Like a reverse of awk 'NR==2 {print ""} 1 which adds a new line under the first and prints whatever there. I can't replace the number after NR== with a fixed value because I want to be able to do this at file including any amount of lines. I mean I suppose I could do something that counts the amount of lines and then make a variable which is the number of the second last line, but I don't want to "reinvent the wheel".
Example input:
1
2
3
4
Example output:
1
2
3
something added here
4
sed '$i\something'(there's a newline after the backslash)..