recording total runtime in bash script

Question

I have the following script. When it runs it prints the Start and end Times to the file 'result.txt'.

However, I also want to record the total runtime (end-start), but where I am doing echo runtine at the end, it just returns runtime: with nothing recorded next to it. Am I doing something incorrect?

#!/bin/bash
clear
echo "Test 001" > result.txt
echo "start time: " $(date +%T) >> result.txt
start=`date +%s`
#DO STUFF HERE
end=`date +%s`
echo "end time: " $(date +%T) >> result.txt
runtime=$((end-start))
echo "runtime: " $(runtime) >> result.txt
echo " - - - "

Answer 1

You have a typo in your echo statement. It should be

#!/bin/bash
#clear
echo "Test 001" > result.txt
echo "start time: " $(date +%T) >> result.txt
start=`date +%s`
#DO STUFF HERE
end=`date +%s`
echo "end time: " $(date +%T) >> result.txt
runtime=$((end-start))
echo "runtime: $runtime" >> result.txt
echo " - - - "

Answer 2

First of all, you are reinventing the wheel. That's what the time command is for:

$ time script.sh
real    0m0.005s
user    0m0.000s
sys     0m0.004s

Then, you have a syntax error:

echo "runtime: " $(runtime) >> result.txt

The $(foo) syntax is command substitution, it will try to run foo. What you meant was

 echo "runtime: $runtime" >> result.txt

By the way, you should always include the error messages you get in your question.