1filetest(3pm)          Perl Programmers Reference Guide          filetest(3pm)
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NAME

6       filetest - Perl pragma to control the filetest permission operators
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SYNOPSIS

9           $can_perhaps_read = -r "file";      # use the mode bits
10           {
11               use filetest 'access';          # intuit harder
12               $can_really_read = -r "file";
13           }
14           $can_perhaps_read = -r "file";      # use the mode bits again
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DESCRIPTION

17       This pragma tells the compiler to change the behaviour of the filetest
18       permission operators, "-r" "-w" "-x" "-R" "-W" "-X" (see perlfunc).
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20       The default behaviour of file test operators is to use the simple mode
21       bits as returned by the stat() family of system calls.  However, many
22       operating systems have additional features to define more complex
23       access rights, for example ACLs (Access Control Lists).  For such
24       environments, "use filetest" may help the permission operators to
25       return results more consistent with other tools.
26
27       The "use filetest" or "no filetest" statements affect file tests
28       defined in their block, up to the end of the closest enclosing block
29       (they are lexically block-scoped).
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31       Currently, only the "access" sub-pragma is implemented.  It enables (or
32       disables) the use of access() when available, that is, on most UNIX
33       systems and other POSIX environments.  See details below.
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35   Consider this carefully
36       The stat() mode bits are probably right for most of the files and
37       directories found on your system, because few people want to use the
38       additional features offered by access(). But you may encounter
39       surprises if your program runs on a system that uses ACLs, since the
40       stat() information won't reflect the actual permissions.
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42       There may be a slight performance decrease in the filetest operations
43       when the filetest pragma is in effect, because checking bits is very
44       cheap.
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46       Also, note that using the file tests for security purposes is a lost
47       cause from the start: there is a window open for race conditions (who
48       is to say that the permissions will not change between the test and the
49       real operation?).  Therefore if you are serious about security, just
50       try the real operation and test for its success - think in terms of
51       atomic operations.  Filetests are more useful for filesystem
52       administrative tasks, when you have no need for the content of the
53       elements on disk.
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55   The "access" sub-pragma
56       UNIX and POSIX systems provide an abstract access() operating system
57       call, which should be used to query the read, write, and execute
58       rights. This function hides various distinct approaches in additional
59       operating system specific security features, like Access Control Lists
60       (ACLs)
61
62       The extended filetest functionality is used by Perl only when the
63       argument of the operators is a filename, not when it is a filehandle.
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65   Limitation with regard to "_"
66       Because access() does not invoke stat() (at least not in a way visible
67       to Perl), the stat result cache "_" is not set.  This means that the
68       outcome of the following two tests is different.  The first has the
69       stat bits of /etc/passwd in "_", and in the second case this still
70       contains the bits of "/etc".
71
72        { -d '/etc';
73          -w '/etc/passwd';
74          print -f _ ? 'Yes' : 'No';   # Yes
75        }
76
77        { use filetest 'access';
78          -d '/etc';
79          -w '/etc/passwd';
80          print -f _ ? 'Yes' : 'No';   # No
81        }
82
83       Of course, unless your OS does not implement access(), in which case
84       the pragma is simply ignored.  Best not to use "_" at all in a file
85       where the filetest pragma is active!
86
87       As a side effect, as "_" doesn't work, stacked filetest operators ("-f
88       -w $file") won't work either.
89
90       This limitation might be removed in a future version of perl.
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94perl v5.36.3                      2023-11-30                     filetest(3pm)
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