Add a new line at the end of file<>
This question already has an answer here:
To insert a newline character to the end of all text files in your working dir:
sed -i -e '$G' *.txt
If you want to append a line to a set of files, it is pretty easy even in a shell:
for file in $files ; do echo "$line" >> "$file" done
If you want to add a line in the middle of the files, you have to use a temp file.