detect input available from command line

[mifflin]

I have a program that may try to get data from the command line using

while (<>) {
}

However, there may not be any input. How do I detect if there is any input so the read will not block? Something like...

if (data is available then) {
    while (<>) {
    }
}
else {
    # do something else
}

UPDATE:

Here is what I ended up doing get the behavior I was looking for...

    my $stdin = IO::Select->new(\*STDIN);
    open my $fh, '|-', $cmd or die $!;
    if (@ARGV || $stdin->can_read(0)) {
        while (<>) {
            print {$fh} $_ or die $!;
        }
    }
    else {
        print {$fh} "\n" or die $!;
    }
    close $fh or die $!;

Now, now matter how the user wants to give my prog the data like...

$ echo "text" | myprog

or

$ myprog <<EOT
some text
EOT

or

$ myprog afile

I get text without blocking.

[Fletch]

Perhaps the -t test or the PBP-blessed IO::Interactive would help?

[jwkrahn]

if ( @ARGV ) {
    while (<>) {
    }
}
else {
    # do something else
}

[mifflin]

What if the input is not coming from @ARGV but from stdin?
example:

$ ./testprog -t [e-mail] -s test5 <<EOT;
> stuff
> typed
> in 
> here
> EOT

In this case @ARGV is empty but there is still something to read. How would I detect this?

[Eliya]

You could test

if (@ARGV or !-t STDIN and !eof()) {
    while (<>) {
        ...

(but any non-input file arguments would need to be removed from @ARGV first, of course)

This should work with input file arguments on the command line, heredoc input (<<), input redirect (<), and piping into the program (|).

Update: actually, I think you don't even need the eof().

[jwkrahn]

I have a program that may try to get data from the command line using

What if the input is not coming from @ARGV but from stdin?

If the input is coming from STDIN then it is not on the command line.

[BioLion]

I think OP needs to clarify whether he wants to prompt for input e.g prompt function or take arguments from the command line e.g. @ARGV or getopt::long...

Hope this helps!

[mifflin]

I want to know when

while (<>) {
}

has something available to read. I'd like to not execute this code when there isn't something available because it will block.

This idiom will read when @ARGV is populated or when there is some data available to read on STDIN.

I can handle detecting whether @ARGV has data. What I'm not sure on is how to detect whether STDIN has something available to read. Still testing but I'm not convinced, yet, that -t, as suggested by others, will work here.

[Anonymous Monk]

One thing you can do is start a second thread. Let that thread do the blocking read, and pass results to the main thread as they become available.