On Linux, how to tell how many cores of the machine are active?

You can use top to list the utilization of each core. Press 1 if necessary to split the CPU row into a separate row for each core.

You can also add a column that shows the last-used core for each process. Press f to bring up the field list, then j to activate the "P" column. Then press space to return to the live view.

ps has a field called psr to tell you which processor a job is running on.

So you could use something like:

ps -e -o psr= | sort | uniq | wc -l

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Note that merely running ps like this will of course make at least one core active.

Probably better is to run this:

tmp=/tmp/ps.$$
ps -e -o psr= > /tmp/ps.$$
sort -u "$tmp" | wc -l
rm "$tmp"

that way the sort and wc do not increase the count.

htop

This command works good in both ubuntu and centos and shows graphically how many CPUs and how are they being used.

for centos:

yum install htop

for ubuntu:

apt-get install htop