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From: "Michel LaBarre" <michel DOT labarre AT rogers DOT com>
To: <cygwin AT cygwin DOT com>
Subject: Bash monitor mode remains enabled if disabled in .bashrc
Date: Wed, 30 Aug 2017 19:26:38 -0400
Message-ID: <000701d321e7$6ed52220$4c7f6660$@rogers.com>
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Playing with set -m/+m and shopt -s lastpipe, in an interactive bash shell,
I came across a situation 
which is minor unless  it has broader implications for others that I do not
know about.

Specifically if "set +m" is present in  .bashrc, it looks like job control
(monitor mode) is off 
in an interactive shell (as per "echo $-") but it is actually on.

I ran into this with "shopt -s lastpipe" behaviour with and without job
control.
I can also reproduce it with "sleep 25&" then "fg" will fail with "no job
control".

If I "set +m" within the interactive shell rather than, or in addition to,
.bashrc, both the 
flags ($-) and behaviour (lastpipe and fg) reflect job control being off.

I include only an example with lastpipe.

FYI - from bash reference for shopt:
lastpipe If set, and job control is not active, the shell runs the last
command
	of a pipeline not executed in the background in the current shell
	environment.

My .bashrc includes "shopt -s lastpipe" and "set +m".  
My objective was to produce the same conditions as within a script for
debugging a messy statement at a command line
and I had no need for job control while doing this and I would sometimes
forget to disable monitor mode after a new bash.
If somebody trying to debug some bash statements and is unaware that a
second "set +m" is required, they
might be confused for a bit.  (An option to make an interactive shell behave
the same as a non-interactive shell
might facilitate such debugging.)

C:\mybin>bash
$ echo "$-"                       # no "m" meaning monitor mode is off i.e.
no job control...
hiBHs
$ echo abc | read zot     # zot is therefore in a sub-process hence value
not available in this process
$ echo $zot

$ set +m                
$ echo "$-"                        # still no m but behaviour is
different...
hiBHs
$ echo abc | read zot    # read zot now running in this process so value is
available
$ echo $zot
abc
$ bash --version
GNU bash, version 4.4.12(3)-release (x86_64-unknown-cygwin)

By the way, the "s" set option ($- above) is not defined in the bash
reference manual that I have and is not accepted 
by the "set" built into bash so I don't know how it gets set.  (i.e."set -s
s" or "set +s s" both fail with "invalid option".)

Michel LaBarre


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