Comparing two files with the diff utility is an easy task
diff <file1> <file2>
But if you want to compare the output of two commands without temporarily saving it to a file, it's a bit more complicated, because diff needs two inputs:
diff -u <(echo output1) <(echo output2)
--- /proc/self/fd/11 2013-07-29 10:59:06.037561642 +0200
+++ /proc/self/fd/12 2013-07-29 10:59:06.037561642 +0200
@@ -1 +1 @@
-output1
+output2
As we can see, the shell creates two file-descriptors from the stdin input.
