Tech Bites

Using Ghostscript to concatenate two pdf files

2010-12-06T02:35:00.000-08:00

Install Ghostscript and use the following syntax - gs -dBATCH -dNOPAUSE -q -sDEVICE=pdfwrite -sOutputFile=final.pdf file1.pdf file2.pdf

Extracting fields from multiple files using awk

2010-02-07T11:43:00.001-08:00

Say, you have two files file1 and file2 containing data in the tabular form in 3 columns each and you want to extract column 2 of file1 and column 1 of file2. We can do it using the combination of paste and awk commands as follows:

paste file1 file2 | awk '{print $2 " " $4}'

We can do more! Suppose you want to add column 1 of file1 to column 3 of file2. This can be accomplished using the following command:

paste file1 file2 | awk '{print $1-$6}'

Installing Mathabx package for latex in Ubuntu

2009-09-27T23:42:00.000-07:00

Mathabx is an important symbol library for latex. The full list of available symbols in latex can be found here. Following steps will help one to install mathabx in Ubuntu 9.04.

Download mathabx.zip from the following link.
$ unzip mathabx.zip
$ cd mathabx
$ sudo mkdir $TEXMF/fonts/source/public/mathabx
In Ubuntu 9.04, TEXMF is /usr/share/texmf-texlive
sudo cp source/*.mf $TEXMF/fonts/source/public/mathabx
sudo cp texinputs/* $TEXMF/tex/generic/misc/
Update the database
$ sudo texconfig rehash
Update the format file
$ sudo fmtutil --refresh
When you build your tex document, you might get an error that /Asterisk is defined again - To avoid that, comment the line in mathabx.dcl where /Asterisk is defined.
$ sudo gedit $TEXMF/tex/generic/misc/mathabx.dcl
Search for line where /Asterisk is present in mathabx.dcl and put a % sign at the start of the line.
You are all set!

Scheduled Shutdown using cron

2009-05-14T01:47:00.000-07:00

Use the command

$ crontab -e

In the editor window write

0 9 * * * /sbin/shutdown -h now

This will shutdown your computer at 9 hrs. After this restart cron service:

sudo /etc/init.d/cron restart

Change file permissions recursively without affecting directories

2009-05-12T03:44:00.000-07:00

If you want to change the permissions of the file to 600 without affecting the directories permissions, use the following command:

find . ! -type d -exec chmod 600 {} \;

Starting enumerate with a specified number in Latex

2008-11-25T14:20:00.000-08:00

If you want to start your enumerate with a specified number but 1, you should use \newcounter tag. The following example illustrates the use of the tag if, for example, we need to start our enumeration from 6 and not 1:

\begin{enumerate}
\setcounter{enumi}{5}
\item Hello
\item Bye
\end{enumerate}

TRICK TO GET FONTS CORRECT IN FEDORA 9

2008-11-18T03:07:00.000-08:00

Install libQtCore and libQtGui first. Then,

On a 32-bit machine

rm -f /usr/local/Wolfram/Mathematica/6.0/SystemFiles/Libraries/Linux/libQtCore.so.4

rm -f /usr/local/Wolfram/Mathematica/6.0/SystemFiles/Libraries/Linux/libQtGui.so.4

ln -s /usr/lib/libQtCore.so.4 /usr/local/Wolfram/Mathematica/6.0/SystemFiles/Libraries/Linux/libQtCore.so.4

ln -s /usr/lib/libQtGui.so.4 /usr/local/Wolfram/Mathematica/6.0/SystemFiles/Libraries/Linux/libQtGui.so.4

On a 64-bit machine,

rm -f /usr/local/Wolfram/Mathematica/6.0/SystemFiles/Libraries/Linux/libQtCore.so.4

rm -f /usr/local/Wolfram/Mathematica/6.0/SystemFiles/Libraries/Linux/libQtGui.so.4

ln -s /usr/lib64/libQtCore.so.4 /usr/local/Wolfram/Mathematica/6.0/SystemFiles/Libraries/Linux/libQtCore.so.4

ln -s /usr/lib64/libQtGui.so.4 /usr/local/Wolfram/Mathematica/6.0/SystemFiles/Libraries/Linux/libQtGui.so.4

Mathematica : Segmentation fault while starting

2008-11-18T02:52:00.000-08:00

If you have just started using mathematica on Fedora x (> 4) and the first time you run it from the terminal and it crashes giving segmentation fault at the end, here is what you need to do. The problem arises because the location of Xlib folders are changed from what it was in previous versions.

Login as root
Make sure that libX11 is installed,
rpm -q libX11

If some version is installed, then proceed
else install it

mkdir /usr/X11R6/
mkdir /usr/X11R6/lib/
mkdir /usr/X11R6/lib/X11/
cd /usr/X11R6/lib/X11/
ln -s /usr/share/X11/XKeysymDB XKeysymDB
ln -s /usr/share/X11/locale/ locale

Things should be fine now!

Skype : Problem with Audio Playback

2008-11-18T02:47:00.000-08:00

If you are getting the message "Problem with Audio Playback" while trying to make call with skype 2.0 on Linux, then you need to do the needful.

Go to - Options (By pressing Ctrl+O or by clicking Skype symbol on the bottom left and then clicking Options)

Then go to Sound devices and change Sound In and Sound options to pulse instead of Default device. Say apply and then close.

Things should be fine !

Getting Started with Joomla! 1.5 on Fedora 9

2008-11-16T06:48:00.000-08:00

Joomla! is a great open source tool for making great websites with very little effort and absolutely no cost !

I wanted to host a website on my computer and I was really impressed with the features offered by Joomla! I found it to be a pain in the neck to install it on my Fedora 9 desktop though. So, I thought of writing a beginner's review of how to install Joomla! on a linux machine. The key steps in the installation are

1) Login as root

$ su -

2) Installing Apache server, MySQL server and php

$ yum -y install mysql mysql-libs php-mysql mysql-server php-cli php-common php-pdo php-mysql php-ldap httpd php

3) Configuring Apache server

Add following lines to /etc/httpd/conf/httpd.conf

#**********************************************
ServerName SERVERNAME
NameVirtualHost IP_ADDRESS

DocumentRoot PATH

DocumentRoot PATH
ServerName SERVERNAME

#**********************************************

Replace the bold entries with the appropriate values.

4) Starting Apache and MySQL services

$ service httpd start
$ service mysqld start

5) Make sure to configure firewall to allow http connections to your computer

$ system-config-firewall &
Check the WWW(HTTP) 80/TCP box

6) Joomla Installation

$ mkdir /home/www/
$ cd /home/www/
$ mkdir Joomla
$ cd Joomla
$ untar Joomla 1.5
$ chmod -R 755 /home/www/

Making configuration.php writable
$ touch configuration.php
$ chmod 777 configuration.php

Open firefox (or any other web browser) and type
http://IP_ADDRESS/Joomla/

Complete Installation following page 8 of Joomla! v 1.5 Quick Start Guide written by Kevin Hayne.

$ rm -rf installation/

Extracting Audio from Video Files

2008-11-13T01:44:00.001-08:00

Using Mplayer (Low Quality)
mplayer -dumpaudio video_file -dumpfile audio_file.mp3

To extract audio from live stream, use -dumpstream option instead of -dumpaudio option.

Using ffmpeg (Quality Control)
ffmpeg -i video_file -ab 128 -ar 44100 audio_file.mp3

No change in command structure for ffmpeg to extract from live stream.