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Re: `unset' and bash...

From: Paul Eggert
Subject: Re: `unset' and bash...
Date: Sun, 03 Sep 2006 12:45:45 -0700
User-agent: Gnus/5.1008 (Gnus v5.10.8) Emacs/21.4 (gnu/linux)

> OK to install the patch below?  Or should we just go all the way and
> recommend the exact code that is _AS_UNSET_PREPARE?

I guess the latter, though it's a close call -- does anybody still
use Bash 2.01?

> I don't quite understand the Bash 2.01 bug; was that setting MAIL to
> something, or unsetting MAIL that triggered the segfault?

It was unsetting mail, plus unsetting MAILCHECK.
You can see Bob Dowling's 1998 bug report
and I also vaguely remember running into the problem.

I installed this somewhat-different change.  The newer Bash bug was
fixed after 2.05a came out, according to its changes file.

2006-09-03  Paul Eggert  <address@hidden>

        * doc/autoconf.texi (Limitations of Builtins): Document 'unset'
        bugs of Bash 2.01 and 2.05a.  Problem report and initial patch by
        Ralf Wildenhues.

--- doc/autoconf.texi.~1.1076.~ 2006-09-01 16:15:32.000000000 -0700
+++ doc/autoconf.texi   2006-09-03 12:38:17.000000000 -0700
@@ -12850,21 +12850,27 @@ for @command{true}.
 @item @command{unset}
 @c ------------------
 @prindex @command{unset}
-You cannot assume the support of @command{unset}.  Nevertheless, because
+In some nonconforming shells (e.g., Bash 2.05a), @code{unset FOO} fails
+when @code{FOO} is not set.  Also, Bash 2.01 mishandles @code{unset
+MAIL} in some cases and dumps core.
+A few ancient shells lack @command{unset} entirely.  Nevertheless, because
 it is extremely useful to disable embarrassing variables such as
 @code{PS1}, you can test for its existence and use
 it @emph{provided} you give a neutralizing value when @command{unset} is
 not supported:
-if (unset FOO) >/dev/null 2>&1; then
+# "|| exit" suppresses any "Segmentation fault" message.
+if ( (MAIL=60; unset MAIL) || exit) >/dev/null 2>&1; then
 $unset PS1 || PS1='$ '
address@hidden example
address@hidden smallexample
 @xref{Special Shell Variables}, for some neutralizing values.  Also, see
 @ref{Limitations of Builtins}, documentation of @command{export}, for
 the case of environment variables.

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