Re: [Qemu-devel] [PATCH v6 1/6] iotests: allow Valgrind checking all QEMU processes

[Eric Blake <eblake@redhat.com>]

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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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