print inside a loop

[nbezzala]

#!/usr/bin/perl

foreach my $i ( 1..3 ) {
    print "\nChecking something - ";
    sleep 3;  
    print "OK";
}

print "\n";

It prints this line all at once - "Checking something - OK"

Why does it not print "Checking something - " first, and then add "OK" after 3 seconds?

[hackman]

You have to make the output unbuffered. You can do that by adding this:

$|=1;

Before the loop.

[moritz]

See also: Suffering from Buffering.

[Ratazong]

Often printing will be done immediately if you finish your text with an \n:

print "\nChecking something - \n";

HTH, Rata

[Eliya]

Often...

More specifically, when the buffering mode is set to "line-buffered" — which is the default with interactive things like terminals.

In general, and to summarize, there are three buffering modes:

• unbuffered — when explicitly requested, and the default for stderr
• line-buffered — the default in interactive contexts
• block-buffered (aka "fully buffered") — the default for everything else, like writing to files, pipes, etc.

In block-buffered mode, flushing doesn't happen before the respective buffer, e.g. 4k bytes size, has filled up.

For example (block-buffered):

$ strace -ewrite perl -E 'say "X"x99 for 1..100' >/dev/null
write(1, "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 4096) = 4096
write(1, "XXX\nXXXXXXXXXXXXXXXXXXXXXXXXXXXX"..., 4096) = 4096
write(1, "XXXXXXX\nXXXXXXXXXXXXXXXXXXXXXXXX"..., 1808) = 1808

Without the redirect of stdout, i.e. when it's connected to the terminal, you'd see 100 write system calls à 100 bytes... (line-buffered)

[bart]

Can one explicitly set buffering to line- or buffer oriented, or is that chosen by the system, and all you can do is enable/disable buffering by assigning a boolean value to $|?

I guess the system choses, perhaps based on the -t flag, but I'm not sure at all.

[Khen1950fx]

Let's give it an error to make it more interesting:

#!/usr/bin/perl

use strict;
use warnings;

foreach my $i ( 1..3 ) {
    if ($i lt 3) {
        print "\nChecking $i - ", "\n";
        sleep 3;
        print "OK\n";
    }
    else {
        print "\nChecking $i - ", "\n";
        sleep 3;
        print "Not OK\n\n";
    }
}

Updated: changed "something" to $i.
Updated: Found a better unbuffered way...

#!/usr/bin/perl

use strict;
use warnings;

foreach my $i ( 1..3 ) {
    if ($i lt 3) {
        print "\nChecking $i - " and
        sleep 3 and
        print "OK";
    }
    else {
        print "\nChecking $i - " and
        sleep 3 and
        print "Not OK\n";
    }
}