Why does ps used with grep always return an output at least of one line?

Solution Verified - Updated -

Issue

  • Why does ps used with grep always return an output at least of one line?
# ps aux | grep sleep
root      5334  0.0  0.0 112640   960 pts/0    S+   09:37   0:00 grep --color=auto sleep
# echo $?
0

Environment

  • Red Hat Enterprise Linux 5/6/7

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