Though see the Plain Text Accounting[0] movement for something maybe more unixy than Gnucash. I download .csv files from my bank and credit card issuers and import to hledger[1]. hledger has its own rules engine for filtering/transforming imported entries, but you could also preprocess the files using any unix tool before importing if you needed to.
0: https://plaintextaccounting.org/
1: https://hledger.org/