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BASH 4.4 mapfile/readarray/read builtins mis-behaving with pipe
- From: BloomingAzaleas <rdbingham at verizon dot net>
- To: cygwin at cygwin dot com
- Date: Sun, 15 Jul 2018 19:01:21 -0400
- Subject: BASH 4.4 mapfile/readarray/read builtins mis-behaving with pipe
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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