unless结构可以看作ifnot()
until可以看作while_not()适当使用这两种结构可以简化条件判断表达式
在perl中执行系统命令$result = `/bin/ps`;
一个用perl写的脚本,用于杀死系统中后台运行的perl程序
1 #!/usr/bin/perl -w 2 3 my $PS = `/bin/ps`; 4 5 if ($PS =~ m/^([0-9]+).*perl/m) 6 { 7 print $1."/n"; 8 my $ret=`kill -n 9 $1`; #Send terminate signal 9 } 10 print $PS;只能杀掉第一个perl运行的程序
表达式修饰符print "$n is a negative number./n" if $n < 0; #注意中间除了空格没有别的分隔符与 if ($n < 0) { print "$n is a negative number./n"; }等效虽然条件表达式在书写上是在语句末尾,仍然整行中最先执行的!!
循环语句也可以写作这种形式 $i *= 2 until $i > $j; print " ", ($n += 2) while $n < 10; #注意print和后一条语句之间是逗号!说明他们在同一个循环块中后面一条语句等价于: while ($n < 10) { print " ", ($n += 2); }
foreach作为表达式修饰符使用时只能使用$_作为循环变量如果需要自定义循环变量,只能用传统的写法
语句块中的局部变量仅在语句块中有效 { print "Please enter a number: "; chomp(my $n = <STDIN>); my $root = sqrt $n; # calculate the square root print "The square root of $n is $root./n"; }这里$n和$root在语句块外不再有效
注意在perl中elsif的拼写 if ( ! defined $dino) { print "The value is undef./n"; } elsif ($dino =~ /^-?/d+/.?$/) { print "The value is an integer./n"; } elsif ($dino =~ /^-?/d*/./d+$/) { print "The value is a _simple_ floating-point number./n"; } elsif ($dino eq '') { print "The value is the empty string./n"; } else { print "The value is the string '$dino'./n"; }
自增自减操作符 my $m = 5; my $n = ++$m; # increment $m to 6, and put that value into $n my $c = --$m; # decrement $m to 5, and put that value into $c
my $d = $m++; # $d gets the old value (5), then increment $m to 6 my $e = $m--; # $e gets the old value (6), then decrement $m to 5
last操作符直接跳出循环体,类似C语言中的break # Print all input lines mentioning fred, until the _ _END_ _ marker while (<STDIN>) { if (/_ _END_ _/) { # No more input on or after this marker line last; } elsif (/fred/) { print; } } ## last comes here ##
next操作符跳过本轮以下所有语句,直接开始下一轮循环是否进行的判断,类似C语言中的continue # Analyze words in the input file or files while (<>) { foreach (split) { # break $_ into words, assign each to $_ in turn $total++; next if //W/; # strange words skip the remainder of the loop $valid++; $count{$_}++; # count each separate word ## next comes here ## } }
print "total things = $total, valid words = $valid/n"; foreach $word (sort keys %count) { print "$word was seen $count{$word} times./n"; }
redo操作符返回本轮循环语句块顶端,在C语言中没有类似操作符 # Typing test my @words = qw{ fred barney pebbles dino wilma betty }; my $errors = 0;
foreach (@words) { ## redo comes here ## print "Type the word '$_': "; chomp(my $try = <STDIN>); if ($try ne $_) { print "Sorry - That's not right./n/n"; $errors++; redo; # jump back up to the top of the loop } } print "You've completed the test, with $errors errors./n";
综合使用三个操作符的例子: foreach (1..10) { print "Iteration number $_./n/n"; print "Please choose: last, next, redo, or none of the above? "; chomp(my $choice = <STDIN>); print "/n"; last if $choice =~ /last/i; next if $choice =~ /next/i; redo if $choice =~ /redo/i; print "That wasn't any of the choices... onward!/n/n"; } print "That's all, folks!/n";
逻辑与 && and逻辑或 || or
利用逻辑语句设置默认值my $last_name = $last_name{$someone} || '(No last name)';如果someone没有在hash中,则被设置为No Last name
问号三元操作符expression ? if_true_expr : if_false_expr求平均值 my $average = $n ? ($total/$n) : "-----"; print "Average: $average/n";如果n未定义或者为0, 结果为-----
利用and or操作符进行流程控制($m < $n) && ($m = $n);等价于if ($m < $n) { $m = $n }
而($m<$n) || ($m =$n);等价于until ($m < $n) { $m = $n }
在perl中打开文件的常用方法: open CHAPTER, $filename or die "Can't open '$filename': $!";
