Re: [Libguestfs] [p2v PATCH 1/4] Makefile.am: factor out "make-physical-machine.sh"
On Tue, Aug 30, 2022 at 07:28:43AM +0200, Laszlo Ersek wrote:
I had grepped the virt-p2v and virt-v2v shell scripts for shebangs:
1 #!/bin/sh
2 #!/bin/bash
4 #!/usr/bin/env perl
17 #!/bin/bash -
1 #!/bin/bash
1 #!/usr/bin/env python3
1 #!/usr/bin/sh
5 #!/usr/bin/env perl
67 #!/bin/bash -
The thinking seems to have been that
- python3 and perl may "move",
- bash is considered always available.
For p2v, yeah, we are pretty much guaranteed to be running on Linux,
and therefore have bash (even if /bin/sh is not bash).
>
>> +
>> +set -e -u -C
>
> My personal opinion is that 'set -e' is a crutch that should be
> avoided because of its unintended interaction with functions;
Can you please elaborate?
POSIX writes, "When a function is executed, it shall have the
syntax-error and variable-assignment properties described for special
built-in utilities in the enumerated list at the beginning of Special
Built-In Utilities", and I've checked that list -- I don't know what you
mean.
Here's a good writeup:
http://mywiki.wooledge.org/BashFAQ/105
One of the simplest demonstrations on that page:
$ bash -c 'set -e; f() { set -e; false; echo huh; }; f; echo survived'
$ bash -c 'set -e; f() { set -e; false; echo huh; }; f && echo survived'
huh
survived
That is, using 'set -e' to end function f early "works", but ONLY in
situations where f itself is not invoked in a context where 'set -e'
is suppressed because of a conditional in an outer scope. And once
'if' or '&&' suppresses 'set -e', you cannot re-enable it
within f
itself. Context-sensitive behavior of your function body based on the
context of the caller is NOT intuitive.
That said, if you KNOW that -e behaves non-intuitively, and plan to
write your script with that in mind, it can be helpful. The complaint
is that because -e is disabled in so many situations, it is harder to
prove that -e catches all the scenarios that you WANTED to be caught
than it is to just write the error handling yourself.
> but I'm
> not adamant enough about it to tell people to rip it out of scripts.
> For short scripts, like this one, it's easy enough to check that we
> aren't tripping over any of -e's surprises.
>
>> +
>> +disk=
>> +
>> +cleanup()
>> +{
>> + set +e
>> + if test -n "$disk"; then
>> + rm -f -- "$disk"
>> + disk=
>> + fi
>> +}
>> +
>> +trap cleanup EXIT
>
> Is it intentional that you are not also cleaning up on signals like
> INT and HUP?
Yes, as EXIT covers those.
Okay; currently true enough for bash, but not portable to other
shells. In fact, the Austin Group visited that topic just this week:
https://austingroupbugs.net/view.php?id=621
After 2018 edition page 2420 line 77499 section 2.14 trap, add a new
paragraph:
The EXIT condition shall occur when the shell terminates normally
(exits), and may occur when the shell terminates abnormally as a
result of delivery of a signal (other than SIGKILL) whose trap
action is the default.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org