceforge.net/tasks/tasks/variable_task.html]

:D

[leecheve]

Java Manifest Files & Classpath issues

The other day I had an issue with a very very very long environment variable on Windows, and it turns out that cmd.exe drops anyting beyond 2K or 8K chars (say Enviroment Variables, Command line arguments, etc) 

Even though the Win32 limitation for environment variables is 32,767 characters, Command Prompt ignores any environment variables that are inherited from the parent process and are longer than its own limitations of either 2047 or 8191 characters (as appropriate to the operating system). For more information about the SetEnvironmentVariable function, visit the following Microsoft Web site:

http://msdn2.microsoft.com/en-us/library/ms686206.aspx

So, this very long env. variable was a CLASSPATH to be used by java to run some testing, thus I was forced to look for a different way to specify a classpath that supported a whole bunch of paths (~330 in this specific case).

I found that one can use a Manifest packaged into a jar file, this manifest will only contain a list of paths to be used as CLASSPATH, in other words a Classpath manifest jar. This is how it works: (you can learn more about java Manifests here)

Create a text file Manifest.txt

In that Manifest.txt file add a Class-Path header followed by a list of jar files. Class-Path: file1.jar dir1/file2.jar dirx/fileb.jar dirZ/dirL/dirF/file3.jar ... However, note that according to the jar file specification (here) there is a limitation:

No line may be longer than 72 bytes (not characters), in its UTF8-encoded form. If a value would make the initial line longer than this, it should be continued on extra lines (each starting with a single SPACE). 

So just type one path per line.   

Then all you need to do is to make a jar file containing that Manifest.txt, use the jar tool for that purpose: (I'm using $> to denote my prompt)

$> jar cfm mynewjar.jar Manifest.txt   c - create a new file f  - specify a file name m - include manifest information from the specified file.

  If you want to look what is inside the jar file, just use:

$> jar tf mynewjar.jar

  And if you want to extract the contents:

$> jar xf mynewjar.jar 

  Now all you need to do is use this mynewjar.jar in your java commandline:

$> java -classpath /path/to/mynew.jar ......

And the most important note (I spent so much time trying to figure this out): If you are doing this on Windows and using absolute paths to your jar files, using: c:/dirA/fileA.jar  As a path to a file wont work!. 

If you use absolute paths make sure you add a leading slash '/' to all your paths: /c:/dirA/fileA.jar That one will work! :) The leading slash (/) is used to prevent "c:" being parsed as a protocol specification. And this is because according to the jar file specification the Class-Path attribute:

The value of this attribute specifies the relative URLs of the extensions or libraries that this application or extension needs. 

This means you could also use: file://c:/dirA/fileA.jar  And will work too. 

[leecheve]

Putty and Private Key Authentication for SSH Connections

The other day using Amazon Web Services (AWS) to setup a Linux instance I needed to connect to my recently created Linux instance using Putty and Private Key Authentication. (see here).

Here is a quick tutorial on how to do that...

In my case AWS created a Private Key for me (called luispk.pem) and I wanted to use PuTTY under Windows to connect to my Linux instance. However PuTTY works with keys in a different format (with extension .ppk)

Therefore you need to convert your .pem key to a .ppk, here are the steps:

Download PuTTYgen here.

Click File -> Load private key

  Click on 'Save private key' and save it in .ppk format

Now you have a .ppk private key file use PuTTY to opan an SSH connection to your Linux instance

You can get PuTTy here.

Open it and Go to Connection -> SSH -> Auth and Click Browse to select your .ppk file

  Now go to Session. Write the IP Address of your Linux instance, Select the radio button for SSH and Click Open

  You should see something similar to this:

And thats it, you are in!

[leecheve]

How to get a free online linux box on Amazon Web Services (AWS)

I'm going to explain how to use the Amazon Web Services Free Tier (http://aws.amazon.com/free/) to setup an instance (Linux machine) and a web server on top of it.

First of all you need an Amazon Web Services (AWS) account, get it at http://aws.amazon.com.

Once you have an account you must enable EC2 By entering your credit card information and a phone number for validation purposes (Your credit card will only be charged if you consume services out of the free tier).

On the AWS Management Console go to EC2 and from the Navigation Menu (left side of the screen) click on Instances

Now click on Launch Instance

Make sure you Select 'Launch Classic Wizard' and Click Continue.

