I have a number of files in a directory hierarchy. For each file, I'd like to add "abcdef" to the beginning, on its own line, and "ghijkl" to the end, on its own line. For example, if the files initially contained:

# one/foo.txt
apples
bananas

# two/three/bar.txt
coconuts

Then afterwards, I'd expect them to contain:

# one/foo.txt
abcdef
apples
bananas
ghijkl

# two/three/bar.txt
abcdef
coconuts
ghijkl

What's the best way to do this?

I've gotten as far as:

# put stuff at start of file
find . -type f -print0 | xargs -0 sed -i 's/.../abcdef/g'

# put stuff at end of file
find . -type f -print0 | xargs -0 sed -i 's/.../ghijkl/g'

but I can't seem to figure out how what to put in the ellipses.