Bash scripting: checking if a variable is a number

Sometime ago I had a simple problem with bash scripting. I liked to check if a variable is a number or not. First I'd looked up into Advanced Bash Scripting Guide but didn't found anything. So my first approach was:

[ -z "`echo $VARIABLE | tr -d "[:digit:]"`" ]

But I didn't like it because of tr usage. So after some discussion with other people I came to the following solution:

[ $VARIABLE -eq $VARIABLE ]

It works great and could be used in a bash script like this:

if [ $VARIABLE -eq $VARIABLE 2> /dev/null ]; then
echo $VARIABLE is a number
else
echo $VARIABLE isn't a number
fi