sed
can operate on an address:
$ sed -i '1s/^/<added text> /' file
What is this magical 1s
you see on every answer here? Line addressing!.
Want to add <added text>
on the first 10 lines?
$ sed -i '1,10s/^/<added text> /' file
Or you can use Command Grouping
:
$ { echo -n '<added text> '; cat file; } >file.new
$ mv file{.new,}