filetest(3p) Perl Programmers Reference Guide filetest(3p) #

filetest(3p) Perl Programmers Reference Guide filetest(3p)

NNAAMMEE #

 filetest - Perl pragma to control the filetest permission operators

SSYYNNOOPPSSIISS #

     $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

DDEESSCCRRIIPPTTIIOONN #

 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 ssttaatt(()) 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 aacccceessss(()) when available, that is, on most UNIX
 systems and other POSIX environments.  See details below.

CCoonnssiiddeerr tthhiiss ccaarreeffuullllyy The ssttaatt(()) 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 aacccceessss(()). But you may encounter surprises if your program runs on a system that uses ACLs, since the ssttaatt(()) 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.

TThhee “"aacccceessss"” ssuubb--pprraaggmmaa UNIX and POSIX systems provide an abstract aacccceessss(()) 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.

LLiimmiittaattiioonn wwiitthh rreeggaarrdd ttoo “”“” Because aacccceessss(()) does not invoke ssttaatt(()) (at least not in a way visible to Perl), tthhee ssttaatt rreessuulltt ccaacchhee “”“” iiss nnoott sseett. This means that the outcome of the following two tests is different. The first has the stat bits of _/_e_t_c_/p_a_s_s_w_d 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 aacccceessss(()), 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 v5.36.3 2014-03-24 filetest(3p)