How to select lines between two marker patterns which may occur multiple times with awk/sed

Use awk with a flag to trigger the print when necessary:

$ awk '/abc/{flag=1;next}/mno/{flag=0}flag' file
def1
ghi1
jkl1
def2
ghi2
jkl2

How does this work?

• /abc/ matches lines having this text, as well as /mno/ does.
• /abc/{flag=1;next} sets the flag when the text abc is found. Then, it skips the line.
• /mno/{flag=0} unsets the flag when the text mno is found.
• The final flag is a pattern with the default action, which is to print $0: if flag is equal 1 the line is printed.

For a more detailed description and examples, together with cases when the patterns are either shown or not, see How to select lines between two patterns?.