Awk

I wanted to find out how much resident memory xfce4 was using on my system, and it appears most xfce applications start with "xf":

$ ps auwx | awk '/xf/{print $5}'
15924
14668
11948
11764
12944
16264
1860

I wanted to use awk to add the results together instead of doing it manually:

$ ps auwx | awk '/xf/{ tot += $5 } END { print tot }'
69108

N.B. This can be misleading in the case of programs that use large amounts of shared memory (like java).