show only matched strings - grep - regex

Question

I have two files. File1 is as follows
Apple
Cat
Bat
File2 is as follows
I have an Apple
Batman returns
This is a test file.
Now I want to check which strings in first file are not present in the second file. I can do a grep -f file1 file2 but thats giving me the matched lines in the second file.

Answer 1

To get the strings that are in the first file and also in the second file:
grep -of file1 file2
The result (using the given example) will be:
Apple
Bat
To get the strings that are in the first file but not in the second file, you could:
grep -of file1 file2 | cat - file1 | sort | uniq -u
Or even simpler (thanks to #triplee's comment):
grep -of file1 file2 | grep -vxFf - file1
The result (using the given example) will be:
Cat
From the grep man page:
-o, --only-matching
Print only the matched (non-empty) parts of a matching line, with each such part on a separate output line.
From the uniq man page:
-u, --unique
Only print unique lines

Answer 2

If you want to show words from file1 that are not in file2, a dirty way is to loop through the words and grep silently. In case of not match, print the word:
while read word
do
grep -q "$word" f2 || echo "$word"
done < f1
To match exact words, add -w: grep -wq...
Test
$ while read word; do grep -q "$word" f2 || echo "$word"; done < f1
Cat
$ while read word; do grep -wq "$word" f2 || echo "$word"; done < f1
Cat
Bat
A better approach is to use awk:
$ awk 'FNR==NR {a[$1]; next} {for (i=1;i<=NF;i++) {if ($i in a) delete a[$i]}} END {for (i in a) print i}' f1 f2
Cat
Bat
This stores the values in file1 into the array a[]. Then, it loops through all lines of file2 checking each single element. If one of them matches a value in the array a[], then this element is removed from the array. Finally, in the END{} block prints the values that were not found.