I want to instert Category:XXXXX into the 2. line

something like this should work, but I have somewhere the wrong sytanx. something with the linebreak goes wrong:

sed "2i\\${n}Category:$cat\n"

Sample:

Titel Blahh Blahh abllk sdhsd sjdhf
Blahh Blah Blahh
Blahh

Should look like this afterwards;

Titel Blahh Blahh abllk sdhsd sjdhf
Category:The Blahh
Blahh Blah Blahh
Blahh

2. Question: how should the syntax be, if the sed command itself should be a variable, like:

insertcat="sed "2i\\${n}Category:$cat\n""

you can use awk

awk '{if(NR==2){print "Category:"$0}{print $0}}' filename

awk 'NR==2{print "Category:TheBlah\n"$0;next}1' file