text manipulation in shellscript

I want to ad开发者_高级运维d a dot to all lines that doesn't start with a dot. How can I do this in shellscript? should I use sed?

example file:
.a
a
b
.c

turns to

.a
.a
.b
.c

---

You can use sed:

sed -i '/^[^.]/s/^/./' text_file