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




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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