http://stackoverflow.com/a/2912711

1.sh

$ echo 'afoobarfoobar' > a.txt
$ echo 'xyzzyfoobarfoobarfoo' > b.txt

# Print all occurrences in all files
$ grep -o foo a.txt b.txt
a.txt:foo
a.txt:foo
b.txt:foo
b.txt:foo
b.txt:foo

# Print number of occurrences per file
$ grep -o foo a.txt b.txt | sort | uniq -c
      2 a.txt:foo
      3 b.txt:foo

# Grep version
$ grep -V
grep (GNU grep) 2.25