How can I replace a specific line of a file in bash script?

Question

I have a file inside the one directory, I want to replace one specific line with another line.

In my case the line is - "password requisite pam_pwquality.so" and I want to make it as

"#password requisite pam_pwquality.so".

I tried like this:

 cat test1 | while read line
    do
       if grep -q "$str" test1; then
            echo "found"
            sed -i -e "s/${str}/${str1}/g" test1

       fi

but it is showing multiple found statements but I am having only one copy of this line.

Answer 1

$ x='password requisite pam_pwquality.so'
$ sed -i "s/\($x\)/#\1/" file