Shell Programming and Scripting

cat list.txt

...
...
...
Hello world 
how r u 
whats up
....
....
...

vi pattern.sh 

cat list.txt | while read line 
do 
pattern=$(awk '/hello/{x=NR+2}(NR<=x){print}' $line)
sed -e s/^/#/g $pattern 
done

...
...
...
#hello world
#how ru 
# whats up 
...
...
...

[user@home-pc ~]$ cat file
...
...
...
Hello world
how r u
whats up
....
....
...
[user@home-pc ~]$ perl -i.bak -lne 'if(/Hello/){$f=1;$l=$.;print "# $_";}elsif($f==1 && $.<=$l+2){print "# $_"}else{print}' file
[user@home-pc ~]$ cat file
...
...
...
# Hello world
# how r u
# whats up
....
....
...

cat file ... how r u 
whats up  ... Hello world how r u whats up .... .... 
how r u 
whats up

sed '/Hello world/,+2s/^/#/' list.txt > list_$$.txt
mv list_$$.txt list.txt

awk '/pattern/ {
        for(N=1; N<=3; N++)
        {
                if(N>1) getline
                print "#", $0;
        }
        next } 1' inputfile > outputfile