This doesn't use sed, but using >> will append to a file. For example:

echo 'one, two, three' >> testfile.csv

Edit: To prepend to a file, try something like this:

echo "text"|cat - yourfile > /tmp/out && mv /tmp/out yourfile

I found this through a quick Google search.

Without sed, you can use >> to append to a file. For example:

echo 'one, two, three' >> testfile.csv

To prepend to a file, try something like:

echo "text"|cat - yourfile > /tmp/out && mv /tmp/out yourfile

This doesn't use sed, but using >> will append to a file. For example:

echo 'one, two, three' >> testfile.csv

This doesn't use sed, but using >> will append to a file. For example:

echo 'one, two, three' >> testfile.csv

Edit: To prepend to a file, try something like this:

echo "text"|cat - yourfile > /tmp/out && mv /tmp/out yourfile

I found this through a quick Google search.