Re: Verifying numeric values

[Greg Wooledge]

On Mon, Oct 18, 2021 at 11:10:32PM -0400, Andy Chu wrote:
> Try this:
> 
> is_valid() {
>   local x=$1
>   local b=$(( 1 <= x && x <= 255 ))
>   return $(( ! b ))   # 0 is true, 1 is false
> }

Using un-validated input in a math context is a code injection in bash.
Given that the purpose of this function is to validate the input, one
must assume the input may be malicious.

unicorn:~$ is_valid 'a[0$(date >&2)]'
Mon Oct 18 23:20:48 EDT 2021

You've also got an issue with variables that point to each other (or to
themselves):

unicorn:~$ is_valid x && echo yes
bash: x: expression recursion level exceeded (error token is "x")

Again, this is a side effect of using potentially unsafe input in a
bash math context.  The value of a variable may be treated as another
variable.

An input validation function has to operate on the input as a string,
until it's been verified to be safely numeric.