inserting text into a specific line

Question

I've got a text file, and using Bash I wish to insert text into into a specific line.

Text to be inserted for example is !comment: http://www.test.com into line 5

!aaaa
!bbbb
!cccc
!dddd
!eeee
!ffff

becomes,

!aaaa
!bbbb
!cccc
!dddd
!comment: http://www.test.com
!eeee
!ffff

Answer 1

sed '4a\
!comment: http://www.test.com' file.txt > result.txt

i inserts before the current line, a appends after the line.