Find common strings in files via awk

find_common.sh

#!/bin/bash
awk 'END {
  # the END block is executed after
  # all the input has been read
  # loop over the rec array
  # and build the dup array indxed by the nuber of
  # filenames containing a given record
  for (R in rec) {
    n = split(rec[R], t, "/")
    if (n > 1)
      dup[n] = dup[n] ? dup[n] RS sprintf("\t%-20s -->\t%s", rec[R], R) : \
        sprintf("\t%-20s -->\t%s", rec[R], R)
    }
  # loop over the dup array
  # and report the number and the names of the files
  # containing the record
  for (D in dup) {
    printf "records found in %d files:\n\n", D
    printf "%s\n\n", dup[D]
    }
  }
{
  # build an array named rec (short for record), indexed by
  # the content of the current record ($0), concatenating
  # the filenames separated by / as values
  rec[$0] = rec[$0] ? rec[$0] "/" FILENAME : FILENAME
  }' $*