3.10. Named Unary and File Test Operators

Some of the "functions" described in Chapter 29, "Functions" are really unary operators. Table 3-2 lists all the named unary operators.

Table 3.2. Named Unary Operators

Unary operators have a higher precedence than some of the binary operators. For example:

sleep 4 | 3;

(sleep 4) | 3;

print 4 | 3;

print (4 | 3);

Another funny thing about named unary operators is that many of them default to $_ if you don't supply an argument. However, if you omit the argument but the token following the named unary operator looks like it might be the start of an argument, Perl will get confused because it's expecting a term. Whenever the Perl tokener gets to one of the characters listed in Table 3-3, the tokener returns different token types depending on whether it expects a term or operator.

Table 3.3. Ambiguous Characters

So a typical boo-boo is:

next if length < 80;

next if length() < 80;
next if (length) < 80;
next if 80 > length;
next unless length >= 80;

Table 3.4. File Test Operators

Note that -s/a/b/ does not do a negated substitution. Saying -exp($foo) still works as expected, however--only single letters following a minus are interpreted as file tests.

The interpretation of the file permission operators -r, -R, -w, -W, -x, and -X is based solely on the mode of the file and the user and group IDs of the user. There may be other reasons you can't actually read, write, or execute the file, such as Andrew File System (AFS) access control lists.[3] Also note that for the superuser, -r, -R, -w, and -W always return 1, and -x and -X return 1 if any execute bit is set in the mode. Thus, scripts run by the superuser may need to do a stat in order to determine the actual mode of the file or temporarily set the UID to something else.

[3]You may, however, override the built-in semantics with the use filetest pragma. See Chapter 31, "Pragmatic Modules".

The other file test operators don't care who you are. Anybody can use the test for "regular" files:

while (<>) {
    chomp;
    next unless -f $_;      # ignore "special" files
    ...
}

The -T and -B switches work as follows. The first block or so of the file is examined for strange characters such as control codes or bytes with the high bit set (that don't look like UTF-8). If more than a third of the bytes appear to be strange, it's a binary file; otherwise, it's a text file. Also, any file containing ASCII NUL (\0) in the first block is considered a binary file. If -T or -B is used on a filehandle, the current input (standard I/O or "stdio") buffer is examined rather than the first block of the file. Both -T and -B return true on an empty file, or on a file at EOF (end-of-file) when testing a filehandle. Because Perl has to read a file to do the -T test, you don't want to use -T on special files that might hang or give you other kinds of grief. So on most occasions you'll want to test with a -f first, as in:

next unless -f $file && -T $file;

Here are a couple of examples:

print "Can do.\n" if -r $a || -w _ || -x _;

stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;

print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;

File ages for -M, -A, and -C are returned in days (including fractional days) since the script started running. This time is stored in the special variable $^T ($BASETIME). Thus, if the file changed after the script started, you would get a negative time. Note that most time values (86,399 out of 86,400, on average) are fractional, so testing for equality with an integer without using the int function is usually futile. Examples:

next unless -M $file > .5;      # files are older than 12 hours
&newfile if -M $file < 0;       # file is newer than process
&mailwarning if int(-A) == 90;  # file ($_) was accessed 90 days ago today

$^T = time;

3.9. Shift Operators		3.11. Relational Operators

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