How can I earn money from open source?

you don't

Donations?

Donations or sponsors.

If you want to make money or an income, have a product worth selling at a subscription level.

Double license.
Pay for commercial license (Qt style)

Freemium, free for a certain level then pay. Of course those paid stuff are not open source or else people would just build from source and get all premium features

As other said: Additional feautures paid support - but also special conditions. Owncloud usually licensed under AGPL, requiring customers to publish their own modifications, the commercials license removes such conditions.

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Become an expert in it and support it for a living. That's worked great for me.

Absolutely donations (If you dare to earn money with open source)

SaaS

You can earn money with it like with any other type of software: just run a business with it.

And if you're afraid someone will 'steal' your ideas, then you could 'license up' to the point where everyone basically can only read your code, but not use it legally. Or split up your system into open and closed parts, where the closed parts contain some of the essential proprietary stuff.

Open source just means that you publish the source. Nothing more, nothing less. However, it opens up a number of possibilities like 3rd party security audits and contributions, to name a few. It really matters why you'd want to go open source and that you believe in that decision.

Work for an employer who contributes to open source. Embedded linux is a safe bet.

The dude that created Vue.js has a good idea. Homeboy created a patreon account once he became known for it and is banking on it

Donations, paid support, paid additional features, SaaS, to name a few.