BASH 4.4 mapfile/readarray/read builtins mis-behaving with pipe

BASH 4.4 mapfile/readarray/read builtins mis-behaving with pipe

[BloomingAzaleas]

Windows 10 Pro 10.0.17134 N/A Build 17134 patched through 15 July 
2018
Cygwin 2.10.0(0.325/5/3)
Bash 4.4.12(3)

Cygwin man pages show:

   mapfile [-d delim] [-n count] [-O origin] [-s count] [-t] [-u 
fd]  [-C  callback]  [-c quantum] [array]
   readarray  [-d delim] [-n count] [-O origin] [-s count] [-t] 
[-u fd] [-C callback] [-c        quantum] [array]
     Read lines from the standard input into the indexed array 
variable  array,  or ...

  read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N 
nchars]  [-p  prompt]  [-t timeout] [-u fd] [name ...]
      One  line  is read from the standard input, or ...

So the expectation is that all bash stdin re-directions such as 
pipes and left chevs should work.  For mapfile/readarry, the 
output array variable MAPFILE is described as being created if 
not array_var argument is given to mapfile.

A) Searched cygwin email list with terms 'mapfile', 'readarray' 
and 'read builtin'.  No obvious hits of recent vintage in summary 
result list.

B) Confirm, not using mapfile/readarry/read, that bash stdin 
redirs work as expected.

echo multi-line_arg | /bin/cat

/bin/cat < some_unix_fmt_file

/bin/cat <<END
some
lines
END

all work as expected.

C) Test mapfile/readarray/read in various stdin redir situations. 
Basic harnesses are :

mapfile/readarry HARNESS:
stdin or fd 0 feed to mapfile
then
: "${MAPFILE[@]:?MAPFILE null or unset}"
if get past the ${:?} test then dump MAPFILE with a simple /for 
/loop as: for ent in "${MAPFILE[@]}" ; do echo ="${ent}"= ; done

read HARNESS:
stdin feed to 'read foo'
then
: "${foo:?foo null or unset}"
if get past the ${:?} test then dump variable foo with echo 
="${foo}"=

CASES:
echo multi-line_arg | mapfile
cat multi-line_unix_fmt_file | mapfile
RESULT:
Fail with MAPFILE null or unset
If 'for' loop forced, "${MAPFILE[@]}" expands to zero words.

CASE:
echo multi-line_arg | read foo
cat multi-line_unix_fmt_file | read foo
  RESULT:
Fail with variable foo null or unset

CASES: left chev re-directions
mapfile < some_unix_fmt_file
mapfile <<END multi-line_here_doc
read < some_unix_fmt_file
read <<END multi-line_here_doc
RESULT:
Work as expected

CASES: mapfile or read in a sub-shell
echo multi-line_arg | ( mapfile ; stuff )
echo multi-line_arg | ( read foo; stuff )
RESULT:
Work as expected.  However, since mapfile/read result vars are 
not accessible in parent shell context, can force a notable 
re-structuring of a script to use sub-shells.

CONCLUSIONS:
For bash builtins mapfile/readarray and read, stdin pipe 
re-direction directly to the builtin fails as if the builtin 
read()/fread() sees a silent pre-mature EOF on the pipe.  In the 
sub-shell tests, the pipe re-direction happens for the sub-shell 
as a whole before commands internal to the sub-shell get 
processed for individual re-dirs, so no re-dir processing 
directly for the builtin.


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Re: BASH 4.4 mapfile/readarray/read builtins mis-behaving with pipe

[Steven Penny]

On Sun, 15 Jul 2018 19:01:21, BloomingAzaleas wrote:
> CASE:
> echo multi-line_arg | read foo
> cat multi-line_unix_fmt_file | read foo
>  =C2=A0RESULT:
> Fail with variable foo null or unset
>
> CASES: left chev re-directions
> mapfile < some_unix_fmt_file
> mapfile <<END multi-line_here_doc
> read < some_unix_fmt_file
> read <<END multi-line_here_doc
> RESULT:
> Work as expected
>
> CASES: mapfile or read in a sub-shell
> echo multi-line_arg | ( mapfile ; stuff )
> echo multi-line_arg | ( read foo; stuff )
> RESULT:
> Work as expected.=C2=A0 However, since mapfile/read result vars are=20
> not accessible in parent shell context, can force a notable=20
> re-structuring of a script to use sub-shells.

