From 124d6e9d9632e66a1ca0f4e77dc471e08b3f9818 Mon Sep 17 00:00:00 2001 From: Stefano Lattarini Date: Thu, 29 Sep 2011 10:36:18 +0200 Subject: [PATCH] docs: korn shells can have $? > 256 for signal-terminated children Some Korn shells, when a child process dies due to signal number n, can leave in $? an exit status of 256+n, instead of the more common 128+n. See also Austin Group issue 0000051: * doc/autoconf.texi (Signal handling): Document the described Korn Shell behaviour, and some of its possible shortcomings. Suggestion by Eric Blake. --- ChangeLog | 11 +++++++++ doc/autoconf.texi | 61 +++++++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 72 insertions(+) diff --git a/ChangeLog b/ChangeLog index be019f52..e03a9b99 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,14 @@ +2011-10-06 Stefano Lattarini + + docs: korn shells can have $? > 256 for signal-terminated children + Some Korn shells, when a child process dies due to signal number + n, can leave in $? an exit status of 256+n, instead of the more + common 128+n. See also Austin Group issue 0000051: + + * doc/autoconf.texi (Signal handling): Document the described Korn + Shell behaviour, and some of its possible shortcomings. + Suggestion by Eric Blake. + 2011-09-26 Eric Blake docs: relax documentation license by dropping cover text diff --git a/doc/autoconf.texi b/doc/autoconf.texi index 91bb50ac..c78e0718 100644 --- a/doc/autoconf.texi +++ b/doc/autoconf.texi @@ -15610,6 +15610,67 @@ and @code{/usr/xpg4/bin/sh} will proceed to exit with status 130 (i.e., 128 + 2). In any case, if there is an active trap associated with @code{SIGINT}, those shells will correctly execute it. +Some Korn shells, when a child process die due receiving a signal with +signal number @var{n}, can leave in @samp{$?} an exit status of +256+@var{n} instead of the more common 128+@var{n}. Observe the +difference between AT&T @code{ksh93} (2011) and @code{bash} 4.1.5 on +Debian: + +@example +$ @kbd{/bin/ksh -c 'sh -c "kill -1 \$\$"; echo $?'} +/bin/ksh: line 1: 7837: Hangup +257 +$ @kbd{/bin/bash -c 'sh -c "kill -1 \$\$"; echo $?'} +/bin/bash: line 1: 7861 Hangup (sh -c "kill -1 \$\$") +129 +@end example + +@noindent +This @command{ksh} behavior is allowed by POSIX, if implemented with +due care; see this @uref{http://www.austingroupbugs.net/view.php?id=51, +Austin Group discussion} for more background. However, if it is not +implemented with proper care, such a behavior might cause problems +in some corner cases. To see why, assume we have a ``wrapper'' script +like this: + +@example +#!/bin/sh +# Ignore some signals in the shell only, not in its child processes. +trap : 1 2 13 15 +wrapped_command "$@@" +ret=$? +other_command +exit $ret +@end example + +@noindent +If @command{wrapped_command} is interrupted by a @code{SIGHUP} (which +has signal number 1), @code{ret} will be set to 257. Unless the +@command{exit} shell builtin is smart enough to understand that such +a value can only have originated from a signal, and adjust the final +wait status of the shell appropriately, the value 257 will just get +truncated to 1 by the closing @code{exit} call, so that a caller of +the script will have no way to determine that termination by a signal +was involved. Observe the different behavior of AT&T @code{ksh93} +(2011) and @code{bash} 4.1.5 on Debian: + +@example +$ @kbd{cat foo.sh} +#!/bin/sh +sh -c 'kill -1 $$' +ret=$? +echo $ret +exit $ret +$ @kbd{/bin/ksh foo.sh; echo $?} +foo.sh: line 2: 12479: Hangup +257 +1 +$ @kbd{/bin/bash foo.sh; echo $?} +foo.sh: line 2: 12487 Hangup (sh -c 'kill -1 $$') +129 +129 +@end example + @node File System Conventions @section File System Conventions @cindex File system conventions -- 2.47.2