[PATCH] hush: Fix assignments being misinterpreted as commands

[Tom Rini <trini@konsulko.com>]

On Tue, Mar 02, 2021 at 08:24:20AM -0500, Sean Anderson wrote:
> On 3/2/21 8:20 AM, Tom Rini wrote:
> > On Mon, Mar 01, 2021 at 06:07:36PM -0500, Sean Anderson wrote:
> > > On 3/1/21 1:26 PM, Heinrich Schuchardt wrote:
> > > > On 3/1/21 3:17 PM, Tom Rini wrote:
> > > > > On Sun, Feb 28, 2021 at 06:51:53PM -0500, Sean Anderson wrote:
> > > > > > On 2/28/21 6:40 PM, Heinrich Schuchardt wrote:
> > > > > > > Am 28. Februar 2021 22:29:51 MEZ schrieb Sean Anderson <seanga2@gmail.com>:
> > > > > > > > If there were no variable substitutions in a command, then initial
> > > > > > > > assignments would be misinterpreted as commands, instead of being
> > > > > > > > skipped
> > > > > > > > over. This is demonstrated by the following example:
> > > > > > > > 
> > > > > > > >      => foo=bar echo baz
> > > > > > > 
> > > > > > > The commit message does not explain why this patch is needed.
> > > > > > 
> > > > > > This is a bug I noticed while writing some tests of hush.
> > > > > > 
> > > > > > > What shall be the value off foo after this line?
> > > > > > 
> > > > > > It should be bar. This is an existing difference when compared with
> > > > > > bash. For example, without this patch, we have
> > > > > > 
> > > > > >      => foo=bar echo $foo
> > > > > >      bar
> > > > > >      => echo $foo
> > > > > >      bar
> > > > 
> > > > This seems really awkward. In bash I get:
> > > > 
> > > > $ foo=bar ./test.sh
> > > > bar
> > > > $ echo $foo
> > > > 
> > > > $
> > > > 
> > > > Where test.sh
> > > > 
> > > > #!/bin/sh
> > > > echo $foo
> > > > 
> > > > I did not expect an assignment made before a command to stick.
> > > 
> > > Yeah, this is because hush does not have the concept of per-command
> > > assignments (scope). So everything happens in the global scope.
> > > 
> > > > > > 
> > > > > > > 
> > > > > > > What will be the output of
> > > > > > > 
> > > > > > > foo=bar echo ${foo}
> > > > > > > 
> > > > > > > with and without your patch?
> > > > > > 
> > > > > > It is the same.
> > > > 
> > > > Please, provide an example where the patch makes a difference.
> > > > 
> > > > Best regards
> > > > 
> > > > Heinrich
> > > > 
> > > > > 
> > > > > bash works as you describe.  dash and busybox-sh both function like
> > > > > this:
> > > > > $ foo=bar echo $foo
> > > > > 
> > > > > $ echo $foo
> > > > > 
> > > > > $
> > > > > 
> > > > > That we error out entirely is different from everyone.  Is that a good
> > > > > thing?  Maybe.  I know I've caught myself making thinkos due to that
> > > > > logic.  It does also violate the principal of least surprise, that we
> > > > > don't act like anything else.  But I would suggest the behavior of
> > > > > busybox-sh (what we forked long long ago) is what we should model here
> > > > > rather than be more bash-like.  I'm not all that firm on this opinion
> > > > > frankly, especially given the one-line nature of the change to bring us
> > > > > that behavior and I assume dash/busybox are acting like pure sh would in
> > > > > this case, which we aren't anyhow.
> > > 
> > > Ok, I'd like to clear things up. Here is the current behavior of U-Boot:
> > > 
> > > 
> > > 	=> foo=bar echo $foo
> > > 	bar
> > > 	=> echo $foo
> > > 	bar
> > > 	=> baz=bar echo qux
> > > 	Unknown command 'baz=bar' - try 'help'
> > 
> > I see this as well (which isn't what I said in another part of this
> > thread, so I re-checked just now).
> > 
> > > with this patch, this changes to
> > > 
> > > 	=> foo=bar echo $foo
> > > 	bar
> > > 	=> echo $foo
> > > 	bar
> > > 	=> baz=bar echo qux
> > > 	qux
> > > 
> > > This patch *only* affects cases where there is an assignment at the
> > > beginning of the line, but there is *no* variable reference in the
> > > command. I know this is an edge case, but the current logic is clearly
> > > wrong here.
> > 
> > OK.  But I don't see that behavior in bash 4.4.20 (Ubuntu 18.04).  Where
> > does one get a shell that works like you're changing our hush to?
> > 
> 
> I'm not changing hush to work like the first two examples, it's already like this.
> 
> Only the third example is affected by this patch.

OK.  But to what end?  Historically we have a buggy but mostly
compatible "hush" that acts like "sh" does.  A more flexible shell could
solve a lot of different use cases including making boot scripts that
people end up writing being clearer and easier to write/debug/maintain.
What I worry about here is making our shell not act like any regular
shell people use.

-- 
Tom
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