Add prefix to each line of output with sed -

okay after googling minutes seems regular way prefix each line of output sed

but error don't understand.

what mean , how can fix this?

$ sed 's/^/#/' test.txt sed: -e expression #1, char 0: no previous regular expression 

what test.txt looks - it's test

### test.txt b c d e f g h 

oh yeah.. version

$ sed --version gnu sed version 4.2.1 copyright (c) 2009 free software foundation, inc. free software; see source copying conditions.  there no warranty; not merchantability or fitness particular purpose, extent permitted law. 

below code same using capturing group won't touch blank lines.

sed 's/^\(.\)/#\1/' test.txt 

try if want add # @ start of blank line also.

sed 's/^\(.\|\)/#\1/' file 

example:

$ cat f  b $ sed 's/^\(.\)/#\1/' f #a  #b $ sed 's/^\(.\|\)/#\1/' f #a # #b