How to reliably get current process CPU usage from command line

wojcieche · 05-01-2012, 06:14 AM

OK. So I have been googling a lot, also this forums, but I still do not have answer for a presumably easy question.

How to reliably get current CPU usage by a process.

I do not mean ps command below - because it returns an average CPU usage in whole process lifetime. I need a snapshot of current usage.

Code:

ps -C processname -o %cpu=

The only solution I came up with is this:

Code:

top -n 1 | grep processname | awk '{print $10}'

But it is not reliable, because top command returns a number of rows limited to the screen hight and sorts by cpu usage decreasingly - so if a process uses 0% of CPU, it is on the end of the top listing and does not show on the list. Therefore in such situation this command returns empty string.

The code above would work if I could sort top's command listing by pid or start time. Is it possible? If it is not possible, how to reliably get current CPU usage of a process?

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P.S. Showing the bigger picture, the point is to make this bash script work:

Code:

#!/bin/bash
cpuLoad=$(top -n 1 | grep processname | awk '{print $10}')

if [ $(echo $cpuLoad) -lt 40 ]
	then echo CPU USAGE IS LOW
fi

This script does not work well, because if cpuLoad variable returns empty string then an error shows up:
./test: line 3: [: -lt: unary operator expected
I can detect the situation where empty string instead of CPU load is returned with such code:

Code:

if [ ! -n "$cpuLoad" ]

but I do not have a clue how to write a code with combined conditions that will work like this:

IF cpuLoad is empty OR cpuLoad < 40 THEN...

The problem with such statement is that upon attempt to evaluate second condition an error will show up because the cpuLoad variable is empty, while value is expected.

wojcieche · 05-01-2012, 06:19 AM

OK, I found a solution for the code in the second part. It should go like this:

Code:

		cpuLoad=$(top -n 1 | grep processname | awk '{print $10}')

		if [ ! -n "$cpuLoad" ]
			then cpuLoad=0
		fi 

		if [ $(echo $cpuLoad) -lt 40 ]
			then CPU USAGE IS LOW
		fi

If the empty string is returned, it means that CPU usage of the process is near or equal to zero, so I just set it to zero, so the second evaluation of the condition can work smoothly.

I found the solution before posting the OP, but decided to publish it as it might be useful to someone.

But the questions from first part of my OP remain: How to reliably get current CPU usage of a process?
Can somebody help?