How can I add a line to a file in a shell script?

To answer your original question, here’s how you do it with sed:

sed -i '1icolumn1, column2, column3' testfile.csv

The “1i” command tells sed to go to line 1 and insert the text there.

The -i option causes the file to be edited “in place” and can also take an optional argument to create a backup file, for example

sed -i~ '1icolumn1, column2, column3' testfile.csv

would keep the original file in “testfile.csv~”.