- ASCII or UTF-8 is assumed for input files.
  - Input files are 1 or 2 files. You can also specify "-" that means STDIO.

Options :

  -i filename  
  -u filename
  -l filename
  -r filename 
       Those 4 above specifies the output file names that will be written
         `intersection', `union', `Left-Only', `Right-Only' data.
  -= 
     ` $0 -=  file1  file2 '  means that both of the 1-st lines of file1 and file2 are ignored. 
     This is useful when files contain header lines as data files.