Date: Sat, 19 Mar 2011 13:12:47 -0400 From: Maxim Khitrov <max@mxcrypt.com> To: Devin Teske <dteske@vicor.com> Cc: FreeBSD <freebsd-questions@freebsd.org>, Andres Perera <andres.p@zoho.com>, 839273@gmail.com Subject: Re: Shell script termination with exit function in backquotes Message-ID: <AANLkTi=W=4hFDHV8ryRcoxT%2BPnwUkWmYnUfuTLyfjNu4@mail.gmail.com> In-Reply-To: <EFA32C5B-1892-41C3-B34B-F96E75CA72CA@vicor.com> References: <AANLkTi=-CFmxRicGcosvzhBbM3DMjbWwQNirMrJ1_KP=@mail.gmail.com> <759A467E-407A-4DB8-9756-08011B5405F0@vicor.com> <AANLkTi=CXLFUBhnY1LuhkeUiGHHGZ43yd%2BMYE9L50_O4@mail.gmail.com> <AANLkTimrnV2rJLyc3M4e3gGy_GUDLXp128f6n8svM3_g@mail.gmail.com> <AANLkTim0GvnAyK3%2B=Bd1Sr=maz0B3Ybgve_c6FKWwfNs@mail.gmail.com> <AANLkTimWxiRQNG3Um__kY-6%2BQ59g5yZT-Kt0qLAqTWOO@mail.gmail.com> <EFA32C5B-1892-41C3-B34B-F96E75CA72CA@vicor.com>
next in thread | previous in thread | raw e-mail | index | archive | help
On Sat, Mar 19, 2011 at 12:44 PM, Devin Teske <dteske@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 <andres.p@zoho.com> wrote= : >>> On Mon, Mar 14, 2011 at 7:46 AM, Maxim Khitrov <max@mxcrypt.com> wrote: >>>> On Mon, Mar 14, 2011 at 3:16 AM, Andres Perera <andres.p@zoho.com> wro= te: >>>>> On Sun, Mar 13, 2011 at 9:49 PM, Devin Teske <dteske@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 tradi= tional 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 nor= m >>>>> >>>>> 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/~masc= heck/ >>> >>> netbsd has =C2=A0been patched to fix `exit 1`, according to the last si= te >> >> Here's another, but related, problem that I just ran into. The man page = reads: >> >> =C2=A0 =C2=A0 Commands may be grouped by writing either >> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 (list) >> =C2=A0 =C2=A0 or >> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 { list; } >> =C2=A0 =C2=A0 The first form executes the commands in a subshell. =C2=A0= Note that built-in >> =C2=A0 =C2=A0 commands thus executed do not affect the current shell... >> >> Here's my script: >> >> ---- >> #!/bin/sh >> >> { A=3D1; }; =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 echo $A >> echo | { B=3D2; }; =C2=A0 =C2=A0 =C2=A0echo $B >> { C=3D3; } > /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 with= in 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
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?AANLkTi=W=4hFDHV8ryRcoxT%2BPnwUkWmYnUfuTLyfjNu4>