To get cpu usage, best way is to read /proc/stat file. See man 5 proc for more help.

There is a useful script written by Paul Colby i found here

#!/bin/bash

# by Paul Colby (http://colby.id.au), no rights reserved ;)



PREV_TOTAL=0

PREV_IDLE=0



while true; do



  CPU=(`cat /proc/stat | grep '^cpu '`) # Get the total CPU statistics.

  unset CPU[0]                          # Discard the "cpu" prefix.

  IDLE=${CPU[4]}                        # Get the idle CPU time.



  # Calculate the total CPU time.

  TOTAL=0



  for VALUE in "${CPU[@]:0:4}"; do

    let "TOTAL=$TOTAL+$VALUE"

  done



  # Calculate the CPU usage since we last checked.

  let "DIFF_IDLE=$IDLE-$PREV_IDLE"

  let "DIFF_TOTAL=$TOTAL-$PREV_TOTAL"

  let "DIFF_USAGE=(1000*($DIFF_TOTAL-$DIFF_IDLE)/$DIFF_TOTAL+5)/10"

  echo -en "

CPU: $DIFF_USAGE%  "



  # Remember the total and idle CPU times for the next check.

  PREV_TOTAL="$TOTAL"

  PREV_IDLE="$IDLE"



  # Wait before checking again.

  sleep 1

done

save it to cpu_usage, add execute permission chmod +x cpu_usage and run:

./cpu_usage

to stop the script, hit Ctrl+c