Why is Gnu grep exit status always 0 on standard input?

Question

On a bash command line in Ubuntu 14.04 :

echo "z" | grep -e 'x' ; echo $PIPESTATUS

always displays 0, even though, obviously, there's no "x" in "z". But:

echo "z" > /tmp/z
grep -e 'x' /tmp/z ; echo $PIPESTATUS 

works as I expect, returning 1 if the pattern is 'x' (as shown), and 0 if the pattern is 'z'.

Why?

Answer 1

PIPESTATUS is actually a BASH array variable containing a list of exit status values from the processes in the most-recently-executed foreground pipeline.

To print all the exit statuses, you need to use it as:

echo "z" | grep -e 'x' ; echo ${PIPESTATUS[@]}
0 1

Where 0 is exit status of first echo command and 1 is the exit status of grep command.

When you use echo $PIPESTATUS it just prints first element of array.

To check the grep command status you don't even need to use PIPESTATUS.

You should just use $?:

echo "z" | grep -q 'x' ; echo $?
1