利用sed指定行數取代字串-replace an entire line in a text file

sed -i 'columns/.*/replacement-word/' file.txt

a=123
sed -i 'columns/.*/'$a'/' file.txt

ex:
$ cat 123.txt
123
234
456
678

$ sed -i '3s/.*/nnn/' 123.txt
$ cat 123.txt
123
234
nnn
678

ref: stack overflow