Shell Programming and Scripting

while read line
do
 ...
 if [ $xyz == "ON" ]; then
 #Append a new line "===" to input.txt file after $line position
 #break
 fi
done < input.txt

$ cat -n input.txt
     1  test
     2  ON
     3  go
     4  hello
     5  hi
     6  IN
     7  ON
-------------------------------------------------
$ cat i.sh
#!/bin/bash
while read line
do
        line_num=$((line_num+1))
        if [ $line = "ON" ]; then
                echo ${line_num}
        fi
done < input.txt
-------------------------------------------------
$ bash i.sh
2
7

line_before_pin_1
line_before_pin_2
..
line_after_pin_1
line_after_pin_2
PIN
line_after_pin_3
..
line_after_pin_end

cat  Input_file
line_before_pin_1
line_before_pin_2
PIN
line_after_pin_1
line_after_pin_2
line_after_pin_3
line_after_pin_end

awk -vxyz=2 '{if($0 ~ /PIN/){while(++a<=xyz){getline;print $0;};print "PIN";a="";next}} 1'   Input_file

line_before_pin_1
line_before_pin_2
line_after_pin_1
line_after_pin_2
PIN
line_after_pin_3
line_after_pin_end