Using composer - basics with localhost Mamp My goal is to try updating drupal/core with Composer and to learn composer basics. Composer is installed on OS X. From terminal i can run 

My-MacBook-Pro:~ osxuser$ composer
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/ /___/ /_/ / / / / / / /_/ / /_/ (__  )  __/ /
\____/\____/_/ /_/ /_/ .___/\____/____/\___/_/
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Composer version 1.4.1 2017-03-10 09:29:45

Usage:
command [options] [arguments]...


So how do I make composer find my drupal8 install in Mamp?

I have tried both:

composer -d, --working-dir=/Applications/MAMP/htdocs/test_d8


and

cd /Applications/MAMP/htdocs/test_d8


If I am running several drupal 8 installs in /Applications/MAMP/htdocs/ how do I tell composer witch one to work on?

From what I've red you don't need drush any more. I am not sure Composer has all it needs to work with Drupal 8 either.

composer show
symfony/css-selector      v3.2.6  Symfony CssSelector Component
symfony/dom-crawler       v2.8.13 Symfony DomCrawler Component
symfony/finder            v3.2.6  Symfony Finder Component
symfony/polyfill-mbstring v1.3.0  Symfony polyfill for the Mbstring extension

composer diagnose
Checking composer.json: WARNING
No license specified, it is recommended to do so. For closed-source software you may use "proprietary" as license.
require.symfony/dom-crawler : exact version constraints (2.8.13) should be avoided if the package follows semantic versioning
Checking platform settings: WARNING
The OpenSSL library (0.9.8zc) used by PHP does not support TLSv1.2 or TLSv1.1.
If possible you should upgrade OpenSSL to version 1.0.1 or above.

Checking git settings: OK
Checking http connectivity to packagist: OK
Checking https connectivity to packagist: OK
Checking github.com rate limit: OK
Checking disk free space: OK
Checking pubkeys: 
Tags Public Key Fingerprint: ---numbers here---
Dev Public Key Fingerprint: ---numbers here---
OK
Checking composer version: OK


Can someone point me in the right direction here? Do I need to use/install other composer submodules/libraries?

Is it possible to do some sort of composer update drupal/core --dry-run as a test?
Posting as an answer instead of a comment.

You run Composer from the project directory, which is where your composer.json and .lock file (which will get generated for you) lives.

I can work on any project at any time in the CLI by cd ing to the project I want to work on. Composer understands that this is the working directory in that execution.

So, no matter where you are in your server, Composer will execute in the directory you are currently in.