Re: Bash expression to detect dying RAID devices

[Roberto C. Sánchez <roberto@debian.org>]

Hi Roger,

On Tue, Dec 24, 2024 at 03:45:31PM +0100, Roger Price wrote:
> File /proc/mdstat indicates a dying RAID device with an output section such
> as
> 
>  md3 : active raid1 sdg6[0]
>        871885632 blocks super 1.0 [2/1] [U_]
>        bitmap: 4/7 pages [16KB], 65536KB chunk
> 
> Note the [U-]. The "-" says /dev/sdh is dead. I would like to scan
> /proc/mdstat and set a flag if [U-], [-U] or [--] occur.  

I am confused. The sample output you provide includes '_' rather than
'-', but your script and all of the tests which it performs are trying
to match '-' (which isn't part of the output).

> My current attempt is
> 
>  #! /bin/bash -u
>  set -x
>  BAD=0;
>  while  read L;
>  do if [[ $L == *"[U-]"* ]]; then B=1; fi;
>     if [[ $L == *"[-U]"* ]]; then B=1; fi;
>     if [[ $L == *"[--]"* ]]; then B=1; fi;
>  done < /proc/mdstat;
>  echo $BAD
> 
Also, you assign BAD=0, then echo $BAD, but your loop assigns B=1
(rather than BAD=1). Even if you managed to find a test that matched,
your script would still always echo 0.

> Far from elegant, but I still can't get it to work.
> The trace contains lines such as
> 
>  + 1164021 1164021 [4]read L
>  + 1164021 1164021 [5][[ 20970368 blocks super 1.0 [2/1] [U_] == *\[\U\-\]* ]]
> 
> The test always fails, but I can't see why.  Any hint would be very welcome.
> 

I think that '==' is the wrong tool. That is testing for string
equality, whilst you are looking for a partial match. This is what I was
able to get working after hacking on it for a minute or two:

#! /bin/bash -u
set -x
BAD=0;
while  read L;
do if [[ $L =~ \[(U_|_U|__)\] ]]; then BAD=1; break; fi;
done < /proc/mdstat;
echo $BAD

Note that I changed to a regex match, and also added a 'break;' after
assigning BAD=1, because there is no need to continue processing the
input at that point.

Regards and Merry Christmas,

-Roberto

-- 
Roberto C. Sánchez