Challenge: a VERY strange problem with command substitution in bash

[Richard Beels via cygwin]

At 07/11/2017 at 15:12, Shakespearean monkeys 
danced on Jürgen Wagner's keyboard and said:
>...
>Using backquotes instead of the command 
>substitution with $(...) does not change the 
>results. I could swear this did work in an 
>earlier version of Cygwin on my Windows 7 machine.
>
>I tried this to see if the code in the parentheses is executed at all:
>
>$ value="$( date 2> foo | cat )"; echo "$? <$value>"
>
>The file "foo" was not created, i.e., it seems 
>the commands don't really get executed.
>
>$ value="$( date && pwd )"; echo "$? <$value>"
>0 <Tue Jul 11 20:49:09 CEST 2017
>/home/juergen>
>
>$ value="$( date || pwd )"; echo "$? <$value>"
>0 <Tue Jul 11 20:32:27 CEST 2017>
>
>both work, so some control structures seem to be 
>permissible... just not a pipe.
>
>What is going on? Some misconfiguration? A 
>Cygwin bug? Some interaction with something 
>weird in Windows 10? I am at loss to understand 
>what could be wrong... and am now most curious 
>whether anybody has an idea what is causing 
>this. Does it work/not work in the same way in your Cygwin installation?
>
>I came across this effect because 
>ssh-host-config did not recognize me as 
>administrator anymore. It's due to a check for a 
>certain user group that uses a command 
>substitution with a pipe. Replacing this with an 
>equivalent command works. The original line used 
>"id -G" and then a "grep -Eq" to check whether a certain group is on that list.
>
>I am VERY curious now! I've rarely been puzzled 
>that much by such a very elementary shell 
>expression (looking back at 34 years of Unix experience).


Hi Jurgen.

90% chance it's what is called bloda in these 
parts.  It's in the FAQ on cygwin.com.  I'll go 
out on a limb and say you might have just 
installed/changed your AV/Firewall software.

And if I want to be super-psychic, can I guess 
comodo?  Because I just changed to comodo a 
couple weeks ago and had the same 
subshell/command substitution/pipeline errors you're mentioning.

If so, you need to exclude your cygwin folder 
from AV scanning. AND...  if the software does 
whitelisting or host intrusion protection (HIPS) 
or "run unknown executables in a 
container/sandbox" or something similar, you also 
need to trust all the executables, too.  Or 
switch to something else that doesn't trip cygwin's trigger.

After doing that and a rebaseall, I haven't had a 
fork error in a week.  I can't wait to run setup 
and come up with an update process, though...

Cheers!


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