filetest - phpMan

NAME
    filetest - Perl pragma to control the filetest permission operators

SYNOPSIS
        $can_perhaps_read = -r "file";      # use the mode bits
        {
            use filetest 'access';          # intuit harder
            $can_really_read = -r "file";
        }
        $can_perhaps_read = -r "file";      # use the mode bits again

DESCRIPTION
    This pragma tells the compiler to change the behaviour of the filetest
    permission operators, "-r" "-w" "-x" "-R" "-W" "-X" (see perlfunc).

    The default behaviour of file test operators is to use the simple mode
    bits as returned by the stat() family of system calls. However, many
    operating systems have additional features to define more complex access
    rights, for example ACLs (Access Control Lists). For such environments,
    "use filetest" may help the permission operators to return results more
    consistent with other tools.

    The "use filetest" or "no filetest" statements affect file tests defined
    in their block, up to the end of the closest enclosing block (they are
    lexically block-scoped).

    Currently, only the "access" sub-pragma is implemented. It enables (or
    disables) the use of access() when available, that is, on most UNIX
    systems and other POSIX environments. See details below.

  Consider this carefully
    The stat() mode bits are probably right for most of the files and
    directories found on your system, because few people want to use the
    additional features offered by access(). But you may encounter surprises
    if your program runs on a system that uses ACLs, since the stat()
    information won't reflect the actual permissions.

    There may be a slight performance decrease in the filetest operations
    when the filetest pragma is in effect, because checking bits is very
    cheap.

    Also, note that using the file tests for security purposes is a lost
    cause from the start: there is a window open for race conditions (who is
    to say that the permissions will not change between the test and the
    real operation?). Therefore if you are serious about security, just try
    the real operation and test for its success - think in terms of atomic
    operations. Filetests are more useful for filesystem administrative
    tasks, when you have no need for the content of the elements on disk.

  The "access" sub-pragma
    UNIX and POSIX systems provide an abstract access() operating system
    call, which should be used to query the read, write, and execute rights.
    This function hides various distinct approaches in additional operating
    system specific security features, like Access Control Lists (ACLs)

    The extended filetest functionality is used by Perl only when the
    argument of the operators is a filename, not when it is a filehandle.

  Limitation with regard to "_"
    Because access() does not invoke stat() (at least not in a way visible
    to Perl), the stat result cache "_" is not set. This means that the
    outcome of the following two tests is different. The first has the stat
    bits of /etc/passwd in "_", and in the second case this still contains
    the bits of "/etc".

     { -d '/etc';
       -w '/etc/passwd';
       print -f _ ? 'Yes' : 'No';   # Yes
     }

     { use filetest 'access';
       -d '/etc';
       -w '/etc/passwd';
       print -f _ ? 'Yes' : 'No';   # No
     }

    Of course, unless your OS does not implement access(), in which case the
    pragma is simply ignored. Best not to use "_" at all in a file where the
    filetest pragma is active!

    As a side effect, as "_" doesn't work, stacked filetest operators ("-f
    -w $file") won't work either.

    This limitation might be removed in a future version of perl.