问题
What's the purpose of the following two lines of perl??
my $host = shift || 'localhost';
my $port = shift || 200;
That should return localhost and port 10. What is the shift keyword??
回答1:
What this piece of code is, is a way to provide default values for $host and $port. It will typically be at the start of a script or a subroutine, and take values from @ARGV and @_ respectively.
That should return localhost and port 10.
No, the || operator is a short circuiting OR, which means that if the LHS operand returns a true value, the RHS operand is ignored. Basically, it means this (and ONLY this): "choose the left hand side value if it is true, otherwise choose the right hand side value."
shift ARRAY will return the first value of ARRAY, or:
If ARRAY is omitted, shifts the @_ array within the lexical scope of subroutines and formats, and the @ARGV array outside a subroutine and also within the lexical scopes established by the eval STRING , BEGIN {} , INIT {} , CHECK {} , UNITCHECK {} and END {} constructs.
Quoted from http://perldoc.perl.org/functions/shift.html
Also, of course, shift removes the value from the array that is shifted. Therefore you can have two shift in a row like this, for very convenient argument handling.
回答2:
The first line shifts from either @_ or @ARGV (depending on where you are in the code), or in the absence of any contents in @_/@ARGV, assigns localhost to $host.
The second one should be self-explanatory now.
Have a look at the shift documentation for details.
回答3:
If no argument is provided, shift will shift @ARGV outside a subroutine and @_ inside a subroutine – that is the argument array passed to either the main program or the subroutine.
In your case, $host is assigned the first element of @ARGV (or @_, if the code is inside a sub) or 'localhost', if the element is false.
This is a very common Perl idiom.
回答4:
shift return the first element of an array and removes it from the array. Like pop, but from the other end.
回答5:
If you use shift, always put the array on it. I've seen experience Perl programmers forget that outside a subroutine, shift works on @ARGV. The more things a programmer has to remember at the same time, the more likely he is to make an error.
来源：https://stackoverflow.com/questions/6080765/perl-shift-operator-simple-question