Substitution on range of lines

Question

I have in 3rd line of a file. I want to replace that with my_dB. How to do this with sed

Answer 1

Each can be done with something similar to this:

sed -i "s/<\/host>/my_db<\/host>/" foo.txt

The -i means that it'll overwrite the file in-place. Remove that flag to have the changes written to stdout.

If this is for a regular task to fill in details for the file, consider adding anchors to the file to make it easier. For example:

DB_HOST

Then the sed would just need to be:

sed -i 's/DB_HOST/my_db/'