Error in API call to delete - Vagrant

These days, I'm heavily working with Vagrant and Chef. During my working sessions, I created several scripts then run "vagrant up", scripts fail then "vagrant destroy" then do some modifications, run "vagrant up" again and this loop occurs for many times. Suddenly, "vagrant up" doesn't run but arises this message:

[node1] Destroying VM and associated drives...
/var/lib/gems/1.8/gems/virtualbox-0.9.1/lib/virtualbox/com/implementer/ffi.rb:106:in `call_and_check': Error in API call to delete: 2147944126 (VirtualBox::Exceptions::COMException)
    from /var/lib/gems/1.8/gems/virtualbox-0.9.1/lib/virtualbox/com/implementer/ffi.rb:80:in `call_vtbl_function'

Looking into log files (${HOME}/.VirtualBox), I found that for an unknown reason "vagrant destroy" can not delete the directory which contains the disk attached to the virtual machine.  Solution: erase that directory by hand. Problem solved!

extracting audio content from video files with open source tools

These days, I have downloaded many music videos and I have enjoyed them but I don't only want to watch them but also listen them. I looked in Internet and found this interesting link which give many tips about how to transform and extract information from multimedia files using ffmpeg.

Then, I created this bash script which extracts audio content from music videos.

#!/bin/bash
OUTPUT=""
if [ $# = 1 ] ; then
    OUTPUT=${1%.*}.mp3
elif [ $# = 2 ] ; then
    OUTPUT=${2}
else
    echo "Usage:"
    echo "     ${0} video_filename output.mp3"
    exit
fi
/usr/bin/ffmpeg -i "${1}" -vn -ar 44100 -ac 2 -ab 192 -f mp3 "${OUTPUT}"

Cut and paste it into your preferred editor (VI). This link explains how to retrieve just the file name without its extension.

Getting high resolution images from PDF files

Today I was preparing a lecture for Thursday (I guess) and I found very illustrative images from a PDF book but I got some problems for extracting them from the book.

• Acroread (for Linux) didn't keep the image in the clipboard 
• The "convert" tool (from Imagemagick) by default, made a poor conversion from PDF to any format

I searched in Internet and I found these two links that gave me a clue

• http://ardvaark.net/useful-pdf-imagemagick-recipes
• http://www.imagemagick.org/discourse-server/viewtopic.php?f=1&t=11476

For my needs "convert -density 300 pdffile.pdf image.png" did the trick. The image.png file gave me an excellent resolution. From that point, I used gimp for cropping the region that I need.