sed replace empty line with character

Question

How do you replace a blank line in a file with a certain character using sed?

I have used the following command but it still returns the original input:

Answer 1

Here is a way with awk. This wouldn't care if you have spaces or blank lines:

awk '!NF{$0=">"}1' file

NF stands for number of fields. Since blank lines or lines with just spaces have no fields, we use that to insert your text. 1 triggers the condition to be true and prints the line:

Test:

$ cat -vet file
ACTCTATCATC$
      $
CTACTATCTATCC$
$
CCATCATCTACTC$
       $

$ are end of line markers

$ awk '!NF{$0=">"}1' file
ACTCTATCATC
>
CTACTATCTATCC
>
CCATCATCTACTC
>