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

[Demi Marie Obenour <demiobenour@gmail.com>]

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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))

?
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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