no mis-behaving: this is intended behavior - you yourself have given
workarounds: either redirect output to a file that can be later read, or pipe to
command grouping ala {} or () and read stdin from inside the subshell

im not sure what you are asking here - it seems you have a grasp already of the
"problem" and "solution" - so nothing more needs to be said really, as nothing
needs to be fixed - another option is "lastpipe"

http://mywiki.wooledge.org/BashFAQ/024


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Re: BASH 4.4 mapfile/readarray/read builtins mis-behaving with pipe [edit] documentation bug

[BloomingAzaleas]

Reply to Steven Penny <svnpenn at gmail dot com>:

    no mis-behaving: this is intended behavior - you yourself have given
    workarounds: either redirect output to a file that can be later read, or pipe to
    command grouping ala {} or () and read stdin from inside the subshell

    im not sure what you are asking here - it seems you have a grasp already of the
    "problem" and "solution" - so nothing more needs to be said really, as nothing
    needs to be fixed - another option is "lastpipe"


If the reported behavior is the intended behavior, then this 
complaint is now a documentation complaint.  Neither the Cygwin 
man pages (quoted in the original report) nor the upstream GNU 
documentation indicate any exception to stdin redirect with pipe 
in the doc paragraphs for the subject builtins.  What confuses 
the situation is that some stdin re-directions work for these 
builtins in a parent shell context (left-chevs)  but not others 
(pipes) UNLESS performed strictly at mid-night under a full moon 
using obscure incantation hidden in pieces throughout a 178-page 
pdf.  In the GNU doc, the sub-shell for builtins in a pipeline 
rule involved is obscured by burial in paragraph six of section 
3.7.3 Command Execution Environment.

I suggest the following adjustment to the man pages inserting a 
parenthetical cue regards behavior in pipes:

   mapfile [-d delim] [-n count] [-O origin] [-s count] [-t] [-u 
fd]  [-C  callback]  [-c quantum] [array]
   readarray  [-d delim] [-n count] [-O origin] [-s count] [-t] 
[-u fd] [-C callback] [-c        quantum] [array]
     Read lines from the standard input (see pipeline 
restriction) into the indexed array  variable  array,  or ...

  read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N 
nchars]  [-p  prompt]  [-t timeout] [-u fd] [name ...]
      One  line  is read from the standard input (see pipeline 
restriction), or ...

At least the reader gets a cue to modify expectations and 
research the "pipeline restriction" compared to all other 
commands that work just peachy with stdin pipe re-direction.  
Regards the shopt lastpipe doc single-sentence paragraph in GNU 
section 4.3.2 The Shopt Builtin, it could be improved by adding: 
"This option will change the parent shell context behavior of 
shell builtins mapfile, readarray and read when they are used as 
the last command of a pipeline."

As to the left-chev work-arounds, yes they work, but that would 
require some script re-structuring.  In any case, "what you are 
asking here" is that there appears to be a difference between a 
plain reading of the doc paragraphs for the builtins and the 
builtins behavior and so a bug against one or the other.  If this 
were a unix/linux kernel environment, the triage emphasis would 
be documentation wording rather than implementation bug.  Given 
the Cygwin context of necessary hidden emulation chicanery on 
Windows, triage emphasis is not that obvious.  I exerted the 
effort to construct a reasonably complete report with multiple 
illustration cases contrasted to the doc to improve the FOSS, be 
it implementation or documentation.  I didn't need too.

Regards.

