Consider the following silly Perl program:
$firstarg = $ARGV[0];
print $firstarg;
$input = <>;
print $input;
I run it from a terminal开发者_JAVA百科 like:
perl myprog.pl sample_argument
And get this error:
Can't open sample_argument: No such file or directory at myprog.pl line 5.
Any ideas why this is? When it gets to the <> is it trying to read from the (non-existent) file, "sample_argument" or something? And why?
<>
is shorthand for "read from the files specified in @ARGV
, or if @ARGV
is empty, then read from STDIN
". In your program, @ARGV
contains the value ("sample_argument")
, and so Perl tries to read from that file when you use the <>
operator.
You can fix it by clearing @ARGV
before you get to the <>
line:
$firstarg = shift @ARGV;
print $firstarg;
$input = <>; # now @ARGV is empty, so read from STDIN
print $input;
See the perlio man page, which reads in part:
The null filehandle <> is special: it can be used to emulate the behavior of sed and awk. Input from <> comes either from standard input, or from each file listed on the command line. Here’s how it works: the first time <> is evaluated, the @ARGV array is checked, and if it is empty, $ARGV[0] is set to "-", which when opened gives you standard input. The @ARGV array is then processed as a list of filenames.
If you want STDIN, use STDIN, not <>
.
By default, perl consumes the command line arguments as input files for <>
. After you've used them, you should consume them yourself with shift;
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