grep -f generating unexpected output

Question

Using "grep -of file1.txt file2.txt" (file contents below), I get output:

and
if
pineapple

Why are 'dif' and 'for' missing? Do I have to use any other switches?

file1.txt

and
dif
for
if
apple
pineapple

file2.txt

andiforpineapple

Answer 1

If you want to re-search the input file for each pattern specified:

$ cat patterns.txt 
and
dif
for
if
apple
pineapple
$ cat source.txt 
andiforpineapple
$ while read; do grep -o -e"$REPLY" source.txt; done <patterns.txt
and
dif
for
if
apple
pineapple

However, this will have a different output line ordering than you apparently want, if the source file has more than one line. Since you've not said what you're using this for, I don't know if that will work for your actual problem.

Answer 2

Its like this:

andiforpineapple
  ^found and
   ^continuing search from i
    ^found if
     ^continuing search from o
       ^found pineapple