Text replace in a file, on 5h line, from position 18 to position 145 - shell

Question

I have this text file:
{
"name": "",
"auth": true,
"username": "rtorrent",
"password": "d5275b68305438499f9660b38980d6cef7ea97001efe873328de1d76838bc5bd15c99df8b432ba6fdcacbff82e3f3c4829d34589cf43236468d0d0b0a3500c1e"
}
Now, I want to be able to replace the d5275b68305438499f9660b38980d6cef7ea97001efe873328de1d76838bc5bd15c99df8b432ba6fdcacbff82e3f3c4829d34589cf43236468d0d0b0a3500c1e using sed for example. (The string has always the exact same length, but the values can be different)
I've tried this using sed:
sed -i 5s/./new-string/18 file.json
That basically replaces text, on the 5th line, starting with position 18. I want to be able to replace the text, exactly starting with position 18 and up to position 154, strictly what's inside the "". The command above will cut the ", at the end of the file and if it's run multiple times, the string becomes every time longer and longer.
Any help is really appreciated.

Answer 1

You can use for example awk for it:
$ awk -v var="new_string" 'NR==5{print substr($0,1,17) var substr($0,146);next}1' file
{
"name": "",
"auth": true,
"username": "rtorrent",
"password": "new_string"
}
but there are better tools for changing a value in a JSON, jq for example:
$ jq '.password="new_string"' file
{
"name": "",
"auth": true,
"username": "rtorrent",
"password": "new_string"
}
Edit: When passing a shell variable $var to awk and jq:
$ var="new_string"
$ awk -v var="$var" 'NR==5{print substr($0,1,17) var substr($0,146);next}1' file
and
$ jq --arg var "$var" '.password=$var'
Edit2: There is always sed:
$ sed -i "5s/\"[^\"]*\"/\"$var\"/2" file