Shell Programming and Scripting

I'm trying to write a bash script to perform basic arithmetic operations but I want to run a comparison on the arguments first to check that they're a number greater than zero.

I want an error to pop up if the arguments args aren't >= 0 so I have:

Code:

if ! [[ $1 -ge 0 ]]; then
echo "bad number: $1"
fi

But whenever I type in something like "blah + 9" it should tell me "bad number: blah", instead it treats "blah" as a zero and it gives the output as 9. If I use -gt 0 it eliminates the "blah" but also doesn't let me use zeros.

Code:

#! /bin/bash

if [[ `echo $1 | egrep "^[0-9]*$"` ]]
then
    echo "good number: $1"
else
    echo "bad number: $1"
fi

Code:

if [[ $1 == ?(+)+([0-9]) ]]
then
echo positive number
else
echo not a positive number
fi

Neither of those cures my problem. It still reads non numbers as zero.

Code:

case "$string" in
[0-9]*) ;;
-[0-9]*) ;;
*) echo "Not integer" ;;
esac

Whoa, I have no idea what that means. Can you please explain the logic behind this? I'll try it now though.

could you please post your input and output with the script.