Maybe have a look at this previous forum thread…
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This is a little different from what you are asking about but it does get to the heart of printing barcode labels directly from the ERPNext application. In the case of the link above, it is using a custom print format to handle the job and the full text of the custom script is posted for you to play with on your own system.
You may need to do a little work to learn how to use the ZPL (Zebra Programming Language) to implement a solution. However by reading through the link it also appears that you could do the same thing with almost any printer programming language that you may already be familiar with in your past experiences.
I thinks it is worth investigating if you are serious about NOT using some other middle-ware application to handle the printer. Most of the middle-ware solutions require a license fee for each connected printer.
Hope this helps…
Please share back here if you find a way to use the information to make your own solution.
If you get really stumped and cannot get it figured out, you might be able to find a developer to write you a simple application that periodically dumps the Item data to a CSV file that can then be used by one of the 3rd party barcode printing programs. The 3rd party programs have some pretty simple-to-use application generators that allow you to print labels based on information it can lookup in the CSV files. The 3rd party programs are not cheap, but it is a way to get things to work.
In the past, I have contacted developers from the list maintained by the erpnext/frappe team on their website. I had one write such an application for me to generate the CSV files every 15 minutes and then used a program called BarTender to make the printing application and handle the printers. All of this will cost you money. Trying to figure out the usage of the information in the above link I posted, is the free alternative and an exercise in learning at the same time.
BKM