Using `sed` to add contents

add.sh

weli@192:/tmp
➤ cat foo.txt                                                                                                                          02:05:38
abc
def

weli@192:/tmp
➤ vi foo.txt                                                                                                                           02:05:41
weli@192:/tmp
➤ sed -e 's/def/xyz/' foo.txt                                                                                                          02:05:48
abc
xyz
weli@192:/tmp
➤ sed -e 's/def/xyz\n/' foo.txt                                                                                                        02:05:51
abc
xyz

weli@192:/tmp
➤ sed -e 's/def/xyz\ndef/' foo.txt                                                                                                     02:05:55
abc
xyz
def
weli@192:/tmp
➤                                                                                                                                      02:06:08