Programming Perl, 3rd Edition

Named Unary and File Test Operators

Some of the "functions" described in Chapter 29 are really unary operators. Table 3.2 lists all the named unary operators.

Table 3-2. Named Unary Operators

-X (file tests)	gethostbyname	localtime	return
alarm	getnetbyname	lock	rmdir
caller	getpgrp	log	scalar
chdir	getprotobyname	lstat	sin
chroot	glob	my	sleep
cos	gmtime	oct	sqrt
defined	goto	ord	srand
delete	hex	quotemeta	stat
do	int	rand	uc
eval	lc	readlink	ucfirst
exists	lcfirst	ref	umask
exit	length	require	undef

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

sleep 4 | 3;

does not sleep for 7 seconds; it sleeps for 4 seconds and then takes the return value of sleep (typically zero) and bitwise ORs that with 3, as if the expression were parenthesized as:

(sleep 4) | 3;

Compare this with:

print 4 | 3;

which does take the value of 4 ORed with 3 before printing it (7 in this case), as if it were written:

print (4 | 3);

This is because print is a list operator, not a simple unary operator. Once you've learned which operators are list operators, you'll have no trouble telling unary operators and list operators apart. When in doubt, you can always use parentheses to turn a named unary operator into a function. Remember, if it looks like a function, it is a function.

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 ...