What I've learned, through various approaches, is that you want to depend on the result of open() (or sysopen()). A file might exist, for instance, because -e "file" returns true, but you still might not be able to open it
But you then go on to use -r in your code to check if a file is readable with the current effective UID/GID. I was going to point you at exactly that operator to solve your "file exists, can I read it" problem. The various -X operators cover all of the various situations the OP mentioned.

In reply to Re: Re: file testing is hard by DrHyde
in thread file testing is hard by jamgill

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