Get CPU usage in bash, without top or sysstat - bash

Question

My test script looks like this:
prev_total_cpu=0
prev_idle_cpu=0
while true
do
tempenv=$(grep "cpu " /proc/stat)
#get cpu times
read cpu user nice system idle <<< "$tempenv"
#calculate total cpu time
let total=$[$user + $nice + $system + $idle]
#calculate delta total and delta idle
prev_total="prev_total_$cpu"
prev_idle="prev_idle_$cpu"
let delta_total=$[$total-${!prev_total}]
let delta_idle=$[$idle-${!prev_idle}]
#calculate cpu usage
printf "$cpu: $[100*($delta_total-$delta_idle)/$delta_total]%% "
#remember total and idle times
let prev_total_$cpu="$total"
let prev_idle_$cpu="$idle"
echo ""
sleep 1
done
Now, the same thing with multiple CPU support:
prev_total_cpu=0
prev_idle_cpu=0
prev_total_cpu0=0
prev_idle_cpu0=0
prev_total_cpu1=0
prev_idle_cpu1=0
while true
do
#loop through cpus
grep "cpu" /proc/stat | while IFS='\n' read tempenv
do
#get cpu times
read cpu user nice system idle <<< "$tempenv"
#calculate total cpu time
let total=$[$user + $nice + $system + $idle]
#calculate delta total and delta idle
prev_total="prev_total_$cpu"
prev_idle="prev_idle_$cpu"
let delta_total=$[$total-${!prev_total}]
let delta_idle=$[$idle-${!prev_idle}]
#calculate cpu usage
printf "$cpu: $[100*($delta_total-$delta_idle)/$delta_total]%% "
#remember total and idle times
let prev_total_$cpu="$total"
let prev_idle_$cpu="$idle"
done
echo ""
sleep 1
done
Doesn't work - it shows the same numbers over and over again. What am I doing wrong?
Sub-question:
I would like to initialize prev_total_$cpuand prev_idle_$cpu in a loop, but I don't know how to set the condition. I need the opposite of sed -ne 's/^cpu\(.*\) .*/\1/p' /proc/stat so I can loop through its output and do the initialization.

Answer 1

When you do
grep "cpu" /proc/stat | while IFS='\n' read tempenv
do
...
done
Your while loop runs on a subshell environment.
One solution would be to use process substitution instead:
while IFS='\n' read tempenv
do
...
done < <(grep "cpu" /proc/stat)
Another way is to enable lastpipe option if Bash supports it:
shopt -s lastpipe
... loop goes next ...
Adding some suggestions to your code, when doing arithmetic operations in Bash you better use (( )) format e.g.:
(( total = user + nice + system + idle ))
It might also be better to use -n than printf:
echo -n "$cpu: $(( 100 * ($delta_total - $delta_idle) / $delta_total ))%% "
Lastly, I do hope you really mean that you run your script with Bash and not other shells like Ksh. They are very different. Bash is a shell but Shell != Bash.