Re: Revisiting Error handling (errexit)

[Robert Elz]

    Date:        Wed, 6 Jul 2022 01:34:53 +0300
    From:        Yair Lenga <yair.lenga@gmail.com>
    Message-ID:  <6B7E3C85-5FCD-459E-A41C-E2803B0E787A@gmail.com>


  | Function main () { 
  |     Local x.   # x is local
  |     For x in a b ; do process $x ; done
  | }
  |
  | Vs.
  | # x is global, all function will see it.
  | For x in a b ; do process $x ; done

That is not how local variables work...

Consider:

cat /tmp/script; echo ===========================
func1()
{
        echo In func1 x=$x
}

func2()
{
        local x=foo

        echo In func2 x=$x
}

func3()
{
        local x=bar

        echo In func3 x=$x
        func1
}

x=anything
echo Globally x=$x

func1
func2
func3

echo Globally x=$x
===========================

Then

bash /tmp/script
Globally x=anything
In func1 x=anything
In func2 x=foo
In func3 x=bar
In func1 x=bar
Globally x=anything

Note the call of func1 from func3 and which x value it sees.

The (ideal) shell model (IMO) for variables is that all variables
are global, always.   If we don't have that then it is hard to see
how to make things like OPTIND or IFS local, and work as expected,
(PATH as well) and explain how those work differently than how you
would expect other variables to work.

What local should do is arrange for any changes to a variable listed as
local to be undone as soon as the function that declared it local is no
longer active (however that happens), so when func3() makes x local,
the old value of x is saved somewhere, x is set (to bar in the above
script) and remains that way while func1 runs, when func3 is done, x
gets restored to its previous value (the effect of the local ends).

The model bash uses isn't exactly that, it is much harder to explain
(and to my mind, irrational, but that's neither here nor there), but
it isn't all that far away either.

In neither of these are local variables in bash anything like local
variables in languages designed as a programming language.

Remember that bash (and all of the other posix style shells, there are
quite a few) is primarily a shell - an environment for users to use to
type commands.   It has its script ability from the genius of Ken Thompson,
who (about 50 years ago, when this idea was revolutionary) recognised that
if users could put the commands they habitually entered into a script, and
have the shell simply run the script, they wouldn't need to type the same
command sequences over and over again.   Seems simple, and obvious, today,
it wasn't then - that's not how other contemporary OS's worked.

Users typing make decisions based upon what happens in earlier commands,
so the scripts needed to be able to do that as well, hence we gained
if (and later the looping commands - originally just if and goto), which
handle any anticipated decisions that users need to make, but users also
deal with unanticipated problems - things that can go wrong which weren't
planned for, that no-one had considered might happen.   The typical user
reaction to one of those when it happens interactively is to just stop,
and either think, or seek help - certainly not just plow on with whatever
was supposed to happen next, which is what a script would do.   That's
where -e (errexit) comes from, when something unanticipated fails, stop.
Note the unanticipated - when something that we expect might fail, fails,
we have already provided the alternate code sequence, for that, so we
don't stop.

This all sounds simple, but when (as you are discovering now I think) you
start to look at all of the weird cases, it isn't so simple - the effect
was that the implementation was a bit strange, and didn't do things quite
the way that people often thought it did - but that was the implementation,
and hence the standard, and now it is more or less locked in - other than
in the simplest cases, -e is useless.

But the solution to this isn't your idea, which will (almost inevitably)
end up being just as convoluted as -e, and just as weird (though differently)
as you try and cope with all the odd cases and make them behave as people
expect should happen.   Instead (if you're writing a shell script, bash or
any other shell) the solution is as (I think) Greg suggested - simply 
anticipate that anything might fail, decide what should happen in each case
if it does fail (which is anything from "I don't care" to "abandon the entire
script" with a whole range of possibilities in between).

If you do that, then neither -e, nor your proposed option, has anything
to do, they're both no-ops, and the code always behaves as implemented by
its author.

But since you seem to be attempting to emulate features of some other
languages, languages not primarily designed to be shells, the better
question is why you're not simply programming in one of those other
languages.   There are lots, both fully compiled, and interpreted
(and semi-compiled) to choose from.   Pick the one best suited for your
application, and use that.   From the way it has been described, that's
very unlikely to be any kind of shell script.

Basically, if a program is complex enough that you need to start out
with writing it as an application, then the shell (and its language)
is not going to be the right solution.

On the other hand, if you start out giving commands on the command line,
and find that you're repeating a similar sequence multiple times, that
is a perfect place for a shell script.   Often once it exists, it will
grow, gain more features, and become more complex than you ever would
have typed - but everything in it (except perhaps some of the control
logic) would be things that you would or could have entered on the
command line, by hand.

Rather than trying to force bash to provide the solution that you need,
just use something which is already suitable.   Long term, you'll be
happier.   So will everyone else who doesn't need to cope with attempting
to understand what your changes mean or how they might work, or not.

kre

ps: note how I avoid showing any prompts, or #! (and hence to path to
bash) to avoid disappointing people here from discovering that the quaint
notion that /bin/bash is something they can expect to work, isn't correct.