In addition to the usual cut/paste/sort/awk stuff, we've had really powerful "stream-operator" databases based on flat text data files for decades. They used to be somewhat slow. Not anymore, esp when running from RAMdisks.
And there's even a book on the subject, centered on the /rdb implementation by the late RSW software. But I warn you, reading this WILL permanently change the way you think about databases: https://www.amazon.com/Relational-Database-Management-Prenti...