Perl 5 version 30.0 documentation
- -X EXPR
- -X DIRHANDLE
A file test, where X is one of the letters listed below. This unary operator takes one argument, either a filename, a filehandle, or a dirhandle, 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
1for true and
''for false. If the file doesn't exist or can't be examined, it returns undef and sets $! (errno). With the exception of the
-ltest they all follow symbolic links because they use
lstat()(so dangling symlinks can't be examined and will therefore report failure).
Despite the funny names, precedence is the same as any other named unary operator. The operator may be any of:
- -r File is readable by effective uid/gid.
- -w File is writable by effective uid/gid.
- -x File is executable by effective uid/gid.
- -o File is owned by effective uid.
- -R File is readable by real uid/gid.
- -W File is writable by real uid/gid.
- -X File is executable by real uid/gid.
- -O File is owned by real uid.
- -e File exists.
- -z File has zero size (is empty).
- -s File has nonzero size (returns size in bytes).
- -f File is a plain file.
- -d File is a directory.
- -l File is a symbolic link (false if symlinks aren't
- supported by the file system).
- -p File is a named pipe (FIFO), or Filehandle is a pipe.
- -S File is a socket.
- -b File is a block special file.
- -c File is a character special file.
- -t Filehandle is opened to a tty.
- -u File has setuid bit set.
- -g File has setgid bit set.
- -k File has sticky bit set.
- -T File is an ASCII or UTF-8 text file (heuristic guess).
- -B File is a "binary" file (opposite of -T).
- -M Script start time minus file modification time, in days.
- -A Same for access time.
- -C Same for inode change time (Unix, may differ for other
-s/a/b/does not do a negated substitution. Saying
-exp($foo)still works as expected, however: only single letters following a minus are interpreted as file tests.
These operators are exempt from the "looks like a function rule" described above. That is, an opening parenthesis after the operator does not affect how much of the following code constitutes the argument. Put the opening parentheses before the operator to separate it from code that follows (this applies only to operators with higher precedence than unary operators, of course):
- -s($file) + 1024 # probably wrong; same as -s($file + 1024)
- (-s $file) + 1024 # correct
The interpretation of the file permission operators
-Xis 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: for example network filesystem access controls, ACLs (access control lists), read-only filesystems, and unrecognized executable formats. Note that the use of these six specific operators to verify if some operation is possible is usually a mistake, because it may be open to race conditions.
Also note that, for the superuser on the local filesystems, the
-Wtests always return 1, and
-Xreturn 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.
If you are using ACLs, there is a pragma called filetest that may produce more accurate results than the bare stat mode bits. When under
use filetest 'access', the above-mentioned filetests test whether the permission can(not) be granted using the access(2) family of system calls. Also note that the
-Xtests may under this pragma return true even if there are no execute permission bits set (nor any extra execute permission ACLs). This strangeness is due to the underlying system calls' definitions. Note also that, due to the implementation of
use filetest 'access', the
_special filehandle won't cache the results of the file tests when this pragma is in effect. Read the documentation for the filetest pragma for more information.
-Btests work as follows. The first block or so of the file is examined to see if it is valid UTF-8 that includes non-ASCII characters. If so, it's a
-Tfile. Otherwise, that same portion of the file is examined for odd characters such as strange control codes or characters with the high bit set. If more than a third of the characters are strange, it's a
-Bfile; otherwise it's a
-Tfile. Also, any file containing a zero byte in the examined portion is considered a binary file. (If executed within the scope of a use locale which includes
LC_CTYPE, odd characters are anything that isn't a printable nor space in the current locale.) If
-Bis used on a filehandle, the current IO buffer is examined rather than the first block. Both
-Breturn true on an empty file, or a file at EOF when testing a filehandle. Because you have to read a file to do the
-Ttest, on most occasions you want to use a
-fagainst the file first, as in
next unless -f $file && -T $file.
If any of the file tests (or either the stat or lstat operator) is 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
-lleave values in the stat structure for the symbolic link, not the real file.) (Also, if the stat buffer was filled by an lstat call,
-Bwill reset it with the results of
stat _). Example:
As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file test operators, in a way that
-f -w -x $fileis equivalent to
-x $file && -w _ && -f _. (This is only fancy syntax: if you use the return value of
-f $fileas an argument to another filetest operator, no special magic will happen.)
Portability issues: -X in perlport.
To avoid confusing would-be users of your code with mysterious syntax errors, put something like this at the top of your script:
- use 5.010; # so filetest ops can stack