How long can $DISPLAY environment variable value be?

What is the maximum length that $DISPLAY environment variable value can take?

I understand that it is - hostname:displaynumber.screennumber

So will it be => max($DISPLAY) = max(hostname) + max (displaynumber) + max(screennumber)?

What is max length of hostname, displaynumber and screennumber on Linux?

Solution 1:

Environment variable length is controlled by MAX_ARG_STRLEN, which is a constant value defined as PAGE_SIZE*32 within the Linux kernel.

You can check your PAGE_SIZE value via Terminal like this:

getconf PAGE_SIZE

If you are running stock Ubuntu (or an official flavour), then the value will likely be 4096 (bytes).

So, with this in mind:

4096 * 32 = 131,072

As a result, the maximum length of an environment variable, be it $DISPLAY or any other, is 128KB. This works out to 131,072 single-byte ASCII characters.