How to redirect and restore STDOUT

Oct 27, 2013 by David Farrell

STDOUT is the Perl filehandle for printing standard output. Unless a filehandle is specified, all standard printed output in Perl will go to the terminal. Because STDOUT is just a global variable, it can be redirected and restored. Want to implement logging on a program without changing every print statement in the source code? Want to capture the standard output of a perl CRON job? Read on.

Terminal redirects

Before you launch your favourite text editor and start hacking Perl code, you may just need to redirect the program output in the terminal. On UNIX-based systems you can write to a file using “>” and append to a file using “>>”. Both write and append will create the file if it doesn’t exist.

perl program.pl > /path/to/log.txt
perl program.pl >> /path/to/log.txt

On Windows a similar effect can be achieved using PowerShell using a pipe operator ("|") and “set-content” to write, or “add-content” to append (the pipe will not redirect STDERR).

perl program.pl | set-content /path/to/log.txt
perl program.pl | add-content /path/to/log.txt

Perl solutions

If a terminal redirect is not specific enough for your needs, you can use one of the following Perl solutions. All of the following solutions use autodie which removes the need for the classic “|| or die $!” syntax to be appended to every open statement in the code.

Redirect STDOUT using select

Perl’s built-in function select changes the standard output filehandle to the filehandle provided as an argument. This makes it easy to globally redirect and restore standard output.

use feature qw/say/;
use autodie;

# open filehandle log.txt
open (my $LOG, '>>', 'log.txt');

# select new filehandle
select $LOG;

say 'This should be logged.';

# restore STDOUT
select STDOUT;

say 'This should show in the terminal';

Redirect STDOUT using local

Perl’s local built-in function is another option for redirecting STDOUT. The local function creates a lexically-scoped copy of any variable passed to it. By enclosing local in a do block, the code below limits the STDOUT redirect to the block scope and STDOUT is automatically restored after the closing block brace ("}"). By definition this is not a global solution for redirecting STDOUT.

use feature qw/say/;
use autodie;

do {
    local *STDOUT;

    # redirect STDOUT to log.txt
    open (STDOUT, '>>', 'log.txt');

    say 'This should be logged.';
};
say 'This should show in the terminal';

Redirect STDOUT using a filehandle

A third way to redirect and restore STDOUT is to copy the STDOUT filehandle before replacing it. This copy can then be restored when required. As with select, this will have a global affect on the Perl program.

use feature qw/say/;
use autodie;

# copy STDOUT to another filehandle
open (my $STDOLD, '>&', STDOUT);

# redirect STDOUT to log.txt
open (STDOUT, '>>', 'log.txt');

say 'This should be logged.';

# restore STDOUT
open (STDOUT, '>&', $STDOLD);

say 'This should show in the terminal';

This article was originally posted on PerlTricks.com.