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Re: {varname} redirection for a command or group leaves the file open


From: Pierre Gaston
Subject: Re: {varname} redirection for a command or group leaves the file open
Date: Wed, 10 May 2017 20:52:26 +0300

On Wed, May 10, 2017 at 8:07 PM, Aldo Davide <address@hidden> wrote:

> Configuration Information [Automatically generated, do not change]:
> Machine: x86_64
> OS: linux-gnu
> Compiler: x86_64-pc-linux-gnu-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./include -I. -I./include -I./lib
> -DDEFAULT_PATH_VALUE='/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
> -DSTANDARD_UTILS_PATH='/bin:/usr/bin:/sbin:/usr/sbin'
> -DSYS_BASHRC='/etc/bash/bashrc' -DSYS_BASH_LOGOUT='/etc/bash/bash_logout'
> -DNON_INTERACTIVE_LOGIN_SHELLS -DSSH_SOURCE_BASHRC -O2 -march=native -pipe
> -Wno-parentheses -Wno-format-security
> uname output: Linux mycomputer 4.9.24 #8 SMP PREEMPT Tue Apr 25 11:19:58
> EEST 2017 x86_64 Intel(R) Core(TM) i7-2670QM CPU @ 2.20GHz GenuineIntel
> GNU/Linux
> Machine Type: x86_64-pc-linux-gnu
>
> Bash Version: 4.4
> Patch Level: 12
> Release Status: release
>
> Suppose that you use a "varname" redirection when executing a simple
> command, e.g.:
>
>         ls -lh /proc/self/fd {var}>/dev/null
>
> I was surprised to discover that the file descriptor remains open after
> the command has completed, as evidenced by issuing the following (works
> on linux) immediately afterwards:
>
>         echo "var is $var"
>         ls -lh /proc/$$/fd
>
> This is unlike what happens with standard redirections, e.g.:
>
>         ls -lh /proc/self/fd 57>/dev/null
>         ls -lh /proc/$$/fd
>
> The same problem exists when braces are used to group (possibly)
> multiple commands:
>
>         { ls -lh /proc/self/fd; } {var}>/dev/null
>         echo "var is $var"
>         ls -lh /proc/$$/fd
>
> On the other hand, everything works just fine with subshells:
>
>         (ls -lh /proc/self/fd) {var}>/dev/null
>         echo "var is $var"
>         ls -lh /proc/$$/fd
>
> As a side-note, in the subshell example the variable var will be
> undefined in the second line, but defined inside the subshell. With
> groups it will remain defined after the group:
>
>         { echo "inside the grouping: $var1"; } {var1}>/dev/null
>         echo "outside the grouping: $var1"
>         (echo "inside the subshell: $var2") {var2}>/dev/null
>         echo "outside the subshells: $var2"
>
> So in summary, I would expect groups to work like subshells, both in
> regards to closing the file descriptor but also in regards to the scope
> of the variable.
>
> See:
https://lists.gnu.org/archive/html/bug-bash/2012-11/msg00040.html
Pierre
PS: I'm with you ;)


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