Behaviour in shell script with job control enabled

[Silas Silva]

Hi all.

I was writing a shell script with `while ping6 ...` but I faced problems
that show up only in interactive shells.  After investigating, I figured
out it was due to the -m option (job control -- set on by default when
running shell interactively).  So I wrote the following:

    #!/bin/sh

    set +m
    if ping6 -c 1 fe80::a800:ff:fe13:942f >/dev/null; then
        echo yes
    else
        echo no
    fi

    set -m
    if ping6 -c 1 fe80::a800:ff:fe13:942f >/dev/null; then
        echo yes
    else
        echo no
    fi

In zsh, sh, and ksh output is:

    no
    <blank line here>

In bash, output is:

    no
    no

Using NetBSD 7.0.

Why enabling (or not) job control (-m) should change the script
behaviour?

Thanks.

-- 
Silas