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...
One good one is Strozzi NoSQL (his use of the term NoSQL predates the current use of the term by many years...): http://www.strozzi.it/cgi-bin/CSA/tw7/I/en_US/NoSQL/Home%20P...
Starbase is another, with interesting extensions for astronomical work.
Linux Review article on the concept here: https://www.linuxjournal.com/article/3294
The article that started it all: http://www.linux.it/~carlos/nosql/4gl.ps
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...