Is shell cmd 'top' a good way to monitor process' cpu usage?

Question

I have writen a simple shell script to monitor a process in my linux.

echo `date` ':' `top -d 3 -n 3 -b | grep swiftd | awk -F' ' '{print $9 }'` >> results.log

I run it every 5 minutes by crontab. Everything fine until i find something wrong with my results.log,

Wed Aug 26 18:06:01 CST 2015 : 4.0 3.3 3.3 
Wed Aug 26 18:07:01 CST 2015 : 4.0 3.3 3.7 
Wed Aug 26 18:08:01 CST 2015 : 5.9 3.7 3.3 
Wed Aug 26 18:09:01 CST 2015 : 97.1 3.7 3.0 
Wed Aug 26 18:10:01 CST 2015 : 4.0 3.3 3.7
Wed Aug 26 18:11:01 CST 2015 : 4.0 3.3 3.3
Wed Aug 26 18:12:01 CST 2015 : 2.0 3.7 3.0 

In the fourth line, the cpu usage became 97.1%, then 3.7% in the next iteration. It seems that the usage got an peak in a short time.

My confusion is that,

1. Is that any other reason a process suddenly got a 100% cpu usage?
2. Is 'top' a good command to monitor process' cpu usage?
3. How to get a better average cpu usage for a process

Answer 1

top is not good command in shell script. I recommend ps command to monitor cpu-usage for specific process. This is example.

echo `date` ':' `ps auxww | grep swiftd | awk '{print $3}'` >> results.log