Appending lines
Append line after line 2
$ sed '2a Text after line 2' file.txt
Append line at the end of the file
$ sed '$a THE END!' file.txt
Append line after every 3rd line starting from line 3
$ sed '3~3a Some text' file.txt
Comments
Append line after line 2
$ sed '2a Text after line 2' file.txt
Append line at the end of the file
$ sed '$a THE END!' file.txt
Append line after every 3rd line starting from line 3
$ sed '3~3a Some text' file.txt