Which file is Perl's diamond operator (null file handle) currently reading from?

I'm using Perl's diamond <> operator to read from files specified on the command line.

I'd like to be able to report messages like "Trouble on line $. of file $FILENAME", but how can I tell which file is currently used by the diamond?

See perlvar:

    $ARGV

Contains the name of the current file when reading from <> .

But also consider $. in perlvar. If you do this with perl -n it might not turn out the way you want it, because the counter is not reset in the perl -n use case.

$.

Current line number for the last filehandle accessed.

Each filehandle in Perl counts the number of lines that have been read from it. (Depending on the value of $/ , Perl's idea of what constitutes a line may not match yours.) When a line is read from a filehandle (via readline() or <> ), or when tell() or seek() is called on it, $. becomes an alias to the line counter for that filehandle.

You can adjust the counter by assigning to $. , but this will not actually move the seek pointer. Localizing $. will not localize the filehandle's line count. Instead, it will localize perl's notion of which filehandle $. is currently aliased to.

$. is reset when the filehandle is closed, but not when an open filehandle is reopened without an intervening close(). For more details, see I/O Operators in perlop. Because <> never does an explicit close, line numbers increase across ARGV files (but see examples in eof).

You can also use HANDLE->input_line_number(EXPR) to access the line counter for a given filehandle without having to worry about which handle you last accessed.

Mnemonic: many programs use "." to mean the current line number.

Here's an example:

$ perl -nE 'say "$., $ARGV";' foo.pl bar.pl
1, foo.pl
2, foo.pl
3, foo.pl
4, foo.pl
5, foo.pl
6, foo.pl
7, foo.pl
8, foo.pl
9, foo.pl
10, foo.pl
11, foo.pl
12, foo.pl
13, bar.pl
14, bar.pl
15, bar.pl

If you want it to reset, you need to check for eof at the end of your read loop (thanks @Borodin). Also see the perldoc for eof:

$ perl -nE 'say "$., $ARGV"; close ARGV if eof' foo.pl bar.pl