At first glance, I thought running the data loader through the command line would be complicated. Turns out it is actually quite easy. Essentially all you need to do is create the configuration through the UI and then blast away at the command line:
Create a new folder describing the job; I recommend creating the folder [...]
Describe the function of an External ID
Each object can have a maximum of 3 external ID fields.
The external ID can be a number, text, or email field.
An external ID can be used to prevent duplicate entries when importing data.
An external ID makes it much easier to update data in Salesforce from an [...]
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