Can't Create Bootable DVD Of Ubuntu 18.04

[John_Patrick_Mason]

@CatKiller-AKA-DogLover

If you're able to boot from it, there's a "check for errors" option that does the same thing.

It's been awhile since I've installed from a live DVD -- I'm still on Ubuntu 16.04 -- but I vaguely remember seeing that option somewhere in the past, but I couldn't find it when I tried to boot from the bootable DVD that I just created. Do I need to click on "Try Ubuntu" first, or is it before that?

Edit: Nevermind, I found the option by holding any key down. I usually check the integrity of any live CD I create through K3b, rather than using that method.

[CatKiller]

It's been awhile since I've installed from a live DVD -- I'm still on Ubuntu 16.04 -- but I vaguely remember seeing that option somewhere in the past, but I couldn't find it when I tried to boot from the bootable DVD that I just created. Do I need to click on "Try Ubuntu" first, or is it before that?

Even longer for me: almost all of my machines have no optical drive. I'm pretty sure it's on the very first menu that you see; if the disc's dodgy you don't want to have to load a bunch of (potentially broken) stuff before you get to check. I think they also went through a phase of some menus being hidden behind a keypress, but I don't know if that's still the case.

[John_Patrick_Mason]

Even longer for me: almost all of my machines have no optical drive. I'm pretty sure it's on the very first menu that you see; if the disc's dodgy you don't want to have to load a bunch of (potentially broken) stuff before you get to check. I think they also went through a phase of some menus being hidden behind a keypress, but I don't know if that's still the case.

You're right. I just tested the bootable DVD that I successfully created. The "check CD for defects" option is hidden by default, unless you press a key -- I think, any key will do -- soon after the boot screen. I had almost forgot about it, since that's not usually how I verify the integrity of my live CDs. And that's true for Ubuntu 16.04, as well; I kept my Ubuntu 16.04 installation disc, in case I rendered my Ubuntu partition unbootable -- it happened to me once, while making changes to system files. Ubuntu 14.04 might have been different, but it's no longer supported, anyway.

I should say that the reason I've stuck to DVDs for so long is because when I did try to create a bootable USB stick with Ubuntu 16.04 on it, it wouldn't boot. Ubuntu 18.04 works fine, though -- on a USB stick.

[scdbackup]

Hi,

> by closed, you mean the xorriso-burned DVD can no longer be written to,
> correct?

Yes. A DVD+R can have three states: Blank, Appendable, Closed.
Blank means it is yet unused. Appendable means that it contains data and
still can take another burn run ("session"). Closed means that it cannot
be written any more.

CD-RW and unformatted DVD-RW can be blanked to become writable from scratch
again. CD-R, DVD-R, DVD+R, BD-R stay closed forever. DVD-RAM, DVD+RW, BD-RE,
formatted DVD-RW, formatted CD-RW can be overwritten without blanking,
because they are in the fourth possible state: Overwritable.

> So, what do you think the problem was, a malfunctioning CD/DVD drive?

Obviously. Now the question is: Does it just have some dust on the lense
or is it entirely dead. A good blow with pressured air into the open
drive might work wonders.

--------------------------------------------------------------------------

As for checkreading, i would propose to read the plain DVD+R content instead
of mounting it and checking its files. If the overall image is ok, the files
can hardly be damaged. (libmath says 0 if i try to estimate the probability.
Well, numerical math is always a bit optimistic.)

It is important to read only as many blocks from the DVD as the original
ISO image has. Else the checksum will hardly match.

blocks=$(expr $(stat -c '%s' ubuntu-18.04.3-desktop-amd64.iso) / 2048)
dd if=/dev/sr1 bs=2048 count=$blocks | md5sum

Compare the resultiing hex number with the hex number in line

72491db7ef6f3cd4b085b9fe1f232345 *ubuntu-18.04.3-desktop-amd64.iso

of http://releases.ubuntu.com/18.04/MD5SUMS


> I don't know what a non-matching md5sum looks like.

It will be just some string of 32 hex digits [0-9a-f] which is not the same
as the one you expect. E.g. "hallo\n" has this MD5:

$ echo hallo | md5sum
aee97cb3ad288ef0add6c6b5b5fae48a  -

If you are insecure about comparing 32 digits, let the shell do it for you:

$ test 72491db7ef6f3cd4b085b9fe1f232345 = 72491db7ef6f3cd4b085b9fe1f232345 && echo OK
OK
$ test 72491db7ef6f3cd4b085b9fe1f232345 = 72491db7ef6f3cd4b085b9fe1f232340 && echo OK
$

(I.e. chirping crickets are a negative outcome.)

Have a nice day

Thomas

[John_Patrick_Mason]

blocks=$(expr $(stat -c '%s' ubuntu-18.04.3-desktop-amd64.iso) / 2048)
dd if=/dev/sr1 bs=2048 count=$blocks | md5sum

OK, I've used stat for other purposes, mainly to check access times, which I would think would be useful in a server environment, but, it in this case, stat -c fetches the total number of bytes of the ISO file. Then, what?

The book that I learned the command line from didn't cover the expr command, but I can see that the expr command uses the output of the stat command as input (command substitution), and that the output of the expr command, ends up getting divided by 2048 in an arithmetic expansion. Is that correct?

[scdbackup]

Hi,

> total number of bytes of the ISO file [...] divided by 2048
> Is that correct?

Yes. Ye olde way of doing arithmetics in shell.

$ stat -c '%s' ubuntu-18.04.3-desktop-amd64.iso
2082816000
$ expr 2082816000 / 2048
1017000

The stat run is put in "$(...)" so that it provides its output as argument
for the expr run. This too is put in "$(...)" to provide its output as content
for the variable assignment "blocks=".

The kids would save 3 characters of line length by

blocks=$(($(stat -c '%s' ubuntu-18.04.3-desktop-amd64.iso) / 2048))

Reason for this computation is that dd would be awfully slow if we let
it read and count single bytes (by bs=1). The block size bs=2048 is
the natural granularity of optical data storage and of ISO 9660 filesystems.
So we can safely assume that the division yields an integer result.

The man page of expr can be displaid on a terminal by

man expr

Have a nice day

Thomas