On 7/15/2018 7:01 PM, BloomingAzaleas wrote:
> Windows 10 Pro 10.0.17134 N/A Build 17134 patched through 15 
> July 2018
> Cygwin 2.10.0(0.325/5/3)
> Bash 4.4.12(3)
>
> Cygwin man pages show:
>
>   mapfile [-d delim] [-n count] [-O origin] [-s count] [-t] [-u 
> fd]  [-C  callback]  [-c quantum] [array]
>   readarray  [-d delim] [-n count] [-O origin] [-s count] [-t] 
> [-u fd] [-C callback] [-c        quantum] [array]
>     Read lines from the standard input into the indexed array 
> variable  array,  or ...
>
>  read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N 
> nchars]  [-p  prompt]  [-t timeout] [-u fd] [name ...]
>      One  line  is read from the standard input, or ...
>
> So the expectation is that all bash stdin re-directions such as 
> pipes and left chevs should work.  For mapfile/readarry, the 
> output array variable MAPFILE is described as being created if 
> not array_var argument is given to mapfile.
>
> A) Searched cygwin email list with terms 'mapfile', 'readarray' 
> and 'read builtin'.  No obvious hits of recent vintage in 
> summary result list.
>
> B) Confirm, not using mapfile/readarry/read, that bash stdin 
> redirs work as expected.
>
> echo multi-line_arg | /bin/cat
>
> /bin/cat < some_unix_fmt_file
>
> /bin/cat <<END
> some
> lines
> END
>
> all work as expected.
>
> C) Test mapfile/readarray/read in various stdin redir 
> situations.  Basic harnesses are :
>
> mapfile/readarry HARNESS:
> stdin or fd 0 feed to mapfile
> then
> : "${MAPFILE[@]:?MAPFILE null or unset}"
> if get past the ${:?} test then dump MAPFILE with a simple /for 
> /loop as: for ent in "${MAPFILE[@]}" ; do echo ="${ent}"= ; done
>
> read HARNESS:
> stdin feed to 'read foo'
> then
> : "${foo:?foo null or unset}"
> if get past the ${:?} test then dump variable foo with echo 
> ="${foo}"=
>
> CASES:
> echo multi-line_arg | mapfile
> cat multi-line_unix_fmt_file | mapfile
> RESULT:
> Fail with MAPFILE null or unset
> If 'for' loop forced, "${MAPFILE[@]}" expands to zero words.
>
> CASE:
> echo multi-line_arg | read foo
> cat multi-line_unix_fmt_file | read foo
>  RESULT:
> Fail with variable foo null or unset
>
> CASES: left chev re-directions
> mapfile < some_unix_fmt_file
> mapfile <<END multi-line_here_doc
> read < some_unix_fmt_file
> read <<END multi-line_here_doc
> RESULT:
> Work as expected
>
> CASES: mapfile or read in a sub-shell
> echo multi-line_arg | ( mapfile ; stuff )
> echo multi-line_arg | ( read foo; stuff )
> RESULT:
> Work as expected.  However, since mapfile/read result vars are 
> not accessible in parent shell context, can force a notable 
> re-structuring of a script to use sub-shells.
>
> CONCLUSIONS:
> For bash builtins mapfile/readarray and read, stdin pipe 
> re-direction directly to the builtin fails as if the builtin 
> read()/fread() sees a silent pre-mature EOF on the pipe.  In 
> the sub-shell tests, the pipe re-direction happens for the 
> sub-shell as a whole before commands internal to the sub-shell 
> get processed for individual re-dirs, so no re-dir processing 
> directly for the builtin.
>

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Re: BASH 4.4 mapfile/readarray/read builtins mis-behaving with pipe [edit] documentation bug

[Eric Blake]

On 07/17/2018 08:52 PM, BloomingAzaleas wrote:
> Reply to Steven Penny <svnpenn at gmail dot com>:
> 
>     no mis-behaving: this is intended behavior - you yourself have given
>     workarounds: either redirect output to a file that can be later 
> read, or pipe to
>     command grouping ala {} or () and read stdin from inside the subshell
> 

> I suggest the following adjustment to the man pages inserting a 
> parenthetical cue regards behavior in pipes:

Is the behavior you are complaining about unique to Cygwin, or can it be 
reproduced on a GNU/Linux box?  If the latter, then an upstream bug 
report is better than asking for a cygwin-specific patch.  [Hint - as 
the maintainer of the cygwin bash port, I don't recall adding any 
cygwin-specific tweaks for mapfile - and a quick test on Fedora shows 
the same behaviors]

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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Re: BASH 4.4 mapfile/readarray/read builtins mis-behaving with pipe [edit] documentation bug

[BloomingAzaleas]

Reply to Eric Blake,

At this time, I do not have a Linux image available to me.   If 
you saw the same behavior on Fedora, then I suggest the behavior 
originates upstream at or close the the GNU source-code level.

Mr. Penny's response asserted the observed behavior "is intended 
behavior", in which case there should exist a GNU specification 
document describing the intended pipe STDIN re-direction 
restrictions for 'mapfile', 'read' and possibly other "builtins". 
Lacking such a reference nothing can be said about intent. 
Implementation-as-intent implies errors and conceptual and 
behavioral  inconsistencies in the implementation were intended. 
I decline to think that of the distributed BASH maintenance team. 
He provided a reference to 
http://mywiki.wooledge.org/BashFAQ/024, where, if you read the 
page, the behavior inconsistency I reported is shown under the 
heading of "More broken stuff:".  I take that "More broken 
stuff:" opinion to mean yet others read the same surface 
discrepancy between doc and behavior as I did.

