Append text to file using sed

Use $ a.

sed -i "$ a some text" somefile.txt

Adding a late post because while the accepted answer is the simplest solution to this problem, the question actually highlights a couple of common situations:

1. Need to edit a file via sudo
2. Trying to use sed to modify an empty file.

In short, you can't touch, then edit a file with sed. Sed doesn't work on empty files, but occasionally you need to do the equivalent of

sudo echo "some text" >> somefile.txt

sudo doesn't like the redirect, but there are workarounds:

echo "some text" | sudo tee --append somefile.txt

or if pipes are not an option either, or you simply must use sed, or you just like complexity:

dd if=/dev/zero count=1 bs=1 of=somefile.txt
sed -i '$ a some text' somefile.txt
sed -i '1 d' somefile

On some systems,. you might have to use sed -i -e '$ a ...

If you're just appending text to the end of the file then you wouldn't use sed in the first place.

echo "some text" >> somefile.txt