Add text to file at certain line in Linux [duplicate]

You can use sed to solve this:

sed "15i avatar" Makefile.txt

or use the -i option to save the changes made to the file.

sed -i "15i avatar" Makefile.txt

To change all the files beginning that start Makefile:

sed "15i avatar" Makefile*

Note: In the above 15 is your line of interest to place the text.