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Re: SH bahaviour to not fork a subshell after " | while read "


From: Pierre Gaston
Subject: Re: SH bahaviour to not fork a subshell after " | while read "
Date: Fri, 1 Jun 2012 12:39:17 +0300

On Fri, Jun 1, 2012 at 11:53 AM, freD <address@hidden> wrote:
> Configuration Information [Automatically generated, do not change]:
> Machine: powerpc
> OS: aix5.1
> Compiler: xlc
> Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='powerpc'
> -DCONF_OSTYPE='aix5.1' -DCONF_MACHTYPE='powerpc-ibm-aix
> 5.1' -DCONF_VENDOR='ibm' -DLOCALEDIR='/opt/freeware/share/locale'
> -DPACKAGE='bash' -DSHELL -DHAVE_CONFIG_H  -I.  -I. -I
> ./include -I./lib  -I/opt/freeware/include -O2
> uname output: AIX tsm 1 6 00C530EB4C00
> Machine Type: powerpc-ibm-aix5.1
>
> Bash Version: 3.0
> Patch Level: 16
> Release Status: release
>
> Description:
>
> In bash mode, variable are lost after a while loop:
>
> bash-3.00# T=toto ; du | while read a ; do   T=$a ; done ; echo $T
> toto
>
> I can keep then a little using parenthesis:
>
> bash-3.00# T=toto ; du | (while read a ; do   T=$a ; done ; echo $T) ; echo $T
> 1489648 .
> toto
>
> Repeat-By:
>
> bash-3.00# T=toto ; du | while read a ; do   T=$a ; done ; echo $T
> toto
>
> Fix:
> May be starting in "sh" mode and/or posix mode should behave like a
> real bourne shell
>
> bash-3.00# /usr/bin/sh
> # T=toto ; du | while read a ; do   T=$a ; done ; echo $T
> 1489632 .
> # exit
> bash-3.00#
>
>

You use a relatively old bash.
In the newer bash you can get this beahviour using "shopt -s lastpipe"

Note that this behaviour is not mandated by posix and that a least one
"real bourne shell", which is not posix acts like bash

Eg with the heirloom bourne shell:
$ ./sh
$ T=toto ; du | while read a ; do   T=$a ; done ; echo $T
toto

I think only ksh93 and zsh behaves like that by default. dash, pdksh,
mksh all put the last part of the pipe in a subshell.



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