Close STDOUT/STDERR temporarily

[Superfox il Volpone]

Hello,

I would like to close temporarily STDOUT and STDERR so that an invoked function cannot write on it. For instance something like:

my $fh = *STDOUT;
*STDOUT = undef;
foo(); # foo cannot print into the stdout
STDOUT = $fh; # restore STDOUT

Is there any way to do that?

Thanks,
S.Fox

[LanX]

these are just file handles, you can save the old values° close them and open them temporarily to another file or a var ref

See select , open , close

Especially the docs for open have explicit sample code for exactly this task.

°) called "duplicate"

[kennethk]

In particular, from open:

Here is a script that saves, redirects, and restores STDOUT and STDERR using various methods:

    #!/usr/bin/perl
    open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
    open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
    open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
    open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
    select STDERR; $| = 1; # make unbuffered
    select STDOUT; $| = 1; # make unbuffered
    print STDOUT "stdout 1\n"; # this works for
    print STDERR "stderr 1\n"; # subprocesses too
    open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
    open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
    print STDOUT "stdout 2\n";
    print STDERR "stderr 2\n";

[Superfox il Volpone]

Ok, thanks for the replies.

[sundialsvc4]

Rolf, are you saying that the OP’s strategy would work?   I presume that you are referring to the paragraph in perldoc -f open which begins “Here is a script that saves, redirects, and restores "STDOUT" and "STDERR" using various methods:” but I do get pretty-lost pretty-fast when reading it.   Could you perhaps offer a couple of specific examples that do, specifically, what the OP has in mind to do?   Or, otherwise, elaborate on how you’d do it?

[LanX]

There are plenty of specific and working examples in this thread!

The confusion stems from demonstrating more than one way to do it within the same sample script (like 3 vs 2 argument open AND lexical filehandles vs bare words AND addressing filehandles as globrefs)

I have to admit that Perldocs are overly confusing in this respect.

[stylechief]

Some example code snippets that work...

    # preserve STDOUT
    open $oldSTDOUT, ">&STDOUT" || warn "Can't preserve STDOUT\n$!\n";
+ 
    close STDOUT;
 
    # preserve STDERR 
    open (OLDER, ">&", \*STDERR) || warn "Can't preserve STDERR\n$!\n"
+;
    close STDERR;

    # if you want to capture any messages in the meantime, point STDER
+R to a memory location
    open (STDERR, ">", \$err) || warn "Can't redirect STDERR\n$!\n";

    # restore STDERR 
    open (STDERR, ">&", \*OLDER) || warn "Can't restore STDERR\n$!\n";
    close OLDER;

    # restore STDOUT
    open STDOUT, ">&", $oldSTDOUT;

    # you may want to flush buffers at some point
    select STDERR; $| = 1;
    select STDOUT; $| = 1;