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tldr: I propose we use bash explicitly for all diskimage-builder scripts 
(at least for the short-term - see details below).

This is something that was raised on my linting changes to enable set -o 
pipefail.  That is a bash-ism, so it could break in the 
diskimage-builder scripts that are run using /bin/sh.  Two possible 
fixes for that: switch to /bin/bash, or don't use -o pipefail

But I think this raises a bigger question - does diskimage-builder 
require bash?  If so, I think we should just add a rule to enforce that 
/bin/bash is the shell used for everything.  I know we have a bunch of 
bash-isms in the code already, so at least in the short-term I think 
this is probably the way to go, so we can get the benefits of things 
like -o pipefail and lose the ambiguity we have right now.  For 
reference, a quick grep of the diskimage-builder source shows we have 
150 scripts using bash explicitly and only 24 that are plain sh, so 
making the code truly shell-agnostic is likely to be a significant 
amount of work.

In the long run it might be nice to have cross-shell compatibility, but 
if we're going to do that I think we need a couple of things: 1) Someone 
to do the work (I don't have a particular need to run dib in not-bash, 
so I'm not signing up for that :-) 2) Testing in other shells - 
obviously just changing /bin/bash to /bin/sh doesn't mean we actually 
support anything but bash.  We really need to be gating on other shells 
if we're going to make a significant effort to support them.  It's not 
good to ask reviewers to try to catch every bash-ism proposed in a 
change.  This also relates to some of the unit testing work that is 
going on right now too - if we had better unit test coverage of the 
scripts we would be able to do this more easily.

Thoughts?

Thanks.

-Ben