the empty quarter » Linux

Install Oracle XE on Ubuntu

martin — Sat, 04 Feb 2012 11:21:13 +0000

Steps to install Oracle Express Edition (XE) database 10g on Ubuntu 11.10 (Oneiric).

Download the Oracle XE deb package (free registration is required).
Double click the downloaded file and select to install it.
In terminal run sudo /etc/init.d/oracle-xe configure.
You will be prompted to enter the following parameters: HTTP port number, database listener port number, SYSTEM and SYS database accounts password and whether the service should be started upon boot.
Thereafter the configuration might take a few minutes. That’s it. To start the service in the future run sudo /etc/init.d/oracle-xe start and to stop sudo /etc/init.d/oracle-xe stop.

Column-wise text manipulation

martin — Mon, 17 Oct 2011 17:02:14 +0000

GNU/Linux includes many utilities for working with text files through the shell. In this post we take a quick look at accessing and manipulating text files in a “column-wise” mode.

Suppose you have the following two files, each with two columns separated by the TAB character.
$cat file1 Alice Paris Bob Tokyo Mary London John New York

$cat file2 13 May Orange 19 Oct Blue 11 Nov Black 29 Feb Red

The data in the two files are in fact related, i.e. file2 contains the date of birth and favourite colour of the people mentioned in file1 (assuming also that the files are sorted correctly). It would make sense to combine the two files together so that each row has the full data for each person. The paste command does just that.
$paste file1 file2 > file3 $cat file3 Alice Paris 13 May Orange Bob Tokyo 19 Oct Blue Mary London 11 Nov Black John New York 29 Feb Red

Suppose that we are only interested in the name and date of birth of each person, and we can discard the hometown and favourite colour information. The cut command is what we shall use:
$cut file3 -f 1,3 > file4 $cat file4 Alice 13 May Bob 19 Oct Mary 11 Nov John 29 Feb

Our next and final requirement is to reorder the columns differently. Instead of having the name followed by date of birth, suppose we want to have the columns the other way round. Unfortunately cat -f 3,1 produces exactly the same output as cut -f 1,3, so the cut command will not be sufficient. We have to use sed instead.
$sed -e 's/$[^\t]*$\t$[^\t]*$/\2\t\1/' file4 > file5 $cat file5 13 May Alice 19 Oct Bob 11 Nov Mary 29 Feb John

How does that work? Well $[^\t]*$ is a “named expression” which matches all characters except TAB. The search pattern looks for two of them, separated by TAB (\t). In the replace-with part, they are referred to as \2 and \1, again separated by \t.

Of course if file5 was what we ultimately wanted from the beginning as our output, we could have simply piped commands together:
$paste file1 file2 | cut -f 1,3 | sed -e 's/$[^\t]*$\t$[^\t]*$/\2\t\1/' > file5

or alternatively
$paste file1 file2 | sed -e 's/$[^\t]*$\t$[^\t]*$\t$[^\t]*$\t$[^\t]*$/\3\t\1/' > file5

Remove HTML tags with sed

martin — Mon, 17 Oct 2011 12:16:13 +0000

Sed can be used to strip out all HTML or XML tags from a file and get the plain text version. Suppose you have file gnulinux.html with the following contents:

The combination of GNU and Linux is the GNU/Linux operating system, now used by millions and sometimes incorrectly called simply “Linux“.

Tempting but incorrect – sed finds the longest possible match which in this case is the entire file, and thus will output nothing:
$sed -e 's/<.*>//g' gnulinux.html

Correct version:
$sed -e 's/<[^>]*>//g' gnulinux.html The combination of GNU and Linux is the GNU/Linux operating system, now used by millions and sometimes incorrectly called simply “Linux“.

TrueCrypt tray icon in Unity

martin — Tue, 16 Aug 2011 02:16:35 +0000

If you use TrueCrypt under Ubuntu 11.04 Natty you would have noticed an annoying behaviour. Under previous versions an icon is present in the system tray which remains there whilst a volume is mounted even if the TrueCrypt window is closed. Under Unity the tray icon is not shown. If you accidentally close the window (instead of minimizing it), there’s no easy way of getting back to it. Launching TrueCrypt again results in a error message.

The fix as suggested on shocm.com is to set the systray-whitelist property by running the following through command line (you will need to restart afterwards):

gsettings set com.canonical.Unity.Panel systray-whitelist "['all']"

However, your mileage may vary, and on my netbook this did not fix the issue. It seemed like the icon was placed in the tray, but it was rendered as a very thin strip about 1-2 pixels wide which could not be clicked.

The only way I found of getting out of missing window scenario was to resort to using TrueCrypt through the command line to dismount all mounted volumes.

truecrypt /d

This will also exist TrueCrypt and afterwards it can be launched as usual.

Remove Mono from Ubuntu

martin — Wed, 29 Dec 2010 00:49:49 +0000

After finally getting some time to fully read up on Mono (especially on the excellent The Source) I have decided it is best to remove it from my system. The Open Sourcerer has a nicely written up set of instructions for 10.04 Lucid Lynx and 10.10 Maverick Meerkat.

Remove empty lines from file

martin — Wed, 22 Sep 2010 16:50:07 +0000

Here’s a quick way to remove empty lines from a file using the Linux command line:

cat file1 | sed /^$/d > file2

Where file1 is the input file containing empty lines and file2 is a newly created file with empty lines removed.

