Programming Perl, 4th Edition

Named Unary and File Test Operators

Some of the “functions” described in Chapter 27 are really unary operators. Table 3-2 lists all the named unary operators.

Table 3-2. Named unary operators

–X (file tests)	fileno	lock	setnetent
abs	getc	log	setprotoent
alarm	getgrgid	lstat	setservent
caller	getgrnam	my	shift
chdir	gethostbyname	oct	sin
chomp	getnetbyname	ord	sleep
chop	getpeername	our	sqrt
chr	getpgrp	pop	srand
chroot	getprotobyname	pos	stat
close	getpwnam	prototype	state
closedir	getpwuid	quotemeta	study
cos	getsockname	rand	tell
dbmclose	glob	readdir	telldir
defined	gmtime	readline	tied
delete	hex	readlink	uc
do	int	readpipe	ucfirst
each	keys	ref	umask
eof	lc	reset	undef
eval	lcfirst	rewinddir	untie
exists	length	rmdir	values
exit	local	scalar	write
exp	localtime	sethostent	any ($) sub
fc

Unlike list operators, 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 ...