Copying partitions in terminal mode

[elmo]

Is it practical to use terminal mode to copy an entire ubuntu partition 
to an external hard drive for backup purposes?

I haven't done much with terminal commands except for the simplest ones 
so I'm not comfortable with copying an
entire partition.   I have copied a file or two within a partition but 
copying an entire partition seems like a big job.

After digging around in my notes, a book or two, and some websites, I 
came up with this command:
      cp -aprv  /dev/sda7   /dev/sdc5   

sda7 is where ubuntu is currently installed  and  sdc5 is the target, an 
empty  ext3 formatted
partition on the external hard drive.

The command seems to be too simple to do a complete partition copy.

Which way is the better way to run this kind of command,. from an 
installed ubuntu or from
a ubuntu CD?

elmo