How to change SSH port number on Ubuntu server

martin — Fri, 03 Sep 2010 08:56:48 +0000

Changing the port number of SSH daemon is a quick way of reducing the number of SSH brute force attacks your server might face (check the file /var/log/auth.log to see if there are many failed SSH login attempts).

Just to be on the safe side, create a backup copy of the SSH daemon config file.
sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.vanilla
Edit the config file.
sudo vi /etc/ssh/sshd_config
Change the port number on the following line, e.g. to 2201 or some other unused port. Make sure you note down the port number.
Port 22
Restart the SSH daemon. You might get kicked out of your existing session.
sudo /etc/init.d/ssh restart
When you login next remember to include the correct port.
ssh youruser@yourserver -p 2201

Disable Update Manager pop-up in Ubuntu

martin — Sat, 07 Aug 2010 05:56:20 +0000

Here’s how to disable the annoying Update Manager pop-up (technically a “pop-under”) and instead have the update icon displayed in the notification area.

Hit Alt-F2 to open the “Run Application” dialog.
Type in gconf-editor to run the GNOME Configuration Editor.
Navigate to Apps > Update Notifier.
Untick the auto_launch checkbox.

Thunderbird: Notifications and Indicator Applet in Ubuntu Lucid

martin — Thu, 05 Aug 2010 14:59:53 +0000

This post is an update on integrating Thunderbird with Ubuntu’s notification system. For Ubuntu 10.04 Lucid Lynx it supersedes my earlier post on the topic which was applicable previous versions of Ubuntu (Jaunty and Karmic).

The big difference in Lucid Lynx is the Indicator Applet and the “Me Menu”, both of these incorporating features related to the “social from the start” marketing tagline. In particular, the Indicator Applet integrates with Evolution, Ubuntu’s default mail client, to show new email notifications and allow quick access to the mail client. Once again, no out-of-the-box integration with Thunderbird. So the challenge is twofold: integrate with the notification mechanism (black pop-ups) as well as the Indicator Applet. The Mail Notification extension unfortunately falls short in the latter part.

Luckily, Ruben Verweij has developed libnotify-mozilla, a Thunderbird extension which works just great for both of the above requirements. Be sure to check out the blog (here and here) and the project’s Launchpad page to get the details and latest news. Here are the steps to set up the extension:

Download the Mozilla Notifications Extensions .xpi file and save it in a temporary location on your system.
Open Thunderbird and install the extension (Tools > Add-ons > Install).
Disable the native Thunderbird notifications by unticking “When messages arrive: Show an alert” in Edit > Preferences.
Install libnotify-bin package (execute in Terminal):
sudo apt-get install libnotify-bin
In order to have Thunderbird entry in the menu even when it is not running. First create a new file as follows (run from Terminal):
mkdir -p ~/.config/indicators/messages/applications gksudo gedit ~/.config/indicators/messages/applications/thunderbird
Then paste the following text in the file and save it:
/usr/share/applications/thunderbird.desktop
(optional) If you wish you can remove the Evolution entries from the indicator menu. Note: because of bug 533021, this is a workaround which will remove Evolution from the menu for all users on the system, not just yourself.
mkdir -p ~/.config/indicators/messages/applications-blacklist sudo mv /usr/share/indicators/messages/applications/evolution ~/.config/indicators/messages/applications-blacklist

(Once the above bug is resolved in the future, move the file back to its original location and create a softlink to it from the blacklist directory: ln -s /usr/share/indicators/messages/applications/evolution ~/.config/indicators/messages/applications-blacklist/evolution)
Log out from your user session and back in.

Managing Passwords with KeePass and KeePassX

martin — Sat, 31 Jul 2010 04:07:33 +0000

Password proliferation is a serious problem on the web today. Most websites require users to register and create a username/password combination for future authentication. Unfortunately not many websites support reusable logins such as OpenID.

The main problem is that of human memory. It is of course not reasonable to expect the user to remember 100+ unique passwords. In most cases, the end result is that users will reuse the same password for multiple sites and systems. The issues with this are obvious: if a malicious third party gets hold of one password, it compromises all where the password was used; also, by piecing together the other bits of data entered at registration (such as date of birth, gender, physical address, mother’s maiden name etc.) for different sites, the attacker might be able to create a very accurate profile which could then be used to gain access to even those systems where a different password was used.

That’s why I use a password management program, in particular KeePass/KeePassX. It allows one to securely store credentials and is itself protected by strong encryption and can only be accessed using a password. In reality one finds that on a daily basis it is sufficient to remember 5-10 most frequently used passwords (computer login, email, Google, social network, internet banking and of course the password manager itself) and the rest can be referred to from KeePass. The program has a feature to generate random passwords, which is ideal to have strong passwords for infrequently accessed systems where one might not even realise the account was hacked for a long period of time. KeePass can even be installed in portable mode, which means you can carry the program and your encrypted passwords file on a USB stick.

I found that KeePass (version 1 for Windows) and KeePassX (Linux and Mac) suit my needs perfectly. There is also a version for Android and iPhone (refer to the Wikipedia article). Best of all? It’s free software licensed under GPL.