On Sat, Mar 19, 2011 at 12:44 PM, Devin Teske <dte...@vicor.com> wrote: > > On Mar 19, 2011, at 9:15 AM, Maxim Khitrov wrote: > >> On Mon, Mar 14, 2011 at 6:40 PM, Andres Perera <andre...@zoho.com> wrote: >>> On Mon, Mar 14, 2011 at 7:46 AM, Maxim Khitrov <m...@mxcrypt.com> wrote: >>>> On Mon, Mar 14, 2011 at 3:16 AM, Andres Perera <andre...@zoho.com> wrote: >>>>> On Sun, Mar 13, 2011 at 9:49 PM, Devin Teske <dte...@vicor.com> wrote: >>>>>> If you make the changes that I've suggested, you'll have consistent >>>>>> execution. The reason you're having inconsistent behavior is because >>>>>> Linux has /bin/sh symbolically linked to /bin/bash while FreeBSD has a >>>>>> more traditional shell (we'll call it bourne shell "plus"). >>>>> >>>>> that is misleading because command substitutions have traditionally >>>>> invoked subshells, and freebsd sh(1)/ash is an exception, not the norm >>>>> >>>>> in this case, ksh and bash deviates are clearly closer to standard >>>>> bourne behaviour >>>>> >>>> >>>> Thanks for that explanation. I can understand the benefits of >>>> optimizing away subshell execution, but that can clearly lead to >>>> unexpected behavior. Is there some documentation on when this >>>> optimization is utilized (i.e. the command executed without a >>>> subshell)? Would I be correct in assuming that it is only restricted >>>> to built-in commands that are known not to produce any output, such as >>>> 'exit'? >>> >>> i would check the source, autoconf docs, and http://www.in-ulm.de/~mascheck/ >>> >>> netbsd has been patched to fix `exit 1`, according to the last site >> >> Here's another, but related, problem that I just ran into. The man page >> reads: >> >> Commands may be grouped by writing either >> (list) >> or >> { list; } >> The first form executes the commands in a subshell. Note that built-in >> commands thus executed do not affect the current shell... >> >> Here's my script: >> >> ---- >> #!/bin/sh >> >> { A=1; }; echo $A >> echo | { B=2; }; echo $B >> { C=3; } > /dev/null; echo $C >> ---- >> >> And here's the output: >> >> ---- >> 1 >> >> 3 >> ---- >> >> Where did the '2' go? > > You're learning that there are deviations to the rule as-mentioned in the > man-page.
I've learned this a long time ago :) My point is that these deviations should be noted in the man page to help eliminate such surprises. A single sentence would have sufficed in this case. > The reason for these deviations is quite simple in-fact... > > The shell needs to create a new set of stdin/stdout file-descriptors for the > block of commands that you've created, and executing said commands within a > sub-shell achieves that. Something very similar to this should be noted in the man page. I figured out why my code wasn't working quickly after thinking about how data would be piped to stdin. Others may waste a lot of time trying to figure out why their code doesn't do what the man page states it should be doing. - Max _______________________________________________ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "freebsd-questions-unsubscr...@freebsd.org"
