Re: RFC: Change to the way sh -x works ?

[christos%astron.com@localhost (Christos Zoulas)]

In article <5094.1510597900%andromeda.noi.kre.to@localhost>,
Robert Elz  <kre%munnari.OZ.AU@localhost> wrote:
>    Date:        Sun, 12 Nov 2017 15:34:55 -0500
>    From:        christos%zoulas.com@localhost (Christos Zoulas)
>    Message-ID:  <20171112203456.0380117FDAE%rebar.astron.com@localhost>
>
>  | | Two different "enable trace" options would be a mess, and wouldn't work
>  | | all that well (what do we do if both are enabled?)
>  | 
>  | Error out, ignore the second?
>
>No, I don't think so, either would be ugly -- and just ignoring changes
>to -x because -X happens to be set has the potential to break things more
>badly then the previous proposal ever had, and generating an error would
>make -X impossible to use with some scripts, for no good reason.
>
>I have a slightly different idea to suggest instead (this one is not yet
>fully implemented, it is more complex than the previous, so no demo,
>but I think it will be easy enough to understand how it should work.)
>
>First, "set -x" stays as it has been in the past, and enables/disables
>tracing, and if nothing else is done, follows stderr around as it is
>changed (each trace message will go to whatever happens to be stderr at
>the time it is generated).   Anything currently using -x, will see no
>differences.
>
> [ An aside; just to clarify something which is not changing - tracing
>   happens before any redirects on the command line, so (assuming -x is set)
>   "echo foo 2>/dev/null" does not send the trace output for the echo command
>   to /dev/null, but "{ echo foo ;} >/dev/null does.  That does not make it
>   silent wrt tracing however, as the { } compound is still traced, at
>   least in netbsd-8/current sh. ]
>
>To this a new -X (aka Xtrace) option is added - think of it as a "super x",
>or to use 1940's/1950's British terminology, for at least some classes
>of Brits of the era, "capital x"...   (I think ... that's a bit before my
>time, and not my country, and not quite the way the idiom would have been
>expressed.)
>
>Any change to -X is also (immediately after) performed on -x as well (the
>sequencing is important).   But in addition to turning on command tracing
>(when enabled by "set -X" or -X on the command line), this option also locks
>the trace output to go to whatever stderr is at the time the X option is
>turned on.   This time the intent will be that any time "set -X" is done,
>stderr at the time will become the trace destination - since nothing is
>currently using this option (as it does not yet exist) there is no need
>to deal with unintentional manipulation in existing scripts, which was the
>motivation for the "only when going from off to on" that was in the previous
>proposal, and which was a bit ugly, and which had a special case exception I
>never bothered to mention... and now never need to.)
>
>Existing scripts that want to manipulate (turn on/off) tracing, will work
>just fine with this, $- will show the 'x' option set whenever tracing
>is enabled, and clear when it isn't, and set -x or set +x can be used to
>enable/disable tracing regardless of whether -X is set or not.   $- will
>also show the X option if that is set, but existing scripts will have no
>idea what that means, and should just leave it alone.
>
>What won't work is saving options with "set +o" and restoring them later -
>the options will be saved/restored just fine, but there is no way to work
>out where stderr is being sent (ie: the path name) in order to get that
>back again later (consider "sh -X 2>/tmp/trace-file script" where "script"
>then  uses "set +o" to save the options, and either it, or even some other
>script being run by a different instance of sh, later executes the output
>from the "set +o" to restore the options - all sh knows is "stderr is
>directed to some file somewhere..." it cannot supply a file name.)
>Of course, the previous proposal had that problem as well.
>
>When Xtrace is disabled ("set +X"), tracing is disabled (this does "set +x"
>as a side effect) and then if later enabled again using "set -x" the trace
>output will go back to being to whatever is current stderr.  The idea is
>that users who want the new style, can just use -X/+X as a direct replacement
>for -x/+x (unless they want to do something fancy, in which case making use
>of both the X and x options could be required.)
>
>Does this seem more reasonable (or safer) than the previous suggestion ?

Yes, that sounds good!

>
>kre
>
>ps: I did not think to mention the last time, but in both that proposal, and
>this one, if a function contains "local -" (ie: option changes are local
>to the function) then when the function returns, any changes to -x (or -X)
>that occurred during the function (after that local command, of course),
>including any changes to where the tracing gets sent, will be undone (tracing
>will go back to where it was being sent before the function changed it, if
>it did -- "local -" is more often used in functions that change "-f" or "-e"
>and similar, than those that are playing with -x.)

Great!

christos