Date: Sat, 19 Mar 2011 12:15:26 -0400 From: Maxim Khitrov <max@mxcrypt.com> To: 839273@gmail.com Cc: Devin Teske <dteske@vicor.com>, Andres Perera <andres.p@zoho.com>, FreeBSD <freebsd-questions@freebsd.org> Subject: Re: Shell script termination with exit function in backquotes Message-ID: <AANLkTimWxiRQNG3Um__kY-6%2BQ59g5yZT-Kt0qLAqTWOO@mail.gmail.com> In-Reply-To: <AANLkTim0GvnAyK3%2B=Bd1Sr=maz0B3Ybgve_c6FKWwfNs@mail.gmail.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>
next in thread | previous in thread | raw e-mail | index | archive | help
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> wrote= : >>> 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 ex= ecution. The reason you're having inconsistent behavior is because Linux ha= s /bin/sh symbolically linked to /bin/bash while FreeBSD has a more traditi= onal 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/~masche= ck/ > > netbsd has =C2=A0been patched to fix `exit 1`, according to the last site Here's another, but related, problem that I just ran into. The man page rea= ds: Commands may be grouped by writing either (list) or { list; } The first form executes the commands in a subshell. Note that built-i= n commands thus executed do not affect the current shell... Here's my script: ---- #!/bin/sh { A=3D1; }; echo $A echo | { B=3D2; }; echo $B { C=3D3; } > /dev/null; echo $C ---- And here's the output: ---- 1 3 ---- Where did the '2' go? Again, I have to assume that when stdin is piped to a group of commands, those commands are executed in a subshell despite curly braces. But where is this behavior documented? It seems that there are a lot of corner cases that can only be understood if you are familiar with the shell implementation. Documentation can certainly be improved in places. - Max
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?AANLkTimWxiRQNG3Um__kY-6%2BQ59g5yZT-Kt0qLAqTWOO>