How do I test if a variable is a number in Bash?

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One approach is to use a regular expression, like so:

re="^[0-9]+$"
if ! [[ $yournumber =~ $re ]] ; then
   echo "error: Not a number" >&2; exit 1
fi

If the value is not necessarily an integer, consider amending the regex appropriately; for instance:

^[0-9]+([.][0-9]+)?$

…or, to handle numbers with a sign:

^[+-]?[0-9]+([.][0-9]+)?$

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