Bash monitor mode remains enabled if disabled in .bashrc
- Date: Wed, 30 Aug 2017 19:26:38 -0400
- From: "Michel LaBarre" <michel.labarre@xxxxxxxxxx>
- Subject: Bash monitor mode remains enabled if disabled in .bashrc
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
Specifically if "set +m" is present in .bashrc, it looks like job control
(monitor mode) is off
in an interactive shell (as per "echo $-") but it is actually on.
I ran into this with "shopt -s lastpipe" behaviour with and without job
I can also reproduce it with "sleep 25&" then "fg" will fail with "no job
If I "set +m" within the interactive shell rather than, or in addition to,
.bashrc, both the
flags ($-) 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
of a pipeline not executed in the background in the current shell
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.)
$ echo "$-" # no "m" meaning monitor mode is off i.e.
no job control...
$ 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
$ echo abc | read zot # read zot now running in this process so value is
$ echo $zot
$ 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".)
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