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On booting my computer, I see multiple entries in my GRUB menu such as memtest, various Ubuntu versions and a corresponding recovery option. What does memtest mean and why are there a lot of Ubuntu entries? Does one differ from the other?

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Memtest

Memtest is for testing your system's RAM for errors. Very useful if your computer starts crashing and there is no software explanation.

Ubuntu kernels

The multiple Ubuntu entries are for the multiple kernels you have installed. Kernels are the core of every Linux system; they are what allow your operating system to use your computer's hardware. New kernels generally improve compatibility with new hardware and fix security vulnerabilities but sometimes there will be a regression--hardware that used to work no longer works with a new kernel--and therefore Ubuntu keeps some old kernels and Grub allows you to boot into an old kernel in case the new one has a regression.

Other Operating Systems

If you have other OSes on your system (from Windows to other flavors of Linux to whatever) chances are Grub will detect them and list them as well. Other versions of Linux will also likely have multiple kernels displayed.

Recovery mode

There will also be a recovery option for every kernel. You can use this in case anything ever goes wrong with your system and you can't boot up normally. Recovery mode loads the file system in read-only mode and then offers you these options:

  • resume (resumes into normal mode, sometimes a full reboot will be required to get full graphics capabilities)
  • fsck (remounts your filesystem in read/write mode and checks the filesystem, fixing any errors - useful for checking for and fixing filesystem errors that are preventing a normal boot, if you can't even boot into recovery mode you can also use a live CD/USB for this
  • clean (Tries to create freespace by cleaning the apt cache and auto-removing unneeded packages, if your disk is so full you can't even boot up normally, chances are you'll need to remove more than this, but this is a start at least)
  • apt-snapshots (I've never used these, but in theory if you use apt snapshots this will let you resume to a previous one)
  • failsafeX (This boots into a failsafe graphics mode)
  • grub (This runs update-grub, which I can't really see why if you could use grub to get into recovery mode you would need this.... but it will run all of grub's auto-detection scripts so it could be useful)
  • network (Tries to enable networking)
  • system-summary (Gives you information about your system, specifically (file)System mode, CPU information, Network connectivity state, Detailed disk usage, Software RAID state, LVM state, Detailed memory usage, Detailed network configuration info, and System database (APT) state)

and finally, the most important and useful option:

  • root (which will drop you to a root prompt where you can diagnose and fix just about any (fixable) problem with your system with enough time and patience - note that your filesystem is still mounted in read-only mode so you will need to remount it with write access to make changes)

These might be out of order, I didn't actually reboot and look at the menu I just got the options from the scripts (stored at /usr/share/recovery-mode/) Hope this helps!

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But I did not install any new kernels! Also can you elaborate on the recovery option a bit more? –  harisibrahimkv May 24 '12 at 4:23
    
Update manager will automatically select and download new kernels for you (all you need to do is press download), in addition to adding hardware compatibility there are also security updates in new kernels. Sure, I'll update my answer with some more info on the recovery option. –  adempewolff May 24 '12 at 4:40
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There are three kinds of entries in my system, (dual booting Ubuntu with Windows 7).

The first two entries are for your Linux system:

  1. First one is the Normal boot entry, it boots your Ubuntu system with normal graphics mode, loads all necessary modules, display boot splash etc.

  2. Second one is for recovery options, It normally does not load the modules, (sometimes disables full graphics support, only access to Unity 2d). It gives you options to clean your system, reset password, give a chance to access on root user to fix problems, give a chance to fix dependency problems etc.

  3. Third one loads Windows 7

  4. memtest options are used to inspect any memory problems you have.

Hope this will help.

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