How to change default application for a filetype?

Right click on the file, choose Properties, select Open with tab, change the application.

How to change your MAC Addresses

$ sudo gedit /etc/network/interfaces

add the hwaddress line under the apposite device - for example:

auto eth0
#iface eth0 inet dhcp
hwaddress ether UR:MA:CA:DD:RE:SS


(ether is the type of device, according to ifconfig - "Link encap:Ethernet")


to restart / stop /start networking:

$ sudo /etc/init.d/networking restart
$ sudo /etc/init.d/networking stop
$ sudo /etc/init.d/networking start

ImageMagick converting multiple files

*I do not use this method anymore, although it was a great start.*

After being a book-worm for half a day, I found out how can I write a working script to solve my imagemagick output problem.
It looks like this:

#!/bin/sh

for i in `find -iname '*.jpg'`
do
convert $i -background white -flatten -colorspace Gray -negate -edge 1 -negate -normalize -sharpen 1 -threshold 50% -despeckle -blur 0x.5 -normalize +dither -posterize 8 gif:- >> $i.gif
done

1.) open your favorite text editor
2.) copy paste the script
3.) save as anyname.sh
4.) make it executable

Usage:
1.) copy anyname.sh in the folder containing only the image files you want to make simple
2.) go (cd) to this folder in the terminal
3.) type ./anyname.sh
4.) wait and see.

How to convert this to pdf?

mogrify -format pdf *.JPG

How to join these single-page pdfs?

pdfjoin *.pdf --outfile newname.pdf