Shell Programming and Scripting

$ 
$ cat file1
goat
door
bear
cat
$ cat file2
goat
moose
dog
cat
$ cat file3
goat
yak
tiger
cat
$ cat file4
goat
yak
lion
cat
$ awk '{x[$0]=x[$0]FILENAME} END{for (i in x){if (x[i]=="file1file2file3file4"){print i}}}' file1 file2 file3 file4
cat
goat
$

$ 
$ sort file1 >sorted_file1; sort file2 >sorted_file2
$ sort file3 >sorted_file3; sort file4 >sorted_file4
$ join sorted_file1 sorted_file2 >common_12
$ join sorted_file3 sorted_file4 >common_34
$ join common_12 common_34
cat
goat
$

$ 
$ sort file1 file2 file3 file4 | uniq -c | awk '$1==4 {print $2}'
cat
goat
$ 
$

cat file? | awk ' { word[$1]++ }
END {
       for (i in word) {
           print i,word[i]
           }
      }
' | sort -k 2,2nr

|  head -2

 > result.txt

awk > outfile 'END { 
  for (W in w) if (w[W] == ARGC - 1) print W 
	}
{  f[$0,FILENAME]++ || w[$0]++ }
' infile1 [infile2 ... infilen]

cat file1 | \
        cat - file2 | sort | uniq -d | \
        cat - file3 | sort | uniq -d | \
        cat - file4 | sort | uniq -d

cat
goat

sort file1 file2 file3 file4 | uniq -c | awk '$1==4 {print $2}'