Re: redirections preceded by a word with builtins inconsistent

[Chet Ramey]

On 5/24/16 12:09 PM, Dan Douglas wrote:
> On Tue, May 24, 2016 at 10:23 AM, Chet Ramey  wrote:
>> It's not that Posix `allows' a subshell, it requires a subshell
>> environment:
> 
> Hm sorry I thought it was "may". This is kind of easy to confuse with
> all the other unspecified things about side-effects from assignments
> affecting subsequent assignments and commands when there's no command
> name.

Yeah.  A subshell environment doesn't automatically mean a subshell, but
it means that changes in the subshell environment don't affect the parent.


-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRUc...@case.eduhttp://cnswww.cns.cwru.edu/~chet/

Re: redirections preceded by a word with builtins inconsistent

[Dan Douglas]

On Tue, May 24, 2016 at 10:23 AM, Chet Ramey  wrote:
> It's not that Posix `allows' a subshell, it requires a subshell
> environment:

Hm sorry I thought it was "may". This is kind of easy to confuse with
all the other unspecified things about side-effects from assignments
affecting subsequent assignments and commands when there's no command
name.

Re: redirections preceded by a word with builtins inconsistent

[Chet Ramey]

On 5/24/16 1:41 AM, Dan Douglas wrote:

>> You don't precede it with an `exec', so it doesn't affect the current
>> shell.  So-called `null' commands with redirections are executed in
>> subshells.  This is where you need the `exec'.
> 
> I know POSIX allows for a subshell but don't know what the point is. I
> was confused by this when I first hit it. The subshell isn't obvious.

It's not that Posix `allows' a subshell, it requires a subshell
environment:

"If there is no command name, any redirections shall be performed in a
subshell environment."

Bash produces a subshell environment by creating a subshell.

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRUc...@case.eduhttp://cnswww.cns.cwru.edu/~chet/

Re: redirections preceded by a word with builtins inconsistent

[Dan Douglas]

On Sun, May 22, 2016 at 1:06 PM, Chet Ramey  wrote:
> On 5/21/16 5:16 PM, adonis papaderos wrote:
>
>> Bash Version: 4.3
>> Patch Level: 42
>> Release Status: release
>>
>> Description:
>> When using redirections preceded by a word on builtins
>> 'i.e. : {varname}<&1'
>> the redirection persists to the current shell.
>
> This is by design, and has been discussed previously.  One such discussion
> starts at http://lists.gnu.org/archive/html/bug-bash/2016-01/msg00156.html.
> Maybe it should only do it when preceded by `exec', but it never has.
>
>> Later using {varname}<&- does not close fd.
>
> You don't precede it with an `exec', so it doesn't affect the current
> shell.  So-called `null' commands with redirections are executed in
> subshells.  This is where you need the `exec'.

I know POSIX allows for a subshell but don't know what the point is. I
was confused by this when I first hit it. The subshell isn't obvious.

 $ bash -c 'echo "$BASHPID"
{x[$((pid=BASHPID))$(echo "2: $pid" >&2),0]}<<<"$((pid=BASHPID))$(echo
"1: $pid" >&2)"'
3907
1: 3908
2: 3908
 $ bash -c 'echo "$BASHPID"
{x[$((pid=BASHPID))$(echo "2: $pid" >&2),0]}<<<"$((pid=BASHPID))$(echo
"1: $pid" >&2)"'
3911
1: 3911
2: 3911

ksh being the only other shell that supports auto-fd assignment for
null commands doesn't create one.

 $ bash -c 'typeset -li y=2
{x[y*=2]}<&$((y*=2,0)); typeset -p x y'
bash: line 1: typeset: x: not found
declare -il y="2"
 $ bash -c 'typeset -li y=2
: {x[y*=2]}<&$((y*=2,0)); typeset -p x y'
declare -a x='([8]="10")'
declare -il y="8"
 $ ksh -c 'typeset -li y=2
{x[y*=2]}<&$((y*=2,0)); typeset -p x y'
typeset -a -i x=([8]=11)
typeset -l -i y=8
 $ ksh -c 'typeset -li y=2
: {x[y*=2]}<&$((y*=2,0)); typeset -p x y'
typeset -a -i x=([8]=11)
typeset -l -i y=8

ksh and zsh are hard to test since they do a horrible job parsing the
{fd} in all but trivial cases.

