How to split a huge csv file based on content of first column?

Question

• I have a 250MB+ huge csv file to upload
• file format is group_id, application_id, reading and data could look like

Answer 1

Sed one-liner:

sed -e '/^1,/wFile1' -e '/^2,/wFile2' -e '/^3,/wFile3' ... OriginalFile 

The only down-side is that you need to put in n -e statements (represented by the ellipsis, which shouldn't appear in the final version). So this one-liner might be a pretty long line.

The upsides, though, are that it only makes one pass through the file, no sorting is assumed, and no python is needed. Plus, it's a one-freaking-liner!