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functionperlauthenticationtesting

Perl - getlogin, getpwuid, and $<

Wanted to understand the example line of code given @ perldoc.perl.org for getlogin

$login = getlogin || getpwuid($<) || "Kilroy";

It seems like it tries to get the user name from getlogin or getpwuid, but if either fails, use Kilroy instead. I might be wrong, so please correct me. Also, I've been using getlogin() in previous scripts - is there any difference between getlogin() and getlogin?

What is this code safeguarding against? Also, what purpose does $< serve? I'm not exactly sure what to search for when looking up what $< is and what it does.

EDIT
found this in the special variables section - still don't know why it is needed or what is does in the example above

$<

The real uid of this process. (Mnemonic: it's the uid you came from, if you're running setuid.) You can change both the real uid and the effective uid at the same time by using POSIX::setuid(). Since changes to $< require a system call, check $! after a change attempt to detect any possible errors.

EDIT x2

Is this line comparable to the above example? (it is currently what I use to avoid any potential problems with "cron" executing a script - i've never run into this problem, but i am trying to avoid any theoretical problem)

my $username = getlogin(); if(!($username)){$username = 'jsmith';}

Solution

  • You're exactly right. If getlogin returns false it will test getpwuid($<) if that returns false it will set $login to "Kilroy"

    $< is the real uid of the process. Even if you're running in a setuid environment it will return the original uid the process was started from.

    Edit to match your edit :)

    getpwuid returns the user's name by the UID (in scalar context, which would be the case here). You would want $< as an argumnent in case the program switched UID at some point ($< is the original one it was started with)