On 8/28/19 5:58 PM, John Snow wrote: >> +++ b/tests/qemu-iotests/common.rc >> @@ -60,61 +60,132 @@ if ! . ./common.config >> exit 1 >> fi >> >> +# Unset the variables to turn Valgrind off for specific processes, e.g.
That's not unsetting, that's setting to the empty string.
>> +# $ VALGRIND_QEMU_IO= ./check -qcow2 -valgrind 015
>> +
>> +: ${VALGRIND_QEMU_VM='y'}
>> +: ${VALGRIND_QEMU_IMG='y'}
>> +: ${VALGRIND_QEMU_IO='y'}
>> +: ${VALGRIND_QEMU_NBD='y'}
>> +: ${VALGRIND_QEMU_VXHS='y'}
>> +
>
> I have to admit to you that I'm not familiar with this trick. I'm
> looking it up and I see := documented, but not = alone.
It's been a repeated complaint to the bash developer that the manual is
doing a disservice to its users by not documenting ${var=val} in an
easily searchable form. It IS documented, but only by virtue of
${var:=val} occurring under a section header that states:
When not performing substring expansion, using the forms
documented
below (e.g., :-), bash tests for a parameter that is unset or
null.
Omitting the colon results in a test only for a parameter
that is
unset.
So the choice is whether you want to special case a variable set to an
empty string the same as an unset variable, or the same as a variable
with a non-empty value.
>
> It doesn't seem documented here at all:
> https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html
>
> I see it here, though:
> https://www.tldp.org/LDP/abs/html/parameter-substitution.html
>
> And it seems to work, but I'm not sure if this works with BSD or OSX's
> sh. I see Eric comment on that compatibility a lot, so maybe I'll let
> him chime in.
It's quite portable; POSIX requires it, and autoconf relies on it.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
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