Redis aof BGREWRITEAOF problem

i am learning redis,and i want to see aof rewrite,so i useBGREWRITEAOF command to see the change。then has some problem。i have search some days,can`t find the answer so i open this issue.

i use docker run -itd --name redis-test redis redis-server --appendonly yes。

file appendonly.aof is some command this is right, but after i use BGREWRITEAOF it changed,seems to become a binary file

use redis-check-aof --fixto check this file,tell me is valid.

so my question is this is right or somewhere done error.
Thanks a lot!
this is a gif

Comment From: antirez

Hello, you have RDB preamble enabled. Basically when this function is on (by default), the AOF is rewritten in terms of an RDB binary file plus an AOF tail of what will come next.