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* doc/autoconf.texi (Limitations of Builtins): Some information

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about `trap'.
Document the Free BSD bug observed by Pavel.
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Akim Demaille 2000-11-10 16:56:22 +00:00
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@ -1,3 +1,9 @@
2000-11-10 Akim Demaille <akim@epita.fr>
* doc/autoconf.texi (Limitations of Builtins): Some information
about `trap'.
Document the Free BSD bug observed by Pavel.
2000-11-10 Pavel Roskin <proski@gnu.org> 2000-11-10 Pavel Roskin <proski@gnu.org>
* autoscan.pl (scan_files): Eliminate a warning if no C files * autoscan.pl (scan_files): Eliminate a warning if no C files

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doc/autoconf.texi
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@ -5401,6 +5401,7 @@ as the string composed of a backslash and an n.
@item @command{exit} @item @command{exit}
@cindex @command{exit} @cindex @command{exit}
@c FIXME: A better merging between this item and `trap' is welcome.
Some shell scripts, such as those generated by @command{autoconf}, use a Some shell scripts, such as those generated by @command{autoconf}, use a
trap to clean up before exiting. If the last shell command exited with trap to clean up before exiting. If the last shell command exited with
nonzero status, the trap also exits with nonzero status so that the nonzero status, the trap also exits with nonzero status so that the
@ -5609,6 +5610,55 @@ expr "x$ac_feature" : '.*[^-a-zA-Z0-9_]' >/dev/null &&
"X@var{foo}" | grep "^X@var{bar}"}, because it avoids problems when "X@var{foo}" | grep "^X@var{bar}"}, because it avoids problems when
@samp{@var{foo}} contains backslashes. @samp{@var{foo}} contains backslashes.
@item @command{trap}
@cindex @command{trap}
It is safe to trap at least the signals 1, 2, 13 and 15. You can also
trap 0, i.e., have the trap run when the script end (either via an
explicit @command{exit}, or the end of the script).
Although @sc{posix} is not absolutely clear on that point, it is widely
admitted that when entering the trap @samp{$?} should be set to the exit
status of the last command run before the trap. The ambiguity can be
summarized as: ``when the trap is launched by an @command{exit}, what is
the @emph{last} command run: that before @command{exit}, or exit itself.
Bash considers @command{exit} was the last command, while Zsh and
Solaris 8 @command{sh} consider that when the trap is run it is
@emph{still} in the @command{exit}, hence it is the previous exit status
that the trap receives:
@example
% cat trap.sh
trap 'echo $?' 0
(exit 42); exit 0
% zsh trap.sh
42
% bash trap.sh
0
@end example
The portable solution is then simple: when you want to @samp{exit 42},
run @samp{(exit 42); exit 42}, the first @command{exit} being used to
set the exit status to 42 for Zsh, and the second to trigger the trap
and pass 42 as exit status for Bash.
Note that in Bourne shell an unqualified @command{exit} is equivalent to
@samp{exit $?}, hence you may actually abbreviate it as @samp{(exit 42);
exit}.
The shell in FreeBSD 4.0 has the following bug: @samp{ $?} is reset to 0
by empty lines if the code in inside trap.
@example
$ trap 'false
echo $?' 0
$ exit
0
@end example
@noindent
Fortunately this bug affects only trap.
@item @command{unset} @item @command{unset}
@cindex @command{unset} @cindex @command{unset}