How to compute double sha256 shown in examples of bip 143?

Question

How do I compute the double sha256 shown in example of bip 143?

The example in Native P2WPKH

  hashPrevouts:
     dSHA256(fff7f7881a8099afa6940d42d1e7f6362bec38171ea3edf433541db4e4ad969f00000000ef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57b90ec68a01000000)
     = 96b827c8483d4e9b96712b6713a7b68d6e8003a781feba36c31143470b4efd37

I tried

  export PREV_OUT=fff7f7881a8099afa6940d42d1e7f6362bec38171ea3edf433541db4e4ad969f00000000ef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57b90ec68a01000000
  echo -n $PREV_OUT |openssl dgst -sha256 -binary |openssl dgst -sha256

This is what I am getting

  cf4cff3b2196ae87636af6088736f0f9102106375f306c032804845a2d3b92d4

Answer 1

Do this:

echo -n $PREV_OUT | xxd -r -p | openssl dgst -sha256 -binary | openssl dgst -sha256

Explanation

You want to do this:

prevout (raw bytes) -> sha256 -> sha256 -> convert to hex

You're doing this:

prevout (raw bytes) -> convert to hex -> sha256 -> sha256 -> convert to hex

xxd -r -p reverses the hex conversion.