0 
#!/usr/bin/env perl

use Term::ReadKey;
ReadMode 4;
END {
    ReadMode 0; # Reset tty mode before exiting
}

while (<>) {
    $key = ReadKey(0);
    $key == "\x04" and last; # Ctrl+D breaks the loop 
    print $key;
}

When I had it without the while loop, it was printing back what I typed in.

It doesn't even produce any output at the end (if it was buffering it or something). Like I'd run it and type a few letters and hit Ctrl+D. It prints nothing.

I'm trying to make a program to convert mouse scroll escape codes into keypresses. I hope I'm not barking up the wrong tree.

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3
  • 5
    String equality is tested with the eq operator. The == converts the arguments to numbers first, which will usually be zero. You would have been warned had you put use strict; use warnings at the top of your script. Commented Apr 14, 2013 at 9:39
  • That may be true and my perl-fu is weak indeed, but i'm nearly certain that is not the true issue. echo "abc" | this_program still prints nothing. However if I take the loop out and have it read and print 2 char's, echo "abc" | this_program will in fact print ab Commented Apr 14, 2013 at 9:49
  • 1
    Oh, now I see it: You read a whole line of input (incl. obligatory \n) with the <> operator. Then, you read a single keypress with ReadKey(0) which you may echo. The line (in $_) is discarded. Change your loop condition to while(defined(my $key = ReadKey(0))){...} and really use strict; use warnings;. Commented Apr 14, 2013 at 9:58

2 Answers 2

Reset to default
3

This line

while (<>)

reads a line from STDIN (assuming you ran the program with no command line arguments). Once a line has been read, it enters the body of the while loop. Whatever you typed up to and including the newline is now in $_.

Now, you press a key, it's stored in $key and numerically compared to CTRL-D. Since neither is numeric, they both end up being zero, the loop terminates.

This is why you should turn on warnings which would have told you:

Argument "^D" isn't numeric in numeric eq (==) at ./tt.pl line 15,  line 1.
Argument "c" isn't numeric in numeric eq (==) at ./tt.pl line 15,  line 1.

Of course, it would make sense to put the loop-termination condition where it belongs as well:

#!/usr/bin/env perl

use strict;
use warnings;

use Term::ReadKey;
ReadMode 4;
END {
    ReadMode 0; # Reset tty mode before exiting
}

my $input;
{
    local $| = 1;
    while ((my $key = ReadKey(0)) ne "\x04") {
        print $key;
        $input .= $key;
    }
}

print "'$input'\n";
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4 Comments

As it is, this code upon exiting will print out all input a second time, yes?
Of course ... to show what was put into $input ... I am assuming you want to do something with the keys the user presses, right?
yes. I'm remapping keys and stuff. I'm now trying to figure out how to deal with EOF. echo "abc" | myprogram | hexdump -C shows lots of \x00 going in (and it does not exit). What is the real convention for breaking out of the loop? clearly unix programs do not send a \x04 byte to signal it, i am only receiving the byte corresponding to Ctrl+D.
I guess I should just break out on a null byte as well.
3

Just replace the while condition to:

while(1) {
   # ...
}
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1 Comment

I think this also needs to set the autoflush $|=1; for the key echo to happen immediately

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