I want to add the following lines

if ($http_user_agent ~* "somewebsite" ) 
 return 444;
 

to all my vhosts in NGINX as follows

server 
 location / 
 want_to_add_those_3_lines_here
 

Any ideas?

Using GNU sed

This task can be can be done with sed.

However, if you are restricted to the POSIX standard of sed, it is not so easy to handle this problem because you have to compare several lines with corresponding pattern to decide whether or not an insertion has to be done.

The GNU implementation of sed offers some extensions that simplify this kind of multi-line problems.

But the most helpful tool in this context is the -z option that GNU sed offers and that we take great benefit of below.

You can tell whether your version of sed is a GNU implementation by running

sed --version

Turning a multi-line into a single-line problem

With the -z option that GNU sed offers, you have a choice of what sed thinks is a line.

The usual perspective is that a file is split into lines right after each of the newline characters therein. In contrast, with the -z option, sed considers lines to be NULL byte terminated instead.

It is quite likely that sed -z will see only one single long line in each one of your files. Fortunately, GNU sed does not impose any limit on the line length that it is able to process.

Solution

Let's chose sed-script as the name of the following script:

#!/bin/bash

pattern="
(
server n
 location / n
)(
 n
)
"

replacement="
 if ($http_user_agent ~* "somewebsite" ) n
 return 444;n
 n
"

sed -z -e 's!'"$pattern"'!1'"$replacement"'2!g' "$1" > "$2"

If you make this script executable and call it like this:

./sed-script INFILE OUTFILE

it will process INFILE looking for every occurrence of

server 
 location / 
 

replaces it by

server 
 location / 
 if ($http_user_agent ~* "somewebsite" ) 
 return 444;
 
 

and puts the result into OUTFILE.

The only reason why we use a script here is to keep the structure of the sed command clear.

Since the search pattern contains a slash (/), an exclamation mark (!) is used to delimit the fields of the sed s command.