Re: [PATCH v2] scripts/make-gpt.sh: allow setting partition size

[Alyssa Ross <hi@alyssa.is>]

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On Fri, Nov 28, 2025 at 04:31:21PM -0500, Demi Marie Obenour wrote:
> On 11/28/25 16:26, Alyssa Ross wrote:
> > Demi Marie Obenour <demiobenour@gmail.com> writes:
> >
> >> On 11/28/25 16:14, Alyssa Ross wrote:
> >>> Demi Marie Obenour <demiobenour@gmail.com> writes:
> >>>
> >>>> On 11/28/25 08:22, Alyssa Ross wrote:
> >>>>> We want non-minimally-sized partitions to leave space for updates.
> >>>>> All such partitions will also be labelled, so we can just add another
> >>>>> optional field at the end.
> >>>>>
> >>>>> Since we don't parse partition specifications in sh, we can't keep a
> >>>>> running total any more, so instead we just go through the table at the
> >>>>> end and add up all the sizes, taking advantage of our knowledge that
> >>>>> the size will always be the last thing in each line in our tables.
> >>>>>
> >>>>> Signed-off-by: Alyssa Ross <hi@alyssa.is>
> >>>>> Message-ID: <20251127174054.2056835-2-hi@alyssa.is>
>
> (snip)
>
> >>>>>  # Keep 1MiB free at the start, and 1MiB free at the end.
> >>>>> -gptBytes=$((ONE_MiB * 2))
> >>>>> -for partition; do
> >>>>> -	sizeMiB="$(sizeMiB "${partition%%:*}")"
> >>>>> -	table="$table${nl}size=${sizeMiB}MiB,$(awk -f "$scriptsDir/sfdisk-field.awk" -v partition="$partition")"
> >>>>> -	gptBytes="$((gptBytes + sizeMiB * ONE_MiB))"
> >>>>> -done
> >>>>> +gptMiB=2
> >>>>> +while read -r partition; do
> >>>>> +	# Here we rely on sfdisk-field.awk always putting size last.
> >>>>> +	: $((gptMiB += ${partition##*=}))
> >>>>> +done <<EOF
> >>>>> +$table
> >>>>> +EOF
> >>>>
> >>>> Nit: should ${partition##*=} be in double quotes?
> >>>
> >>> I tried changing it, but it is apparently an "arithmetic syntax error"
> >>> to use double quotes here.
> >>
> >> I'd use a separate variable then.
> >
> > Doesn't seem to make a difference.  As an experiment:
> >
> > partition="size=2 + 3"
> > size=${partition##*=}
> > printf "%s\n" $(($size))
> >
> > This prints 5 even though a separate variable is used.  It doesn't seem
> > to be possible to get $(( … )) *not* to interpret arithmetic in
> > variables.
> >
> > In bash, but not busybox ash as used in Nix builds, it is possible to
> > use double quotes like you suggested, but $(("$size")) is /still/ 5.
>
> That's actually required by POSIX.  What about:
>
> size=${partition##*=}
> : $((gptMiB += size))

How's that better?  Arithmetic will still be expanded inside size —
or do you have some other concern with the parameter expansion inside
the arithmetic expansion?

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