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A file test, where X is one of the letters listed below. This unary operator takes one argument, either a filename or a filehandle, and tests the associated file to see if something is true about it. If the argument is omitted, tests $_, except for -t, which tests STDIN. Unless otherwise documented, it returns 1 for TRUE and '' for FALSE, or the undefined value if the file doesn't exist. Despite the funny names, precedence is the same as any other named unary operator, and the argument may be parenthesized like any other unary operator. The operator may be any of:

-r	File is readable by effective uid/gid.
-w	File is writable by effective uid/gid.
-x	File is executable by effective uid/gid.
-o	File is owned by effective uid.

-R	File is readable by real uid/gid.
-W	File is writable by real uid/gid.
-X	File is executable by real uid/gid.
-O	File is owned by real uid.

-e	File exists.
-z	File has zero size.
-s	File has nonzero size (returns size).

-f	File is a plain file.
-d	File is a directory.
-l	File is a symbolic link.
-p	File is a named pipe (FIFO), or Filehandle is a pipe.
-S	File is a socket.
-b	File is a block special file.
-c	File is a character special file.
-t	Filehandle is opened to a tty.

-u	File has setuid bit set.
-g	File has setgid bit set.
-k	File has sticky bit set.

-T	File is a text file.
-B	File is a binary file (opposite of -T).

-M	Age of file in days when script started.
-A	Same for access time.
-C	Same for inode change time.


    while (<>) {
	next unless -f $_;	# ignore specials

The interpretation of the file permission operators -r, -R, -w, -W, -x, and -X is by default based solely on the mode of the file and the uids and gids of the user. There may be other reasons you can't actually read, write, or execute the file. Such reasons may be for example network filesystem access controls, ACLs (access control lists), read-only filesystems, and unrecognized executable formats.

Also note that, for the superuser on the local filesystems, the -r, -R, -w, and -W tests always return 1, and -x and -X return 1 if any execute bit is set in the mode. Scripts run by the superuser may thus need to do a stat() to determine the actual mode of the file, or temporarily set their effective uid to something else.

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 -T and -B switches work as follows. The first block or so of the file is examined for odd characters such as strange control codes or characters with the high bit set. If too many strange characters (>30%) are found, it's a -B file, otherwise it's a -T file. Also, any file containing null in the first block is considered a binary file. If -T or -B is used on a filehandle, the current stdio buffer is examined rather than the first block. Both -T and -B return TRUE on a null file, or a file at EOF when testing a filehandle. Because you have to read a file to do the -T test, on most occasions you want to use a -f against the file first, as in next unless -f $file && -T $file.

If any of the file tests (or either the stat() or lstat() operators) are given the special filehandle consisting of a solitary underline, then the stat structure of the previous file test (or stat operator) is used, saving a system call. (This doesn't work with -t, and you need to remember that lstat() and -l will leave values in the stat structure for the symbolic link, not the real file.) Example:

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

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 _;