filetest(3pm) Perl Programmers Reference Guide filetest(3pm)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.
perl v5.12.5 2012-09-11 filetest(3pm)
