user10377: I was trying to get strong feedback that I really understood a portion of the Mastering Bitcoin Book concerning HD wallets. The picture below is the reference for the question.

I checked out the functionality exposed by a Javascript Brain Wallet. By entering "123" (is a very poor secret) into a field for a HD brainwallet and the following extended private key emerged: xprv9s21ZrQH143K2hYgcmBxsUfRw3Kbu3eEGXFJrM8Cqtewuqa6Y6nXbB7tv9HnrBajqjptPsRjL9FmnwLa317jHWJ5dpwTc99C2AiPztPRaxk.
Clicking the button to the right of the private key got me to another page indicating the master private chain code is 40675733fc35303c1c75bc598716f24af225528da686192ceb567c88313f8885 and the master private key is L4h3ZSQ3yfFNbovQjTGumvUGEfB1a7oFAhbkhMMFGsdDKoTSwwU8.  
Given these last two pieces of information and an index of 0, is it possible to issue a number a number of bx commands and concatenate their output results to feed a HMAC-SHA512 hash to arrive at private WIF key of KwMsDozFYQG7SZFwfvFRwWDhfuMJjkE669WxP6SqrpDPsCBrPV4p public address 15jbcT7wE81s8n2dbEGRFGf65zp7PHrjxh?  If so, how is this accomplished or does bx need to be complemented by some other command that renders HMAC-SHA512 functionality?

user2306: No need for an external program at all:
$ KEY=xprv9s21ZrQH143K2hYgcmBxsUfRw3Kbu3eEGXFJrM8Cqtewuqa6Y6nXbB7tv9HnrBajqjptPsRjL9FmnwLa317jHWJ5dpwTc99C2AiPztPRaxk
$ echo $KEY | bx hd-public -i 0 | bx hd-to-address
15jbcT7wE81s8n2dbEGRFGf65zp7PHrjxh

Or, because this is non-hardened, you can use the extended public key:
$ KEY=xpub661MyMwAqRbcFBd9iniyEccAV5A6JWN5dkAuejXpQEBvnduF5e6n8ySNmP8uWeqNbwFXLHEQkV1shXbNtBV8YCUJPMSuE7bGCvDaZJfUBuP
$ echo $KEY | bx hd-public -i 0 | bx hd-to-address
15jbcT7wE81s8n2dbEGRFGf65zp7PHrjxh


10377: <p>I was trying to get strong feedback that I really understood a <a href="https://github.com/aantonop/bitcoinbook/blob/develop/ch04.asciidoc#CKDpriv" rel="nofollow noreferrer">portion</a> of the Mastering Bitcoin Book concerning HD wallets. The picture below is the reference for the question.</p>

<p><img src="https://i.stack.imgur.com/HPlYl.png" alt="HD Private Child Key Derivation"></p>

<p>I checked out the functionality exposed by a <a href="https://coinb.in/#newHDaddress" rel="nofollow noreferrer">Javascript Brain Wallet</a>. By entering "123" (is a very poor secret) into a field for a HD brainwallet and the following extended private key emerged: <strong>xprv9s21ZrQH143K2hYgcmBxsUfRw3Kbu3eEGXFJrM8Cqtewuqa6Y6nXbB7tv9HnrBajqjptPsRjL9FmnwLa317jHWJ5dpwTc99C2AiPztPRaxk</strong>.</p>

<p>Clicking the button to the right of the private key got me to another page indicating the master private chain code is <strong>40675733fc35303c1c75bc598716f24af225528da686192ceb567c88313f8885</strong> and the master private key is <strong>L4h3ZSQ3yfFNbovQjTGumvUGEfB1a7oFAhbkhMMFGsdDKoTSwwU8</strong>.  </p>

<p>Given these last two pieces of information and an index of 0, is it possible to issue a number a number of bx commands and concatenate their output results to feed a HMAC-SHA512 hash to arrive at private WIF key of <strong>KwMsDozFYQG7SZFwfvFRwWDhfuMJjkE669WxP6SqrpDPsCBrPV4p</strong> public address <strong>15jbcT7wE81s8n2dbEGRFGf65zp7PHrjxh</strong>?  If so, how is this accomplished or does bx need to be complemented by some other command that renders HMAC-SHA512 functionality?</p>
 2306: <p>No need for an external program at all:</p>

<pre><code>$ KEY=xprv9s21ZrQH143K2hYgcmBxsUfRw3Kbu3eEGXFJrM8Cqtewuqa6Y6nXbB7tv9HnrBajqjptPsRjL9FmnwLa317jHWJ5dpwTc99C2AiPztPRaxk
$ echo $KEY | bx hd-public -i 0 | bx hd-to-address
15jbcT7wE81s8n2dbEGRFGf65zp7PHrjxh
</code></pre>

<p>Or, because this is non-hardened, you can use the extended public key:</p>

<pre><code>$ KEY=xpub661MyMwAqRbcFBd9iniyEccAV5A6JWN5dkAuejXpQEBvnduF5e6n8ySNmP8uWeqNbwFXLHEQkV1shXbNtBV8YCUJPMSuE7bGCvDaZJfUBuP
$ echo $KEY | bx hd-public -i 0 | bx hd-to-address
15jbcT7wE81s8n2dbEGRFGf65zp7PHrjxh
</code></pre>
