NRPE script for monitoring load average
This looks pretty good. Some suggestions:


[[ is preferred over [ in Bash.
This script could benefit from set -o errexit -o nounset -o pipefail. You'll need to set cpu_cores=0 before referring to it in the loop, but that's just best practice.
Dividing by one isn't going to do much :)
The quotes are a cute feature, but I'd much rather see a summary of the point in them than the full sentences. Something like


  Treat 15 minute load average above 1 as critical.


A useful tactic I've found when dealing with comments is to think how you could name things to avoid comments altogether. Code can't lie, and names are very much like comments enshrined in code, so if you for example rename minute15 to quarter_load_average or something else human readable you should be able to remove the


  
  Find out a load was before 15 minutes.
  


comment above it.
Since you're only interested in one row from /proc/cpuinfo I would grep for it. Your goal of making the script stand-alone should be weighed against at least


maintainability, which will suffer greatly if you only use builtin commands,
speed, which is already not great for read, and
availability of grep, which is pretty much universal by now.

You can redirect a line to standard input using some_command &lt;&lt;&lt; "$line" to avoid redundant echos. So echo "$quarter_load_average*100/1" | bc could instead be bc &lt;&lt;&lt; "$quarter_load_average*100/1".