bash prepend a header file into a file

prepend

assuming that the file you want to edit is my.txt

$cat my.txt    
this is the regular file
And the file you want to prepend is header

$ cat header
this is the header
Be sure to have a final blank line in the header file.
Now you can prepend it with

$cat header <(cat my.txt) > my.txt
You end up with

$ cat my.txt
this is the header
this is the regular file
As far as I know this only works in 'bash'.