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implicit redirection of background within pipeline

From: Martin D Kealey
Subject: implicit redirection of background within pipeline
Date: Sun, 10 Jan 2016 20:23:59 +1300 (NZDT)
User-agent: Alpine 2.00 (DEB 1167 2008-08-23)

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64'
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-pc-linux-gnu'
-DCONF_VENDOR='pc' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL
-DHAVE_CONFIG_H   -I.  -I../. -I.././include -I.././lib  -D_FORTIFY_SOURCE=2
-g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat
-Werror=format-security -Wall
uname output: Linux treehug.home.kurahaupo.gen.nz 3.13.0-55-generic
#94-Ubuntu SMP Thu Jun 18 00:27:10 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 4.3
Patch Level: 11
Release Status: release

        The first backgrounded element within a pipeline component has its
        stdin redirected to /dev/null; this does not apply to the second and
        subsequent background elements.

        On a Linux system this can be observed with:
        echo x | ( ls -ld /proc/self/fd/0 &
                   ls -ld /proc/self/fd/0 &
                   wait )

        On other systems it is apparent simply with:
        echo x | ( cat & wait )
        # no output is produced

        # this command produces no output:
        echo x | ( cat & wait )

        # this is only a work-around:
        echo x ( :& cat & wait )

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