How do I apt-get -y dist-upgrade without a grub config prompt?

The /etc/default/grub file is generated at package install time, which is necessary because it integrates with debconf. This means that it can not treated as a dpkg conf file, and so dpkg's configuration file handling doesn't know about it.

Instead, it uses ucf, a more sophisticated Debian tool for handling configuration. This, unfortunately, doesn't understand dpkg options, so setting Dpkg::Options::="--force-confdef" won't help. It does have its own way of doing no-prompt upgrades, though, through the UCF_FORCE_CONFFNEW and UCF_FORCE_CONFFOLD environment variables.

ucf uses debconf for prompting, so setting the debconf interface to noninteractive will also silence the message. If you really want non-interactive updates you'll need to do this anyway - arbitrary packages may ask debconf questions (although they generally won't during upgrades).

You can set the debconf interface as a one-off by adding DEBIAN_FRONTEND=noninteractive to your environment, or can set it permanently by running dpkg-reconfigure debconf and selecting the noninteractive frontend. If you're using the noninteractive frontend you'll get the default answer for any questions a package might ask.

For ucf, the default answer is “keep the existing file”.

So, the full command to do a really, 100% guaranteed¹ no-prompting update would be.

sudo DEBIAN_FRONTEND=noninteractive apt-get -y -o Dpkg::Options::="--force-confdef" -o Dpkg::Options::="--force-confold" dist-upgrade

¹: It's technically possible for packages to use another method of prompting than debconf, but this is against Debian policy. If you run across such a package, file a bug.

going off of RAOF's answer and after spending countless hours searching on the web to be able to perform a completely hands-off update & dist-upgrade on Ubuntu 12.04, i came up with this thanks to the fact this post (https://bugs.launchpad.net/ubuntu/+source/grub/+bug/239674/comments/1) points out that grub adheres to UCF and not Dpkg Options when you want to use the package maintainers grub menu.lst instead of any possible local menu.lst edits.

i left the Dpkg force-confnew options in for other packages that aren't grub.

#!/bin/bash

unset UCF_FORCE_CONFFOLD
export UCF_FORCE_CONFFNEW=YES
ucf --purge /boot/grub/menu.lst

export DEBIAN_FRONTEND=noninteractive
apt-get update
apt-get -o Dpkg::Options::="--force-confnew" --force-yes -fuy dist-upgrade

The previous solutions wouldn't work with 16.04. This works from this answer on stack overflow:

sudo DEBIAN_FRONTEND=noninteractive apt-get -y -o DPkg::options::="--force-confdef" -o DPkg::options::="--force-confold"  install grub-pc

After much testing, the only thing that worked was:

dpkg-reconfigure debconf -f noninteractive -p critical
UCF_FORCE_CONFFOLD=YES apt -o Dpkg::Options::="--force-confdef" -o DPkg::Options::="--force-confold" -y dist-upgrade

For what I was doing it (Ubuntu 16.04 in Vagrant with provisioning), the DEBIAN_FRONTEND=noninteractive was not enough. Thanks others in this thread, I needed to add UCF_FORCE_CONFFOLD=YES for grub-pc

I've been wrangling with the same issue on Ubuntu 18.04 the past few days. On launching a new EC2 instance (specifically ami-00035f41c82244dab), I run an automated provisioning script (via User Data config supplied at the time of initialisation), one of the first steps of which is to run apt update/upgrade.

The script gets blocked while the user is prompted about modified GRUB files - first /etc/default/grub, and then /boot/grub/menu.lst. Since this is running in an unattended mode when supplied as User Data, the process stalls and never recovers.

From lots of Googling, it seems this has been a long-running GRUB issue in one form or another, with fixes being applied and then latter regressing again, as far as I can tell.

Ultimately, the only workaround I have been able to apply successfully is the following ugly hack in my provisioning script. Hopefully it might get somebody else out of a bind though!

#!/bin/bash 

apt update

### Workaround: Pre-update /etc/default/grub and remove /boot/grub/menu.lst to avoid 'file changed' prompts from blocking completion of unattended update process
patch /etc/default/grub <<'EOF'
10c10
< GRUB_CMDLINE_LINUX_DEFAULT="console=tty1 console=ttyS0"
---
> GRUB_CMDLINE_LINUX_DEFAULT="console=tty1 console=ttyS0 nvme.io_timeout=4294967295"
19c19
< GRUB_TERMINAL=console
---
> #GRUB_TERMINAL=console
EOF
rm /boot/grub/menu.lst

apt upgrade -y

### Workaround part 2: re-generate /boot/grub/menu.lst
/usr/sbin/update-grub-legacy-ec2 -y

I can only assume the problem I encountered is quite specific to the Ubuntu 18.04 AMI version currently available, and any updated version that incorporates newer GRUB packages may not be subject to the same issue. In particular, the nature of the changes to /etc/default/grub are not likely to be applicable to newer versions of the AMI. Just putting this out there anyway.