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From: Geir Hauge <geir.hauge@gmail.com>
Newsgroups: gnu.bash.bug
Subject: Re: trap DEBUG and $_
Date: Mon, 12 Oct 2015 09:13:58 +0200
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On Mon, Oct 12, 2015 at 07:24:00AM +0200, isabella parakiss wrote:
> On 10/11/15, Dan Stromberg <dstromberglists@gmail.com> wrote:
> > Is there a way of outputting a datestamp to shell stderr at the _beginning_
> > of the execution of a command, that won't wipe out $_?
> >
> > I use $_ quite a bit for the last argument to the previous command,
> > interactively.  And I'd like to datestamp all my commands. Datestamping
> > after with $PS1 is easy, but datestamping before is a little harder.
> >
> > I've tried using:
> >
> >    function preexec { if tty -s; then echo "$(tput smso)cmd started $(date
> > +%Y\ %a\ %b\ %d\ %r)$(tput rmso)"; fi; }
> >    trap preexec DEBUG
> >
> > ...but as you might suspect, I'm finding that $_ is always "preexec" with
> > this enabled.
> >
> > Any suggestions?
> >
> > Thanks!
> >
> trap '__=$_; preexec; : "$__"' DEBUG

And since the preexec function ignores its arguments, it can be
shortened to:

    trap 'preexec "$_"' DEBUG

-- 
Geir Hauge