This step is important! on Step 1 of the wizard go to 'Community AMIs', Filter results by 'EBS Images' and Type 'ubuntu-lucid' as shown in the image below. Using Ubuntu will keep things free.

Select the AMI that fits your needs, I choose Ubuntu lucid 10.4 amd64. And Click Continue.

This is another important step! Make sure your Instance Type is Micro (Micro is part of the free tier) and Click Continue

Click Continue again until you get a step called 'CREATE KEY PAIR'. If you already have a Key Pair use that, if not then create a new key pair and download the key file to your system. Click Continue

At the 'CONFIGURE FIREWALL' take the default Security Group and Click Continue.

Finally Click Launch at the 'REVIEW' step of the wizard.

Wait a few minutes and your instance should be up and running (see the image below).

Now you need to assign an IP Address to your instance, click on "Elastic IPs" in the Navigation menu at the left of the screen.

Click on "Allocate New Address" and select EC2 on the popup window and clock Yes.

IMPORTANT! Non-Associated IP Addresses have a cost $$$. Make sure you always have your Elastic IP associated to an instance at all times. 

Right-click on your recently created IP Address and Click 'Associate'

Select your instance in the "Associate Address" window and Click "Yes, Associate".

That's it you have an online instance with an IP Address associated to it! 

I'll make another post later today with instructions on how to log in, using SSH, to your newly created instance (ubuntu linux in my case).

Remember this:

Only 1 instance at any time (running or not).

Only 1 IP at any time, and such IP must be always attached/associated to an instance (unattached IPs cost you money!).

Always use public/ebs AMIs. If you want Red Hat, Amazon Linux, SuSE, you will have to pay for those OSes.

Update: I did that post about using PuTTY and SSH to login into your Linux instance, and you can find it here.

Update 2:

Now it is time to create a username for yourself using the command 'useradd'.

To create a username called luis I used the following command 

sudo useradd -G adm,admin -m -d /usr/luis luis 

-G specifies a list of supplementary groups. In this case adm and admin -m tells the command to create the user home directory -d specifies the user home directory (to be created as specified by -m), in this case I want the home directory to be /usr/luis

ubuntu@domU:~$ sudo useradd -G adm,admin -m -d /home/luis luis ubuntu@domU:~$ ls /usr/ bin  games  include  lib  lib64  local  luis  sbin  share  src

Now set a password to that user you just created using 'sudo passwd <username>'

ubuntu@domU:~$ sudo passwd luis Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully

It is recommended that you do not login as root, you should create an account (as you just saw) and use that account to do sudo whenever you need it.

And for security reasons disable ssh root login, or someone could try to brute force your root password and login as root!

Edit the file /etc/ssh/sshd_config:

 sudo vi /etc/ssh/sshd_config

Locate the line:

PermitRootLogin yes

And change it to 

PermitRootLogin no

Also don't forget to check Password Authentication is enabled

PasswordAuthentication yes

Now restart the ssh service

ubuntu@domU:~$ sudo /etc/init.d/ssh restart

 * Restarting OpenBSD Secure Shell server sshd      [ OK ]

That's it! Now you can open a ssh connection to your linux box and login using your new credentials!

[leecheve]

ubuntu - list of installed packages

$ dpkg --get-selections $ dpkg --get-selections | grep php

[leecheve]

Vim enable mouse

" Enable mouse, to copy to clipboard using mouse use shift + drag

set mouse=a

The mouse can be enabled for different modes: n Normal mode v Visual mode i Insert mode c Command-line mode h all previous modes when editing a help file a all previous modes r for |hit-enter| and |more-prompt| prompt Normally you would enable the mouse in all four modes with: :set mouse=a

http://vimdoc.sourceforge.net/htmldoc/options.html#'mouse'

[leecheve]

VIM wildmenu

http://vim.wikia.com/wiki/Great_wildmode/wildmenu_and_console_mouse

Put these two in your vimrc, and you'll enjoy the greatest wildmode that isn't fully documented in the Vim help.

set wildmenu set wildmode=list:longest,full

"set wildmenu" enables a menu at the bottom of the vim/gvim window.

The meaning of "list:longest,full" is so that when you do completion in the command line via tab, these events will happen:

1. (on the first tab) a list of completions will be shown and the command will be completed to the longest common command.

2. (on second tab) the wildmenu will show up with all the completions that were listed before.

To test it out, type ":spe" and then hit tab once to see 1 and hit tab again to see 2.