可以编写一个Perl脚本来从（1）文件，（2）stdin，（3）重定向中的任何一个接收数据吗？

Question

Several unix utilities, such as fmt, head, and cat, can receive data in any of 3 ways: a filename; a pipe from standard input; or a redirect "<". For instance:

printf '%b' 'dog \ncat \nrat\n' > dogcatrat
fmt dogcatrat
cat dogcatrat  | fmt
fmt < dogcatrat

Can one write a perl script that will behave with the same versatility? Or is there a good reason not to attempt this? And is "pipe from standard input" the right way to refer to the line of code that starts with cat?

I want to write myfmt.pl, to be used in any of these three ways.

Answer 1

The ARGV special filehandle will do this by default. It is also the handle used by readline (aka the <> and <<>> operators) when not given a handle. So this is actually pretty common in Perl scripts.

#!/usr/bin/env perl
use 5.022;
use warnings;
while (my $line = <<>>) {
  # $line from one of the filenames passed as an argument, otherwise STDIN
  # $ARGV is the current filename, or - when reading from STDIN
}

You can use the <> operator instead to support older versions of Perl, but the <<>> operator added in Perl 5.22 is a better option for this task if available, because the standard <> operator allows passing strange things like date| to run processes rather than read files.

For safer filename-only operation while supporting older versions of Perl, you could use ARGV::readonly or emulate the <<>> operator like the following:

#!/usr/bin/env perl
use strict;
use warnings;
unshift @ARGV, '-' unless @ARGV;
while (my $file = shift) {
  my $fh;
  if ($file eq '-') {
    $fh = \*STDIN;
  } else {
    open $fh, '<', $file or die "open $file failed: $!";
  }
  while (my $line = <$fh>) {
    # ...
  }
}

(Technically the <<>> operator also does not allow passing - as an argument to read STDIN, but it is your choice if you want to allow that.)

Answer 2

It appears that the following script fills the bill.

#!/usr/bin/perl
use strict;
use warnings;
use 5.18.2;
local $/ = ""; # input record separator: one paragraph at a time
while (<>) {
    print;
    print "\n";
    say '-' x 30;
}

Example:

printf '%b' 'dog \ncat \nrat\n' > aaa
try.pl aaa
cat aaa | try.pl
try.pl < aaa