Question

Why is the output empty?

echo "a b c d" | read X Y Z V
echo $X

I thought it would be a.

Answer 1

The issue is that, in order to run the read command with its input redirected from the echo, a new subshell process is spawned. This process reads the values, assigns them to the variables – and then exits; then the second echo command is run. To demonstrate this you can do the second echo and the read both from a subshell:

$ echo "a b c d" | ( read X Y Z V; echo $X )
a