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Take a look at cat /proc/stat
grep 'cpu ' /proc/stat | awk '{usage=($2+$4)*100/($2+$4+$5)} END {print usage "%"}'
EDIT please read comments before copy-paste this or using this for any serious work. This was not tested nor used, it's an idea  for people who do not want to install a utility or for something that works in any distribution. Some people think you can "apt-get install" anything.
NOTE: this is not the current CPU usage, but the overall CPU usage in all the cores since the system bootup. This could be very different from the current CPU usage. To get the current value top (or similar tool) must be used.
Current CPU usage can be potentially calculated with: 
awk '{u=$2+$4; t=$2+$4+$5; if (NR==1){u1=u; t1=t;} else print ($2+$4-u1) * 100 / (t-t1) "%"; }' \
<(grep 'cpu ' /proc/stat) <(sleep 1;grep 'cpu ' /proc/stat)

