Re: What shells fail to work if comparing with ?
On 02/10/2011 11:56 AM, Dr. David Kirkby wrote: I know its considered bad practice to check for an empty string with something like: if [ $STR = ] ; then but what shells do actually break with this, and under what conditions? At least Solaris /bin/sh mishandles particular $STR: $ /bin/sh -c 'test \( = '; echo $? /bin/sh: test: argument expected 1 Do you really want to be polluting stderr if $STR happens to be (? I was proposing someone change a test like that to if [ x$STR = ] ; then Won't work. With only two arguments, that provokes syntax errors in most versions of test. Did you mean: [ -z $STR ] or [ ! $STR ] If so, that still won't work, as there are some test implementations that get order of precedence wrong for some $STR. Did you mean: [ x$STR = x ] if so, then you can see why that is the exact same recipe that the autoconf manual is adamant about recommending, since the three-argument form with a prefix (usually x) that guarantees that the first and last argument can't be misinterpreted as operations and cause spurious syntax errors. but someone has argued against this, saying he knows of no shell where the former is not acceptable. Well, then he doesn't know about Solaris /bin/sh; there are other broken shells still in the wild, as well. I realise this issue is probably more of a problem with older shells, but can anyone give me any examples of where the former will break, but the latter will be ok? Still very much a problem to be aware of with today's portable shell programming. -- Eric Blake ebl...@redhat.com+1-801-349-2682 Libvirt virtualization library http://libvirt.org signature.asc Description: OpenPGP digital signature ___ Autoconf mailing list Autoconf@gnu.org http://lists.gnu.org/mailman/listinfo/autoconf
Re: What shells fail to work if comparing with ?
Hello David, * Dr. David Kirkby wrote on Thu, Feb 10, 2011 at 07:56:24PM CET: I know its considered bad practice to check for an empty string with something like: if [ $STR = ] ; then but what shells do actually break with this, and under what conditions? It's not shells that break. It's these kinds of issues: http://thread.gmane.org/gmane.comp.sysutils.autoconf.general/13706 Cheers, Ralf ___ Autoconf mailing list Autoconf@gnu.org http://lists.gnu.org/mailman/listinfo/autoconf
Re: What shells fail to work if comparing with ?
On Thursday 10 February 2011, Dr David wrote: I know its considered bad practice to check for an empty string with something like: if [ $STR = ] ; then but what shells do actually break with this, and under what conditions? Solaris 10 /bin/sh breaks with [ $var != ] for some (very corner-case) values of $var: $ /bin/sh -c 'var=(; [ $var != ]'; echo st = $? /bin/sh: test: argument expected st = 1 $ /bin/sh -c 'var=!; [ $var != ]'; echo st? = $? /bin/sh: test: argument expected st = 1 With [ $var = ], it doesn't really break, but misbehaves by printing spurious diagnostic: $ /bin/sh -c 'var=!; [ $var = ]'; echo st = $? /bin/sh: test: argument expected st = 1 $ bin/sh -c 'var=(; [ $var != ]'; echo st = $? /bin/sh: test: argument expected st = 1 The exit status is correct in this cases, though. I was proposing someone change a test like that to if [ x$STR = ] ; then I guess you mean: if [ x$STR = x ] ; then right? but someone has argued against this, saying he knows of no shell where the former is not acceptable. I realise this issue is probably more of a problem with older shells, Solaris 10 /bin/sh is not really old. but can anyone give me any examples of where the former will break, but the latter will be ok? HTH, Stefano ___ Autoconf mailing list Autoconf@gnu.org http://lists.gnu.org/mailman/listinfo/autoconf
Re: What shells fail to work if comparing with ?
On 10 February 2011 18:56, Dr. David Kirkby david.kir...@onetel.net wrote: I know its considered bad practice to check for an empty string with something like: if [ $STR = ] ; then but what shells do actually break with this, and under what conditions? I was proposing someone change a test like that to if [ x$STR = ] ; then Oops, I meant to put if [ x$STR = x ] ; then ___ Autoconf mailing list Autoconf@gnu.org http://lists.gnu.org/mailman/listinfo/autoconf
Re: What shells fail to work if comparing with ?
On 02/10/2011 12:40 PM, Ralf Wildenhues wrote: Hello David, * Dr. David Kirkby wrote on Thu, Feb 10, 2011 at 07:56:24PM CET: I know its considered bad practice to check for an empty string with something like: if [ $STR = ] ; then but what shells do actually break with this, and under what conditions? It's not shells that break. It's these kinds of issues: http://thread.gmane.org/gmane.comp.sysutils.autoconf.general/13706 Ralf pointed out a completely different point from the Solaris /bin/sh syntax issue - that is, that in autoconf scripts, [ and ] are special to m4, and require extra effort to use, so you are more portable using: test x$STR = x than you are using: [ x$STR = x ] so that m4 doesn't eat the [ and ]. -- Eric Blake ebl...@redhat.com+1-801-349-2682 Libvirt virtualization library http://libvirt.org signature.asc Description: OpenPGP digital signature ___ Autoconf mailing list Autoconf@gnu.org http://lists.gnu.org/mailman/listinfo/autoconf
Re: What shells fail to work if comparing with ?
On Thu, 10 Feb 2011, Stefano Lattarini wrote: but someone has argued against this, saying he knows of no shell where the former is not acceptable. I realise this issue is probably more of a problem with older shells, Solaris 10 /bin/sh is not really old. I do see the issues you mention with recently-patched /bin/sh on Solaris 10. However, /usr/xpg4/bin/sh passes your tests with flying colors on Solaris 10. The description of /usr/xpg4/bin/sh is considerably different than /bin/sh: The /usr/bin/sh utility is a command programming language that executes commands read from a terminal or a file. The /usr/xpg4/bin/sh utility is a standards compliant shell. This utility provides all the functionality of ksh(1), except in cases discussed in ksh(1) where differences in behavior exist. Interestingly, on Solaris 10, /bin/sh is a link to /sbin/sh, implying that it is intended for purposes like system initialization scripts. Bob -- Bob Friesenhahn bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer,http://www.GraphicsMagick.org/ ___ Autoconf mailing list Autoconf@gnu.org http://lists.gnu.org/mailman/listinfo/autoconf
