The echo command is the easiest way to append a new line to the end of a file. In this example, "Hello World" will be appended to the end of example.txt.

echo "Hello World" >> example.txt

The sed command with the a (append) flag can be used to append lines to a file or variable. For example, let's say you have a file or variable with the following text.

Hello World

The following command will add "New Text" to the file or variable. The a flag in this command is used to append.

sed -i '/Hello World/a New Text' $foo

In this example, the file or variable will have the following output.

Hello World
New Text