sed - How to insert a line in a file between two blocks of known lines (if not already inserted previously), using bash?

Question

I wrote a bash script which can modify php.ini according to my needs.
Now I have to introduce a new change, and I cannot find a clear solution to it.

I need to modify php.ini in order to insert (if not already inserted previously)

extension="memcache.so" 

between the block

;;;;;;;;;;;;;;;;;;;;;;
; Dynamic Extensions ;
;;;;;;;;;;;;;;;;;;;;;;

and the block

;;;;;;;;;;;;;;;;;;;
; Module Settings ;
;;;;;;;;;;;;;;;;;;;

possibly just before the second one.
Can anyone help me please? Thanks in advance

EDITED: solved by using

if ! grep -Fxq 'extension="memcache.so"' 'php.ini'; then
    line=$(cat 'php.ini' | grep -n '; Module Settings ;' | grep -o '^[0-9]*')
    line=$((line - 2))
    sudo sed -i ${line}'iextension="memcache.so"' 'php.ini'
fi

Answer 1

Get the line number using grep -n:

line=$(cat php.ini | grep -n 'Module Settings' | grep -o '^[0-9]*')

Calculate the line to insert the text to:

line=$((line - 3))

Insert it using sed or awk. Examples to insert "newline" on line 45:

sed '45i
ewline' file
awk 'NR==45{print "newline"}1'