$ENV{LOGNAME} is setted at login time, so if someone log in as popeye and the do
su
his $ENV{LOGNAME} is again popeye. But if popoye do
su -
his $ENV{LOGNAME} is again popeye.

In reply to about LOGNAME by Luca Benini
in thread How do you grab username/login id in Unix? by jeremyw

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