[PATCH v2 03/10] base-files: upgrade: add get_image_dd()

[Yousong Zhou]

On Tue, 10 Nov 2020 at 13:35, Philip Prindeville
<philipp at redfish-solutions.com> wrote:
>
> Comments…
>
>
> > On Nov 9, 2020, at 7:22 PM, Yousong Zhou <yszhou4tech at gmail.com> wrote:
> >
> > This is mainly to handle stderr message "Broken pipe", "F+P records
> > in/out" by common pattern "xcat | dd .."
> >
> > Ref: https://bugs.openwrt.org/index.php?do=details&task_id=3140
> > Reported-by: Philip Prindeville <philipp at redfish-solutions.com>
> > Signed-off-by: Yousong Zhou <yszhou4tech at gmail.com>
> > ---
> > package/base-files/files/lib/upgrade/common.sh | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> > diff --git a/package/base-files/files/lib/upgrade/common.sh b/package/base-files/files/lib/upgrade/common.sh
> > index e12dfc9678..6c09908ee3 100644
> > --- a/package/base-files/files/lib/upgrade/common.sh
> > +++ b/package/base-files/files/lib/upgrade/common.sh
> > @@ -94,6 +94,12 @@ get_image() { # <source> [ <command> ]
> >       $cmd <"$from"
> > }
> >
> > +get_image_dd() {
> > +     local from="$1"; shift
> > +
> > +     ( get_image "$from" | dd "$@" ) 2>&1 | grep -v -E ': Broken pipe| records (in|out)'
>
>
> Can you through stdout onto another descriptor first, like 3, then move 2 to 1, run the grep, and then after the grep put 3 back onto 1?

Well, I tried a bit and I think it works ;)

root at OpenWrt:/tmp# cat a.sh
echo hello before
(exec 3>&2; sh /tmp/b.sh 2>&1 1>&3 | grep -v stderr; ) 2>&1
echo hello after
root at OpenWrt:/tmp# cat b.sh
echo hello stderr >&2
echo hello stdout >&1
root at OpenWrt:/tmp# sh a.sh
hello before
hello stdout
hello after
root at OpenWrt:/tmp# sh a.sh >/dev/null
root at OpenWrt:/tmp# sh a.sh 2>/dev/null
hello before
hello stdout
hello after
root at OpenWrt:/tmp#

>
> I don’t like the idea of co-mingling stdout and stderr…

I also had this concern and made sure all current users of
get_image_dd had "of=x" present.  That said, it's better if the
constraint is not there at all.

I will wait a while for more comments and send v3 with the redirection
trick if all goes well.

Regards,
               yousong