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)