What can I do to get my Unity environment back on Ubuntu 12.10 from the start, without using the terminal everytime ? and what should I do to prevent the all system to freeze once done ?
That sounds like something might be corrupted. You might want to try reinstalling Unity.
sudo apt-get purge unity should do it (note: I haven't actually tried it, and I'm not on an Ubuntu machine right now, so the package name might be something slightly different than "unity"). Then, reinstall it with
sudo apt-get install unity (or whatever the exact package name turns out to be).
Is there a way to merge those two bars ? Or is there a way to hide the Activities bar when I'm not using it like on Unity environment ?
The WebUpd8 team has an global menu extension (see their LaunchPad page for installation details). I don't know how well it works (the last post I saw on it was for Ubuntu 11.10, when the extension was still in alpha; the PPA says it's on version 0.9), but it's easy enough to install and try.
If that doesn't work, it is possible to tweak the bar through other methods. There are a few ways that you may be able to accomplish basically what you want:
Install Gnome-Tweak-Tool (Easy, may not get you what you want) This is probably the easiest way, though I don't know if it will have what you want. Just run
sudo apt-get install gnome-tweak-tool to get it, then open it up and find the settings for the Activities bar. Whatever features GTT is set up to change, you'll be able to change them there.
Resize the bar (Easy, may not get you what you want) A similar question over at Unix & Linux offers up a simple way to resize the bar:
Hold both "Windows key + Alt" then "Right-click" the bar.
Then click "Properties".
In the "General Tab", simply resize the bar.
Install a different theme (Medium; easy to do, but a lot of work involved; may or may not get what you want) Here's a good tutorial for doing so. Basically, install Gnome-Tweak and Gnome-Extensions, and find a theme you like, install it, and enable it in Gnome-Tweak.
Find your theme's CSS file This will depend on your install, and whether you're using a custom theme. I recommend Googling something like "Ubuntu 12.10 gnome shell theme location", though the above Unix & Linux cites
/usr/share/gnome-shell/theme/gnome-shell.css as the location (this may be different if you have a custom theme). Then, open it with Gedit, or your favorite text editor (may need
sudo for this).
#panel block Once you've found the
#panel block, you can change a number of its attributes, using CSS. Adjusting the font size will make it shorter (it's set up so its height is based on the text size).
I'm not totally sure if this will work, since the Gnome devs have only implemented some of the CSS properties, but you may be able to do this to show/hide the top bar:
height: 1px; /* Adjust to desired minimize height, while still allowing you to hover over it. */
/* Other properties. */
height: 12px; /* Adjust to desired full size. */
Reload Gnome Shell Press
alt+F2, then type
r in the box, and hit
enter. This will reload the Gnome Shell (yes, it may disappear momentarily, but it will reappear).
For more things to tweak in the CSS, there's this awesome little post on the Ubuntu forums.