If

Exit status:
0 is true, everything else is false
exit n  command in shell

     if  cmp  file1.txt  file2.txt;  then
         echo  "files  same"
     elif  diff  -i  file1.txt  file2.txt;  then
         echo  "files  same  except  case"
     else
         echo  "files  different"
     fi