How to use a bash script/command to insert a line before another line?

Question

bash

Good afternoon.
There is a t.txt file with the content:

AAA
BBB
DDD
EEE

How can I insert the string "CCC" between "BBB" and "DDD" using a bash script/command?
Can you advise how to implement this? The main nuance is that the file has already been created with certain content and, therefore, the command

echo "
AAA
BBB
CCC
DDD
EEE" > t.txt

the issue is not resolved)
Thanks in advance)

Answer 1

# после паттерна
sed -i.bak '/BBB/a CCC' /path/to/file

# перед паттерном
sed -i.bak '/BBB/i CCC' /path/to/file