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Re: Unset array doesn't work


From: don fong
Subject: Re: Unset array doesn't work
Date: Sat, 3 Mar 2018 10:16:08 -0800

Robert Elz:

> And yet when that change to the entrenched behaviour was made,
> there were no complaints?   And there's no option to switch back to
> the previous way?   Kind of suggests just how important everyone
> believes the original method was, doesn't it?


doesn't the same argument apply even more strongly to your proposed
change?  from the fact that the "bug" has been in existence for decades
without a huge clamor for it to be "fixed", your own reasoning would imply
that "fixing" it can't be that important.

Koichi Murase:

> I don't agree with changing the default behavior that removes the
> placeholder of previous-context variables. The reason is just backward
> compatibility, but it's important.


speaking as a mere bash user, i want to give a huge +1 to Koichi's point.
after decades of bash doing unset one way, changing the behavior carries a
risk
of breaking existing scripts.

the upside seems small considering that if people aren't even aware of this
behavior, it can't be much of a pain point now can it?  but it can become a
pain point if bash changes something that people have been relying on,
even unknowingly.

IMHO, testability should be a primary factor when considering any new bash
features.  from a testing standpoint there are "too many" shell options
already.
and it's not just about testing bash itself.  how many existing shell
scripts are
going to break because some user unknowingly ran the script with a
combination of options that the script author wasn't aware of?

so even if it were easy to implement as a shell option, it doesn't sound
like a good idea to me.  i might feel differently if bash had comprehensive
test coverage.  AFAIK the tests are nowhere near strong enough.

changing the default setting of the proposed option seems even more risky.


On Fri, Mar 2, 2018 at 11:23 PM, Robert Elz <address@hidden> wrote:

>     Date:        Fri, 2 Mar 2018 14:43:02 -0500
>     From:        Chet Ramey <address@hidden>
>     Message-ID:  <address@hidden>
>
> My final comments on this subject:
>
>   | Perhaps. But bash has never done this. Not from day one. That's 30
> years.
>
> That a bug (be it a design bug, or a coding bug) has existed a long tiime
> does not make it any less a bug.
>
> I have been using bash for essentially all that time (from before you took
> over maintainership) and I never knew it worked like that.   From comments
> here (where some people far more knowledgable about bash and its
> internals than I are to be found) I suspect that very few other people know
> about it either.
>
>   | This is how bash dynamic scoping works. The exception for the
> declaration/
>   | unset at the current scope was added 16 years ago, and the existing
>   | behavior was already entrenched.
>
> And yet when that change to the entrenched behaviour was made,
> there were no complaints?   And there's no option to switch back to
> the previous way?   Kind of suggests just how important everyone
> believes the original method was, doesn't it?
>
>   | I can see doing this and allowing it to be toggled by a shell option.
>
> A suggestion:   Do that for bash 5, and in the alpha release, make
> the option default to cause things to work the opposite way than
> happens now (so the option needs to be explicitly changed to get
> the current behaviour).   I know that's the opposite of what would
> usually be done in order to retain backwards compat, but for this,
> I think it would be a useful test to see if anyone notices the difference.
> You can always change it for beta/final releases if there are issues.
> If not, perhaps the option can just go away (then or later.)
>
>   | > Lastly, where does the notion of "remove" come from?
>   |
>   | As a way to describe the historical bash behavior, it works.
>
> Yes, that I understand.   My issue is that I believe this is colouring
> your thoughts on just what "unset" is - same as the "appear/be"
> (trivial seeming) semantic issue you commented on in another message.
>
> That is, it appears to me as if you believe that "unset" (as a state, not
> the command here) implies "non-existing".   That's never been correct.
>
> The converse is correct - a variable that does not exist appears as
> an unset variable when referenced.
>
> There are (even ignoring the unset command) too many ways
> (in bash, as well as other shells) to get variables that patently
> obviously "exist" in some form or other but are unset.
>
> The most obvious example is
>
>         export newvar
>
> after that
>
>         echo ${newvar-unset}
>
> prints "unset".   Sometime later if we give newvar a value, it, and its
> new value are exported - demonstrating that the export attribute was
> remembered (ie: "newvar" existed before it was set - it must have done
> in order to retain an atttribute).
>
> jinx$ export newvar
> jinx$ echo ${newvar-unset}
> unset
> jinx$ newvar=set
> jinx$ printenv newvar
> set
> jinx$ echo $BASH_VERSION
> 4.4.12(1)-release
>
> All shells that function correctly behave that way.   In bash (and in
> several other shells, though not all, due to unrelated differences)
> the same is true of the local command in a function...
>
> var=set
> func() { local var; echo ${var-unset}; }
> echo $var ; func; echo $var
>
> prints "set" "unset" "set"  showing that in the function there is
> a var (which must still exist as its value is retained in the global
> scope) which is unset.
>
> Even if the model was that the "var" in the function is something
> completely unrelated to the global "var" (as it would be in C for
> example) we can still see that the local command must have
> created some state for it, as otherwise the echo would be
> referring to the global "var".
>
> This can be more obviously seen in
>
> var=set
> func() { [ -n "$1" ] && local var; echo ${var-unset}; }
> echo $var ; func x; func; echo $var
>
> The same code either accesses the local or global var
> depending upon whether we happened to have executed
> the local command or not - indicating that command sets
> some atttibute, and still makes an unset var.
>
> Again this is all as it should be (given the assumption that "local"
> creates an unset version of the variable, which is a rational choice,
> if not the one made by all shells - a topic we have discussed in
> another context.)
>
> The best model for all of this is that set/unset is an atttibute of a
> shell variable, just like exported, read-only (and any more that
> exist in various shells) and that when we reference a variable
> which does not (or perhaps even just did not) exist at all, then
> it is made to appear as if it were an unset variable, and this is
> the special case, rather than considering leaving some state
> around to remember attributes of vars that happen to be unset
> but must continue to exist as being what is unusual or special.
>
> An analogy is the unlink sys call, which is not "remove".   If you
> think of it as remove, and that its function is to remove files, then
> things get way too complex to describe the behaviour.   But if
> it just alters an attribute of the file (in that case, its link/ref count)
> it all becomes simple.   And then when there is no longer any
> way to access the file, it can be removed as a space saving
> optimisation.     Treat the unset command in the shell the same
> way.  If, after it, there is nothing about the variable that is different
> than a variable which does not exist, then we can remove it
> because that changes nothing, and saves memory  - but if there
> remains anything about the variable which is still needed, then
> we just cause the unset atttibute to be set (however that is
> implemented) and continue.
>
> kre
>
>
>


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