== Bashisms in _#!/bin/sh_ scripts are not necessarily bugs

In the wake of [[Shellshock
http://en.wikipedia.org/wiki/Shellshock_%28software_bug%29]], any number
of people have cropped up in any number of places to say that you should
always be able to change a system's _/bin/sh_ to something other than
Bash because Bashisms in scripts that are specified to use _#!/bin/sh_
are a bug. It is my heretical view that these people are wrong in general
(although potentially right in specific situations).

First, let us get a trivial root out of the way: a Unix distribution
is fully entitled to assume that you have not changed non-adjustable
things. If a distribution ships with _/bin/sh_ as Bash and does not have
a supported way to change it to some other shell, then the distribution
is fully entitled to write its own _#!/bin/sh_ shell scripts so that
they use Bashisms. This may be an unwise choice on the distribution's
part, but it's not a bug unless they have an official policy that all
of their shell scripts should be POSIX-only.

(Of course the distribution may act on {{AB:RFE:Request For
Enhancement}}s that their _#!/bin/sh_ scripts not use Bashisms. But
that's different from it being a *bug*.)

Next, let's talk about user scripts. On a system where _/bin/sh_ is
always officially Bash, ordinary people are equally entitled to assume
that your systems have not been manually mangled into unofficial states.
As a result they are also entitled to write their _#!/bin/sh_ scripts
with Bashisms in them, because these scripts work properly on all
officially supported system configurations.  As with distributions,
this may not be a wise choice (since it may cause pain if and when
they ever move those scripts to another Unix system) but it is not a
*bug*.  The only case when it even approaches being a bug is when the
distribution has officially included large warnings saying '_/bin/sh_ is
currently Bash but it may be something else someday, you should write
all _/bin/sh_ shell scripts to POSIX only, and here is a tool to help
with that'.

There are some systems where this is the case and has historically been
the case, and on those systems you can say that people using Bashisms
in _#!/bin/sh_ scripts clearly have a bug by the system's official
policy. There are also quite a number of systems where this is or has
not been the case, where the official _/bin/sh_ is Bash and always
has been. On those systems, Bashisms in _#!/bin/sh_ scripts are not a
bug.

(By the way, only relatively recently have you been able to count
on _/bin/sh_ being POSIX compatible; see [[here
../unix/PortableShThreeUnixes]].  Often it's had very few guarantees.)

By the way, as a [[pragmatic matter ../unix/PortableShThreeUnixes]]
a system with only Bash as _/bin/sh_ is likely to have plenty of
_/bin/sh_ shell scripts with Bashisms in them even if the official
policy is that you should only use POSIX features in such scripts.
This is a straightforward application of [[one of my aphorisms of
system administration SysadminAphorismII]] (and perhaps also [[this
one SysadminAphorismIII]]). These scripts have a nominal bug, but
of course people are not going to be happy if you break them.