how to add line inside a file code example

Example: how to add line inside a file

how to add lines inside a file

sed -i -e "48r../../../../folder1/lines_to_add.txt" a.txt 

a.txt - file you want to change
48r is line number of a.txt 
some lines are inside lines_to_add.txt file
../../../../scripts3_2d/lines_to_add.txt 
-i update a.txt, try without -i before runthe code,
be carefull with newlines, 
"keep a newline at the end of lines_to_add.txt"