I believe what we have at this point is:

A) GNU doc lacking nuance, attention to consistent terminology, 
and helpful rule-statement to code example/counter-example 
illustrations adjacent to rule statements.  For a doc 
counter-example, the Open Group doc at 
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html 
does make an effort to distinguish behaviors between "2.14 
Special Built-In Utilities" and "2.9.1 Simple Commands" whereas 
the GNU doc indiscriminately mixes the words "commands" (section 
3.2) and "builtin" and "command" and the phrase "builtin command" 
(section 4) as if their behaviors are identical under the section 
4 title of "Shell Builtin Commands" 
(https://www.gnu.org/software/bash/manual/html_node/Shell-Builtin-Commands.html#Shell-Builtin-Commands). 
The GNU doc for shopt lastpipeis fairly opaque unless you have 
deep knowledge and that knowledge is cued when considering the 
possible meanings of "current shell environment" for built-ins 
(same process) vs. external child-process executables.

B) However conceptually inconsistent, an obsessive BASH doc 
reader could imply the observed bash built-ins behavior by 
integrating multiple hints and rule statements scattered across 
the GNU doc and then, crucially, doubting the plain meaning of 
the unqualified doc statements of "Read lines from the standard 
input..." .

Thank you for your response.

Regards,

UN*X Since '85

On 7/20/2018 12:08 PM, Eric Blake wrote:
> On 07/17/2018 08:52 PM, BloomingAzaleas wrote:
>> Reply to Steven Penny <svnpenn at gmail dot com>:
>>
>>     no mis-behaving: this is intended behavior - you yourself 
>> have given
>>     workarounds: either redirect output to a file that can be 
>> later read, or pipe to
>>     command grouping ala {} or () and read stdin from inside 
>> the subshell
>>
>
>> I suggest the following adjustment to the man pages inserting 
>> a parenthetical cue regards behavior in pipes:
>
> Is the behavior you are complaining about unique to Cygwin, or 
> can it be reproduced on a GNU/Linux box?  If the latter, then 
> an upstream bug report is better than asking for a 
> cygwin-specific patch.  [Hint - as the maintainer of the cygwin 
> bash port, I don't recall adding any cygwin-specific tweaks for 
> mapfile - and a quick test on Fedora shows the same behaviors]
>

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Re: BASH 4.4 mapfile/readarray/read builtins mis-behaving with pipe [edit] documentation bug

[Steven Penny]

On Sat, 21 Jul 2018 19:36:38, BloomingAzaleas wrote:
> At this time, I do not have a Linux image available to me.

its not that hard to get:

1. http://virtualbox.org
2. http://cdimage.debian.org/debian-cd/current-live/amd64/iso-hybrid

> you saw the same behavior on Fedora, then I suggest the behavior
> originates upstream at or close the the GNU source-code level.

the behavior is documented by POSIX. Command grouping example:

    $ bash -c 'q=2; (q=3); echo "$q"'
    2

and reference:

> Execute compound-list in a subshell environment [...] Variable assignments and
> built-in commands that affect the environment shall not remain in effect after
> the list finishes.

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_04_01

pipe example:

    $ bash -c 'q=2; echo 3 | read q; echo "$q"'
    2

and reference:

> Changes made to the subshell environment shall not affect the shell
> environment. [...] Additionally, each command of a multi-command pipeline is
> in a subshell environment

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_12

> For a doc counter-example, the Open Group doc at
> http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html

its funny that you list this page, as thats exactly where i got my references.
thanks.

> Thank you for your response.
>
> Regards,
>
> UN*X Since '85

finally i would say to stop top posting. for someone who is self declared unix
for 30 years, it seems odd that you cant even run fedora, or adhere to defacto
mailing list etiquette. Cheers.


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Re: BASH 4.4 mapfile/readarray/read builtins mis-behaving with pipe [edit] documentation bug

[Brian Inglis]

On 2018-07-21 18:37, Steven Penny wrote:
> On Sat, 21 Jul 2018 19:36:38, BloomingAzaleas wrote:
>> At this time, I do not have a Linux image available to me.
> its not that hard to get:
> 1. http://virtualbox.org
> 2. http://cdimage.debian.org/debian-cd/current-live/amd64/iso-hybrid

Even easier to install one (or more) of the WSL distros from the MS Store after
enabling developer mode in Settings.

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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