Re: [Help-bash] `set -v` difference in () vs not in ()

[Mike McClain]

On Tue, Jun 12, 2018 at 08:10:27AM -0400, Greg Wooledge wrote:
> On Mon, Jun 11, 2018 at 08:43:34PM -0700, Mike McClain wrote:
> > On Mon, Jun 11, 2018 at 08:27:24AM -0400, Greg Wooledge wrote:
> > > On Sat, Jun 09, 2018 at 05:32:23PM -0500, Peng Yu wrote:
> > > > -v      Print shell input lines as they are read.
> > >
> > > Which is almost entirely useless.  It doesn't help you debug a script.
> > > I don't know why people keep trying to use it.
> > >
> > > I could see it maybe possibly helping Chet debug bash.  But not a script.
> >
> > Hi Greg,
> >     While I don't doubt for a minute that you know what you're talking
> > about, I don't.
> >     How is this different than an echo ""? The bash version of the
> > proverbial 'print statement' used by programmers everywhere?
>
> Bash (and sh) has two different "debug" options: -x and -v.
>
> set -x tells bash (or sh) to print each command as it is being
> *executed*, showing parameter expansions.  It is a useful tool.
>
> set -v tells bash (or sh) to print each *line* of the script as it
> is being *read*.  But this is not useful, because the point at which
> a line of input is read has nothing to do with when the commands
> contained in that line are executed.
>
> As Peng partially demonstrated, any long compound command (e.g. a
> function definition, or a while loop) is read all at once, before
> any execution happens.  set -v shows you the shell reading the lines
> of the script.  So what?  It's not executing them.  It's just reading
> them.  It doesn't reveal any useful information.
>
> Adding echo/printf commands to show the contents of variables (or the
> simple fact that execution has reached a specific point) is a useful tool,
> and you should continue doing so wherever it's helpful.  This is similar
> to how set -x helps you -- it shows you what the shell is actually doing.
>
> "declare -p varname" is also useful for showing the contents of
> variables in many cases.  In a very small number of cases, even that
> may not reveal enough information, and then something like
>
>   printf %s "$varname" | od -c -An
>
> (or your favorite hexdumper) may be what's needed.

    That's a very clear explanation.
Thank you,
Mike
--
It's not always polite to speak your mind.