It would be way simpler to only allow redirects at the end of a
command and require each redirect to be a separate word. That
eliminates a ton of ambiguities for {fd}>x, &>x and >

Maybe add a `set -k` work-alike for legacy scripts and people that insist on
`echo >&2 i like surprise redirec{t}

Re: redirections preceded by a word with builtins inconsistent

[Greg Wooledge]

On Sun, May 22, 2016 at 09:48:47PM +0300, adonis papaderos wrote:
> > > When using redirections preceded by a word on builtins
> > > 'i.e. : {varname}<&1'
> > > the redirection persists to the current shell.

That surprises me, but if Chet says it's not a bug, then so be it.

Simply stop doing it.  It's not good shell code.  If you want to open
or close an FD for the current shell, use exec.

exec {varname}<&1
exec {varname}<&-

Re: redirections preceded by a word with builtins inconsistent

[adonis papaderos]

I had read this before sending this message. The discussion you mention
concerns leaking named redirections from function calls. This is not what I
was trying to say. I was talking about the difference in behaviour between
the creation and the destructions of the fd. There is also the difference
in behaviour between builtins and external programs that confuses me, but
in this case I just wanted to understand the creation and the destruction.
Imho, since it is by design, there should be a mention of this is the
documentation. I'm probably nitpicking, but the reasoning behind this is
not obvious to me.

Thanks,
Adonis

2016-05-22 21:06 GMT+03:00 Chet Ramey :

> On 5/21/16 5:16 PM, adonis papaderos wrote:
>
> > Bash Version: 4.3
> > Patch Level: 42
> > Release Status: release
> >
> > Description:
> > When using redirections preceded by a word on builtins
> > 'i.e. : {varname}<&1'
> > the redirection persists to the current shell.
>
> This is by design, and has been discussed previously.  One such discussion
> starts at http://lists.gnu.org/archive/html/bug-bash/2016-01/msg00156.html
> .
> Maybe it should only do it when preceded by `exec', but it never has.
>
> > Later using {varname}<&- does not close fd.
>
> You don't precede it with an `exec', so it doesn't affect the current
> shell.  So-called `null' commands with redirections are executed in
> subshells.  This is where you need the `exec'.
>
> --
> ``The lyf so short, the craft so long to lerne.'' - Chaucer
>  ``Ars longa, vita brevis'' - Hippocrates
> Chet Ramey, ITS, CWRUc...@case.edu
> http://cnswww.cns.cwru.edu/~chet/
>

Re: redirections preceded by a word with builtins inconsistent

[Chet Ramey]

On 5/21/16 5:16 PM, adonis papaderos wrote:

> Bash Version: 4.3
> Patch Level: 42
> Release Status: release
> 
> Description:
> When using redirections preceded by a word on builtins
> 'i.e. : {varname}<&1'
> the redirection persists to the current shell.

This is by design, and has been discussed previously.  One such discussion
starts at http://lists.gnu.org/archive/html/bug-bash/2016-01/msg00156.html.
Maybe it should only do it when preceded by `exec', but it never has.

> Later using {varname}<&- does not close fd.

You don't precede it with an `exec', so it doesn't affect the current
shell.  So-called `null' commands with redirections are executed in
subshells.  This is where you need the `exec'.

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRUc...@case.eduhttp://cnswww.cns.cwru.edu/~chet/

redirections preceded by a word with builtins inconsistent

[adonis papaderos]

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-strong -Wformat
-Werror=format-security -Wall
uname output: Linux venus 4.4.0-22-generic #40-Ubuntu SMP Thu May 12
22:03:46 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 4.3
Patch Level: 42
Release Status: release

Description:
When using redirections preceded by a word on builtins
'i.e. : {varname}<&1'
the redirection persists to the current shell.
Later using {varname}<&- does not close fd.


Repeat-By:
ls -l /proc/self/fd
: {varname}<&1
ls -l /proc/self/fd
{varname}<&-
: ls -l /proc/self/fd