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How can I mix command line arguments and file names for <> in Perl?

Consider the following dumb Perl program:

 $firstarg = $ARGV[0]; print $firstarg; $input = <>; print $input;

I run it from the terminal 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 so? When it gets to <> is it trying to read from a (nonexistent) file, "sample_argument" or something else? And why?

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3 answers

<> is an abbreviation for "read from files specified in @ARGV , or if @ARGV empty, then read from STDIN ". In your program, @ARGV contains the value ("sample_argument") , and so Perl tries to read from this file when you use the <> operator.

You can fix this by clearing @ARGV before going to the <> line:

 $firstarg = shift @ARGV; print $firstarg; $input = <>; # now @ARGV is empty, so read from STDIN print $input;
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See the man perlio page, which is partially read:

The null file descriptor <> is special: it can be used to emulate the behavior of sed and awk. The input from <> comes either from standard input, or from each specified file on the command line. Here's how it works: the first time <> is evaluated, the @ARGV array is checked, and if it's empty, $ ARGV [0] is set to "-", which opens up standard input. The @ARGV array is then processed as a list of file names.

If you want STDIN, use STDIN, not <> .

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By default, perl consumes command line arguments as input files for <> . After you use them, you must use them yourself with shift;

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