How do I detect a failed subprocess in a bash read statement? - bash

Question

In bash we can set an environment variable from a sequence of commands using read and a pipe to a subprocess. But I'm having trouble detecting errors in my processing in one edge case - a part of the subprocess pipeline producing some output before erroring.
A simplified example which takes an input file, looks for a line starting with "foo" and sets var to the first word on that line is:
set -e
set -o pipefail
set -o nounset
die() {
echo $1 > /dev/stderr
exit 1
}
read -r var rest < <( \
cat data.txt \
| grep foo \
|| die "PIPELINE" \
) || die "OUTER"
echo "var=$var"
Running this with data.txt like
blah
zap foo awesome
bang foo
will output
var=zap
Running this on a data.txt file that doesn't contain foo outputs (to stderr)
DEAD: PIPELINE
DEAD: OUTER
This is all as expected.
We can introduce another no-op stage like cat at the end of the process
...
read -r var rest < <( \
cat data.txt \
| grep foo \
| cat \
|| die "PIPELINE" \
) || die "OUTER"
...
and everything continues to work.
But if the additional stage is paste -s -d' ' and the input does not contain "foo" the output is
var=
DEAD: PIPELINE
Which seems to show that the pipeline errors, but read succeeds with an empty line. (It looks like paste -s -d' ' outputs a line of output even when its input is empty.)
Is there a simple way to detect this failure of the pipeline, and cause the main script to error out?
I guess I could check that the variable is not empty - but this is a simplified version - I'm actually using sed and paste to join multiple lines to set multiple variables, like
read -r v1 v2 v3 rest < <( \
cat data.txt \
| grep "^foo=" \
| sed -e 's/foo=//' \
| paste -s -d' ' \
|| die "PIPELINE"
) || die "OUTER"

Answer 1

You could use another grep to see if the output of paste contained something:
read -r var rest < <( \
cat data.txt \
| grep foo \
| paste -s -d' ' \
| grep . \
|| die "PIPELINE" \
) || die "OUTER"

Answer 2

In the end I went with two different solutions depending on the context.
The first was to pipe the results to a temporary file. This will process the entire file before performing the read, and thus any failures in the pipe will cause the script to fail.
cat data.txt \
| grep "^foo=" \
| sed -e 's/foo=//' \
| paste -s -d' ' \
> $TMP/result.txt
|| die "PIPELINE"
read -r var rest < $TMP/result.txt || die "OUTER"
The second was to just test that the variables were set. While this meant
there was a bunch of duplication that I wanted to avoid, it seemed the most bullet-proof solution.
read -r var rest < <( cat data.txt \
| grep "^foo=" \
| sed -e 's/foo=//' \
| paste -s -d' ' \
|| die "PIPELINE"
) || die "OUTER"
[ ! -z "$var" ] || die "VARIABLE NOT SET"