Comment 7 for bug 1868138

Anyway, both

GRUB_CMDLINE_LINUX="moo
bar"

and

GRUB_CMDLINE_LINUX="foo \
bar"

break.

I have looked at OPs claim that this is a new regression, and verified that this is a wrong assertion - it was broken in 8.13:

ubuntu@boo1:~$ sudo apt install ./*.deb
Reading package lists... Done
Building dependency tree
Reading state information... Done
Note, selecting 'grub-common' instead of './grub-common_2.02-2ubuntu8.13_amd64.deb'
Note, selecting 'grub-efi-amd64-bin' instead of './grub-efi-amd64-bin_2.02-2ubuntu8.13_amd64.deb'
Note, selecting 'grub-efi-amd64' instead of './grub-efi-amd64_2.02-2ubuntu8.13_amd64.deb'
Note, selecting 'grub2-common' instead of './grub2-common_2.02-2ubuntu8.13_amd64.deb'
Suggested packages:
  multiboot-doc grub-emu xorriso desktop-base
The following packages will be upgraded:
  grub-common grub-efi-amd64 grub-efi-amd64-bin grub2-common
4 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 0 B/3006 kB of archives.
After this operation, 35.8 kB of additional disk space will be used.
Get:1 /home/ubuntu/grub-efi-amd64_2.02-2ubuntu8.13_amd64.deb grub-efi-amd64 amd64 2.02-2ubuntu8.13 [47.8 kB]
Get:2 /home/ubuntu/grub-efi-amd64-bin_2.02-2ubuntu8.13_amd64.deb grub-efi-amd64-bin amd64 2.02-2ubuntu8.13 [654 kB]
Get:3 /home/ubuntu/grub2-common_2.02-2ubuntu8.13_amd64.deb grub2-common amd64 2.02-2ubuntu8.13 [533 kB]
Get:4 /home/ubuntu/grub-common_2.02-2ubuntu8.13_amd64.deb grub-common amd64 2.02-2ubuntu8.13 [1771 kB]
Preconfiguring packages ...
(Reading database ... 59745 files and directories currently installed.)
Preparing to unpack .../grub-efi-amd64_2.02-2ubuntu8.13_amd64.deb ...
Unpacking grub-efi-amd64 (2.02-2ubuntu8.13) over (2.02-2ubuntu8) ...
Preparing to unpack .../grub-efi-amd64-bin_2.02-2ubuntu8.13_amd64.deb ...
Unpacking grub-efi-amd64-bin (2.02-2ubuntu8.13) over (2.02-2ubuntu8) ...
Preparing to unpack .../grub2-common_2.02-2ubuntu8.13_amd64.deb ...
Unpacking grub2-common (2.02-2ubuntu8.13) over (2.02-2ubuntu8) ...
Preparing to unpack .../grub-common_2.02-2ubuntu8.13_amd64.deb ...
Unpacking grub-common (2.02-2ubuntu8.13) over (2.02-2ubuntu8) ...
Setting up grub-common (2.02-2ubuntu8.13) ...
Installing new version of config file /etc/grub.d/00_header ...
update-rc.d: warning: start and stop actions are no longer supported; falling back to defaults
Setting up grub-efi-amd64-bin (2.02-2ubuntu8.13) ...
Setting up grub2-common (2.02-2ubuntu8.13) ...
Setting up grub-efi-amd64 (2.02-2ubuntu8.13) ...
/etc/default/grub: line 25: unexpected EOF while looking for matching ``'
/etc/default/grub: line 36: syntax error: unexpected end of file
Sourcing file `/etc/default/grub'
/usr/sbin/grub-mkconfig: 36: /etc/default/grub: Syntax error: EOF in backquote substitution
dpkg: error processing package grub-efi-amd64 (--configure):
 installed grub-efi-amd64 package post-installation script subprocess returned error exit status 2
Processing triggers for install-info (6.5.0.dfsg.1-2) ...
Processing triggers for systemd (237-3ubuntu10.39) ...
Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
Processing triggers for ureadahead (0.100.0-21) ...
Errors were encountered while processing:
 grub-efi-amd64
E: Sub-process /usr/bin/dpkg returned an error code (1)