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

i have file inside 1 directory, want replace 1 specific line line.

in case line - "password requisite pam_pwquality.so" , want make as

"#password requisite pam_pwquality.so".

i tried this:

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

but showing multiple found statements having 1 copy of line.

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

TY