Send output from exec to file perl

Question

Problem

When running exec in perl, I am am unable to redirect the output to a file.

I am running vlc in the exec but since i doubt everyone has it set up I have replaced with echo below, it shows same behaviour for the example.

I am only interested in exec 'command','args' format of exec not the one that spawns a shell since it spawns a subshell with vlc which still prints to screen + other problems with killing it cleanly.

Code

use strict;
use warnings;

my $pid = fork;
if (!defined $pid) {
    die "Cannot fork: $!";
}
elsif ($pid == 0) {
    exec "/usr/bin/echo","done";
}

Tried

exec "/usr/bin/echo","done",">/dev/null";

As expected just prints ">/dev/null", but was worth a try.

exec "/usr/bin/echo done >/dev/null";

Runs sh which then runs echo, works here, but not in my actual problem with vlc, thought i would include anyway since someone will surely suggest it.

Question

How do I redirect output from this exec when using 'command','args' to a file?

Extra

Any more info needed please ask.

are you just trying to send the output of the exectuted command to a file? — user7818749
– user7818749, Commented Sep 27, 2017 at 8:55
I think the >/dev/null is part of the shell. If you run exec '/usr/bin/echo', 'done' no shell is involved, which is the idea of the argument list. And without a shell, there is no way to redirect. You need to find a different way to change the handles that the program gets. But I don't know how unfortunately. — simbabque
– simbabque, Commented Sep 27, 2017 at 9:10
@simbabque Looks like changing STDOUT does work, just not with select strangely? — 123
– 123, Commented Sep 27, 2017 at 11:10
It makes sense actually. select does not change STDOUT. It changes the handle that Perl uses to write to by default. I didn't think of that. Well done. :) — simbabque
– simbabque, Commented Sep 27, 2017 at 11:12

123 · Accepted Answer · 2017-09-27 11:07:17Z

4

Turns out you can just change the file descriptors before the exec

use strict;
use warnings;

my $pid = fork;
if (!defined $pid) {
    die "Cannot fork: $!";
}
elsif ($pid == 0) {
    open STDOUT, ">", '/logger/log' or die $!;
    open STDERR, ">", '/logger/log' or die $!;
    exec "/usr/bin/echo","done";
 }

answered Sep 27, 2017 at 11:07

123

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Comments

score 0 · Accepted Answer · 2017-09-27 09:46:47Z

0

I suppose if you just need to print to file, this should work.

easiest possible way to capture the output is by using backticks.

use strict;
use warnings;

open (my $file, '>', 'output.log');
my $pid = fork;
if (!defined $pid) {
    die "Cannot fork: $!";
}
elsif ($pid == 0) {
    print $file `/usr/bin/echo done`;
}

edited Sep 27, 2017 at 9:46

answered Sep 27, 2017 at 9:03

user7818749

2 Comments

simbabque Over a year ago

Again, exec does not return. It's like a fork, it runs and forgets. Your print line in the first example will never be reached. In fact, your Perl throws a warning to tell you that. Run this code, and you'll see what I mean: perl -E 'END{say"end"} exec q{sleep 5 && echo "foo"}; say "bar";'

user7818749 Over a year ago

@simbabque My mistake. Fixed.

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