Well the structure is a bit odd to begin with but basically grep (and most other Unix utilities) is line-oriented. So in your first version, grep sees the input as "xxxyyyzzz" and in the second one, grep sees the input as "xxx\nyyy\nzzz\n". So the first one is similar to echo -e "xxxyyyzzz" | grep "xxx" and the second one is similar to echo -e "xxx\nyyy\nzzz" | grep "xxx"