Re: set -e yet again (Re: saving bash....)

[Eric Blake]

On 08/12/2011 06:51 AM, Greg Wooledge wrote:
> On Thu, Aug 11, 2011 at 11:56:10PM -0700, Linda Walsh wrote:
> > **Exception**
> > declare -i a
> > a=0
> > --
> >         As a is declared to be an integer, it has the results evaluated
> > at assignment time.   a=0 is an integer expression that doesn't set
> > $?=1
> > Neither should:
> > ((a=0))
>
> a=0 is an assignment.  Assignments always return 0.

No they don't.

readonly a
a=0

sets $? to non-zero.

> imadev:~$ ((a=0)); echo $?
> 1
>
> And here, the same thing, but we return false, because the value was 0.
> This is the thing about which you are complaining.  This is also one
> of the things I mention on http://mywiki.wooledge.org/BashFAQ/105 in
> which I describe how horrible and useless set -e is.

And that point has been made several times in this thread.  'set -e' is 
a historical wart - bash has it because POSIX requires it.  If you want 
to use bash extensions, then _don't_ use 'set -e', and you don't have to 
worry about how the unintuitive behavior interacts with extensions.

Greg, you missed one other useful form:

a=$((0))

This is an assignment (sets $? to 0 for all but errors like assigning to 
a readonly variable) of arithmetic expansion.  Also POSIX, and slightly 
shorter than

: $((a=0))

--
Eric Blake   eblake@redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org