SalesforceDX: Log in to sandbox by username/password from command line without browser involved

sfdx force:auth only provides the JWT and web OAuth login methods:

sfdx force:auth commands: (get help with sfdx help force:auth:COMMAND)
 force:auth:jwt:grant      authorize an org using the JWT flow
 force:auth:logout         log out from authorized orgs
 force:auth:sfdxurl:store  authorize an org using an SFDX auth URL
 force:auth:web:login      authorize an org using the web login flow

But... there's a couple of different ways around the lack of a direct username/password login option.

One's to use sfdx force:source:convert to build a Metadata API package. Provided you have configured your sandbox so that CLI access doesn't require completing the verification code (or you're logging in with a security token), you can then use any existing Metadata API deployment tool (Force.com Migration Tool, Force.com CLI, etc) to perform the deploy using the username and password alone.

Another route is to login to the sandbox by hand once, locally, via force:auth:web:login. Then do sfdx force:org:display -u <username> --verbose to get the SFDX authentication URL, which starts with force:// and includes the OAuth refresh token (not the session id). You can store that securely in an encrypted file and later load it into SFDX with sfdx force:auth:sfdxurl:store -f <thefile>. (Credit to CRMScience for documenting this approach).

You could use the auth command from the sfpowerkit plugin:

sfpowerkit auth:login

https://github.com/Accenture/sfpowerkit

It supports login with username-password combination (probably will need security token as well). A functionality that the standard CLI lacks