i have two files

file 1 containing :

aaa
bbb
ccc
.
.
.
zzz

111
222
333
111
222
333
.
.
.

111
222
aaa
111
222
bbb
333
111
222
ccc
.
.
.

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Not sure I get your criteria totally, from your expected output:

111 line 1 from file2
222 line 2 from file 2
aaa line 1 from file 1
111 line 4 from file2  ...   what happened to line 3 from file 2
222 line 5 from file2
bbb line 2 from file 1
333 line 6? from file2
111 line 7 from file2
222 line  8 from file 2
ccc line 3 from file 1

cat file2 | paste - - file1| tr "\t" "\n"

actually Tyatalus

i have file 1 containing different strings each in a differnt line and what i want to do is insert these strings before a particular pattern in file 2

eg file 1 may contain

AAA
BBB
and so on there are 201 entries exactly

now i want to insert AAA after pattern XXX in file 2 then BBB after the next occurence of pattern XXX and so on

ah. in that case I think you want:

#  nawk '/222/{print;getline < "file1"}{print}' file2