in comments from ubuntu-devel
[17:50] <juliank> Continuation lines seem wrong to have in that file
[17:50] <juliank> If you add a continuation line between 8.14 and 8.15,
you can remove the continuation line again
[17:51] <juliank> A future update should probably handle that, but a
continuation line here is not expected, and there's no point rolling it back as
that's a non default unexpected thing
as stated clearly at
https://www.gnu.org/software/grub/manual/grub/html_node/Simple-
configuration.html#Simple-configuration-handling
...
The file /etc/default/grub controls the operation of grub-mkconfig. It
is sourced by a shell script, and so must be valid POSIX shell input; normally,
it will just be a sequence of ‘KEY=value’ lines, but if the value contains
spaces or other special characters then it must be quoted
...
per POSIX
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_02_01
Escape Character (Backslash)
A <backslash> that is not quoted shall preserve
the literal value of the following character, with the exception of a
<newline>. If a <newline> follows the <backslash>, the shell shall
interpret this as line continuation. The <backslash> and <newline> shall
be removed before splitting the input into tokens. Since the escaped
<newline> is removed entirely from the input and is not replaced by any
white space, it cannot serve as a token separator.
all that's to say that continuation lines are, and have been, supported
in posix-shell-compliant /etc/default/grub
that it's "non default unexpected thing" is specious ... at _best_ it's
unused/untested by the pacakgers.
fwiw, with grub2-2.04 on numerous _other_ systems there are no problems
whatsoever with these continuation lines; and Ubu 18LTS didn't either
... until 'recently'.
no, i don't yet have an identifying bisect ...
also, it's been commented that current bug is 'non-destructive'.
that's open to interpretation.
modifying an end-user config file persistently, and without user-
interaction/-confirmmation ... such that a subsequent
(re)upgrade/(re)install doesn't fix the problem, but rather requires a
manual edit/fix of the config in order qualifies as 'destructive'. here,
anyway.
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https://bugs.launchpad.net/bugs/1868138
Title:
18.04LTS upgrade of grub-common:amd64 FAILs in post-install;
incorrectly REWRITES user's /etc/default/grub
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