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

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.

Try the following:

cat /proc/cpuinfo

Here's a link to an Android Java example.

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