DX CLI - Custom Settings

The custom settings metadata can be retrieved or deployed just like any other metadata.

All you need is name of the custom setting

If you are working in orgs that cannot have source tracking enabled

sfdx force:source:retrieve -m CustomObject:<Customsettingname__c>

If you are using source tracked orgs use

sfdx force:source:pull

If you want data within these custom settings then you can use soql

sfdx force:data:soql:query -q "SELECT Id, Name FROM Customsettingname__c"

You can export the data to the local folder using

sfdx force:data:tree:export -q "SELECT Id, Name FROM Customsettingname__c" -x export-customsetting -d /tmp/sfdx-out -p

And then load the exported custom setting data into a target scratch org with

sfdx force:data:tree:import -p /tmp/sfdx-out/export-customsetting.json