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Re: An xtrace variant

From: Robert Elz
Subject: Re: An xtrace variant
Date: Mon, 29 Jun 2020 16:40:24 +0700

    Date:        Sun, 28 Jun 2020 15:19:27 -0400
    From:        worley@alum.mit.edu (Dale R. Worley)
    Message-ID:  <87h7uv0zi8.fsf@hobgoblin.ariadne.com>

  | For a long time, I've wanted a variant of -x that only echoed the simple
  | commands after bash is done executing the startup files.

The NetBSD sh has a -q option (probably all ash derived shells do) which
inhibits -x (and -v) during processing of startup files.

eschwartz@archlinux.org said:
  | Why not just run bash -x script-name without the bash -l option and without
  | $BASH_ENV set? 

That creates a different environment, and isn't (always) useful for
debugging scripts.   Sometimes it works, other times it doeesn't.

Dale again:
  | I finally did a test implementation, naming a new switch "-X".

The NetBSD sh (this time alone I believe) also has a -X (not that part, I
think one or two other shells also have unrelated -X options) which acts
just like -X, except it locks the output to stderr as it is at the time
the -X option is enabled (until -X is unset).  It also enables -x (after
which -x can be enabled/disabled as neede).   The idea is that it is
possible to do
        set -X 2>/tmp/trace-file
and all the -x output then goes to that file, regardless of furtre stderr
redirections (-X can be used on the command line as well of course, causing
the shell's startup stderr to be used).   This is actually a much more useful
option than -q, as in practice, inserting "set -x / set +x" into the script
at relevant points tends to be more useful for anything non-trivial than
simply running the whole thing with -x set - but being able to see the commands
run when stderr is redirected can be very useful.

  | I named the new option "-X" as a mnemonic variant of "-x".

That's why I picked it for mine as well...

  | Its long name is "Xtrace", because I couldn't think of anything better.

Same here, for the same reason - but there were complaints about having
names that differed only in char case, so I eventually changes it to xlock
(NetBSD 8 still has Xtrace though).

  | The code changes are quite simple really.

The ones for our -X as well, mostly mechanical, though a little more
complexity in the file descriptor management area.

The overall change set got bigger, as I also make the -x output show 
redirections, and give some info about some compound commands (there's
still less about case statements than I'd like).

  | Is this a useful idea?  Is there a better way to get an effect like
  | this?

I suspect that the ash shell's -q is a slightly better way, as that option
can be enabled by default - it simply does nothing if the shell is not
run with -x (or -v) on the command line, but works any time those are.

But overall, an option worth having I believe.


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