I have a problem checking whether a certain command in a multi-pipe command chain did throw an error. Usually this is not hard to check but neither set -o pipefail nor checking ${PIPESTATUS[@]} works in my case. The setup is like this:
cmd="$snmpcmd $snmpargs $agent $oid | grep <grepoptions> for_stuff | cut -d',' f$fields | sed 's/ubstitute/some_other_stuff/g'"
Note-1: The command was tested thoroughly and works perfectly.
Now, I want to store the output of that command in an array called procdata. Thus, I did:
declare -a procdata
procdata=( $(eval $cmd) )
Note-2: eval is necessary because otherwise $snmpcmd throws up with an invalid option -- <grepoption> error which makes no sense because <grepoption> is not an $snmpcmd option obviously. At this stage I consider this a bug with $snmpcmd but that's another show...
If an error occurres, procdata will be empty. However, it might be empty for two different reasons: either because an error occurred while executing the $snmpcmd (e.g. timeout) or because grep couldn't find what it was looking for. The problem is, I need to be able to distinguish between these two cases and handle them separately.
Thus, set -o pipefail is not an option since it will propagate any error and I can't distinguish which part of the pipe failed. On the other hand echo ${PIPESTATUS[@]} is always 0 after procdata=( $(eval $cmd) ) even though I have many pipes!?. Yet if I execute the whole command directly at the prompt and call echo ${PIPESTATUS[@]} immediately after, it returns the exit status of all the pipes correctly.
I know I could bind the err stream to stdout but I would have to use heuristic methods to check whether the elements in procdata are valid or error messages and I run the risk of getting false positives. I could also pipe stdout to /dev/null and capture only the error stream and check whether ${#procdata[@]} -eq 0. But I'd have to repeat the call to get the actual data and the whole command is time costly (ca. 3-5s). I wouldn't want to call it twice. Or I could use a temporary file to write errors to but I'd rather do it without the overhead of creating/deleting files.
Any ideas how I can make this work in bash?
Thanks
P.S.:
$ echo $BASH_VERSION
4.2.37(1)-release