linux - Display duplicate lines in two different files -

I have two files and I would like to display the duplicate line I tried to do it but it does not work:

cat id1.txt | While reading the ID; Grep "$ id" id2.txt; Done

I am thinking that there is no other way to display duplicate lines in the file. I have a list of IDs in both files.

files sorted? Can they be sorted?

If sorted:

  comm-12 id1.txt id2.txt   
  bash  4.x:  
  comm-12 & lt; (Sort id 1.txt) & lt; (Sort id 2.txt)   
 If you do not have  bash  4.x and can not be resolved using temporary files.  
 You can type grep -F :  
  grep -F -f id1.txt id2.txt   
 It appears in the words of  id1.txt  which appears in  id2.txt . Only one problem here is making sure that no ID  1  matches any ID containing  1  anywhere. Available in some versions the  -w  or  -x  options will work here.