Perl - select
select returns the currently selected filehandle.
select(FILEHANDLE)
Sets the current default filehandle for output, if FILEHANDLE is supplied.
- A write or a print without a filehandle will default to this FILEHANDLE.
- References to variables related to output will refer to this output channel.
select(RBITS,WBITS,EBITS,TIMEOUT)
This syntax of select() tries to see which file handles or sockets are active
Any of the bit masks can also be undef. The timeout, if specified, is in seconds, which may be fractional.
Usage
Redirect STDOUT temporarily to a scalar
# Open a filehandle on a string
my $scalar_file = '';
open my $scalar_fh, '>', \$scalar_file
or die "Can't open scalar filehandle: $!";
# Select scalar filehandle as default, save STDOUT
my $ostdout = select( $scalar_fh );
# Unbuffered output
$| = 1;
# Now, close scalar filehandle and bring back STDOUT
close( $scalar_fh );
print "ABC\n";
print "DEF\n";
print "GHI...\n";
# Bring STDOUT back
select( $ostdout );
Set unbuffered output recipe
-> Perl - Unbuffered Output or Hot Filehandle
my $old_fh = select(STDERR);
$| = 1; # Set ->autoflush()
select($old_fh);
Use IO::Handle to set unbuffered output
One should not use select($file_handle) syntax to set the “currently selected filehandle” because this will affect all subsequent uses by such functions as print and is a sure-fire way to confuse the maintenance programmer. Instead, use IO::Handle and its methods.
use IO::Handle;
STDERR->autoflush(1);
Perform sub-second sleeps
#You can effect a sleep of 250 milliseconds this way
select(undef, undef, undef, 0.25);
Use Time::HiRes for a simple delay
Avoid the use of select() for performing non-integer sleeps. It’s something that generally requires the reader to read perldoc -f select to figure out what it should be doing.
For a simple delay, one should use Time::HiRes
use Time::HiRes;
sleep( 0.25 );