[ralph]

sed is great but it's a stream editor; try moving the last line to the start of the file.

[tbe]

sed '$G;1h;1!H;$!d'

[ralph]

Nice try, but efficiency aside, it breaks.

    $ sed '$G;1h;1!H;$!d' <<<foo | hd
    00000000  66 6f 6f 0a 0a             |foo..|
    00000005
    $