source: vendor/bash/3.1-p17/examples/functions/isnum2

isnum2()
{
        case "$1" in
        '[-+]' | '')    return 1;;      # empty or bare `-' or `+'
        [-+]*[!0-9]*)   return 1;;      # non-digit with leading sign
        [-+]*)          return 0;;      # OK
        *[!0-9]*)       return 1;;      # non-digit
        *)              return 0;;      # OK
        esac
}

# this one handles floating point
isnum3()
{
        case "$1" in
        '')             return 1;;      # empty
        *[!0-9.+-]*)    return 1;;      # non-digit, +, -, or .
        *?[-+]*)        return 1;;      # sign as second or later char
        *.*.*)          return 1;;      # multiple decimal points
        *)              return 0;;      # OK
        esac
}