Unix command to find lines common in two files

Question

I\'m sure I once found a unix command which could print the common lines from two or more files, does anyone know its name? It was much simpler than diff.

Answer 1

On limited version of Linux (like a QNAP (nas) I was working on):

• comm did not exist
• grep -f file1 file2 can cause some problems as said by @ChristopherSchultz and using grep -F -f file1 file2 was really slow (more than 5 minutes - not finished it - over 2-3 seconds with the method below on files over 20MB)

So here is what I did :

sort file1 > file1.sorted
sort file2 > file2.sorted

diff file1.sorted file2.sorted | grep "<" | sed 's/^< *//' > files.diff
diff file1.sorted files.diff | grep "<" | sed 's/^< *//' > files.same.sorted

If files.same.sorted shall have been in same order than the original ones, than add this line for same order than file1 :

awk 'FNR==NR {a[$0]=$0; next}; $0 in a {print a[$0]}' files.same.sorted file1 > files.same

or, for same order than file2 :

awk 'FNR==NR {a[$0]=$0; next}; $0 in a {print a[$0]}' files.same.sorted file2 > files.same