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How to decide about a variable whether it contains a number or not

Fri, 2007.06.15 - 09:23 — müzso
Let's suppose we have a variable named myvar. How do you decide (in a safe way!) in a standard POSIX shell (or in the almost equivalent /bin/sh of the Debian distros) whether it contains a number or not? Sounds pretty much trivial, not? Here's what I came up with:
mynumber=_
mynumber=$(($myvar)) 2> /dev/null
{ [ "$mynumber" = "_" ] && echo "Myvar is not a number."; } || echo "Myvar is a number."

Or you could rely on the exit value of the $(( )) expression, like this:
mynumber=$(($myvar)) 2> /dev/null
{ [ $? -ne 0 ] && echo "Myvar is not a number."; } || echo "Myvar is a number."