To show the changes between system A and system B do:
$ diff -u systemA.txt systemtB.txt
This will result in output like:
--- a 2014-06-01 21:44:48.501128132 +0100
+++ b 2014-06-01 21:47:26.450197702 +0100
@@ -1,3 +1,4 @@
+aalib install
accountsservice install
acl install
acpi install
@@ -5,7 +6,6 @@
acpi-support install
acpi-support-base install
acpid install
-adduser install
Lines prefixed by +
are packages that were present on system B but not system A.
Lines prefixed by -
are packages that were present on system A but not system B.
To find all of the packages that were installed on one system but not the other you can extract the install lines that begin with +
:
$ diff -u a b|grep install|grep '^+'
+aalib install