Unix – How to append data in a file by dd?

I want to append new data in a file stored in SSD.

dd if=/dev/shm/test of=/data/sdb/test bs=1G oflag=append

But df -h shows the dd command always overwrite the test file, instead appends new data in the test file. I also tried

dd if=/dev/shm/test of=/data/sdb/test bs=1G conv=notrunc

It does not work, either.

Solution:

What about:

 dd if=/dev/shm/test bs=1G >>/data/sdb/test