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IPC::Open3 misbehaving when STDOUT is not FD #1

by sgifford (Prior)
on Jun 05, 2009 at 16:47 UTC ( #768840=perlquestion: print w/ replies, xml ) Need Help??
sgifford has asked for the wisdom of the Perl Monks concerning the following question:

I'm having some trouble with IPC::Open3. Here is a test script that demonstrates the problem:
#!/usr/bin/perl use warnings; use strict; use IPC::Open3; # Basic setup stuff open(SAVE_STDOUT,">&",STDOUT) or die "save stdout failed"; open(DEVNULL,"< /dev/null") or die "open /dev/null failed"; # Now set STDOUT to a filehandle on a new descriptor open(FH1,"> /dev/null") or die "open /dev/null failed"; *STDOUT = *FH1; # Run the command open3(*DEVNULL, *PIPE, undef, "/bin/sh","-c",<<EOF) or die "open3 fail +ed"; printf "out 1\\nout 2\\nout 3\\n" printf "err 1\\nerr 2\\nerr 3\\n" >&2 EOF ; # And copy the output open(STDOUT,">&",*SAVE_STDOUT) or die "restore stout failed"; while (<PIPE>) { print "PIPE: ",$_; } close(PIPE) or die "close pipe failed"; exit(0);
The expected output from this is:
PIPE: out 1 PIPE: out 2 PIPE: out 3 PIPE: err 1 PIPE: err 2 PIPE: err 3
which indicates that the child process had both standard output and standard error sent to the pipe created by open3. Instead, with both IPC::Open3 1.02 and 1.03, I get this output:
out 1 out 2 out 3 PIPE: err 1 PIPE: err 2 PIPE: err 3
This indicates that the standard output from the script didn't go through the pipe. Instead it went directly to my terminal, where file descriptor #1 is connected to in the parent process.

This appears to be the result of a bug in IPC::Open3, which I reported in Perl bug #66224, including a patch. I wanted to see if my fellow Monks had any insight or ideas for a better fix/workaround.

Thanks!

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Re: IPC::Open3 misbehaving when STDOUT is not FD #1
by Khen1950fx (Canon) on Jun 05, 2009 at 20:35 UTC
    printf "out 1\\nout 2\\nout 3\\n" >&2 will give you the desired output.

    Update: While the answer is true, "true" here means "error". Bash is running printf, not perl, so Bash is running into or and die and returning or: command not found and die: command not found; hence, eveything goes to stderr.

      So will
      #!/usr/bin/perl print <<'__EOI__'; PIPE: out 1 PIPE: out 2 PIPE: out 3 PIPE: err 1 PIPE: err 2 PIPE: err 3 __EOI__

      True, but completely unhelpful.

      I posted my comments to the OP in RT.

      True Khen1950fx, that may be a good workaround. I will see if it works in my environment (it will depend on whether STDERR is set to a file descriptor other than 2, which fails in a similar way). If it does it will be easier to support than a patched IPC::Open3.
Re: IPC::Open3 misbehaving when STDOUT is not FD #1
by ikegami (Pope) on Jun 05, 2009 at 22:34 UTC

    Simpler, clearer, more self-contained test:

    #!/usr/bin/perl use warnings; use strict; use IPC::Open3; use Test::More tests => 1; # Hide undesired output. open(STDOUT, '>', '/dev/nul'); open(STDERR, '>', '/dev/nul'); # Associate STDOUT and STDERR with # descriptors other than 1 and 2. open(local *STDOUT, '>', '/dev/nul'); open(local *STDERR, '>', '/dev/nul'); open3( undef, local *PIPE, undef, 'echo out ; echo err >&2', ); my $pipe = ''; $pipe .= $_ while <PIPE>; is($pipe, "out\nerr\n");

    Still unix-specific, though.

      Portable?
      #!/usr/bin/perl use warnings; use strict; use IPC::Open3; use Test::More tests => 1; use File::Spec; my $devnull = File::Spec->devnull; # Hide undesired output. open(STDOUT, '>', $devnull); open(STDERR, '>', $devnull); # Associate STDOUT and STDERR with # descriptors other than 1 and 2. open(local *STDOUT, '>', $devnull); open(local *STDERR, '>', $devnull); open3( undef, local *PIPE, undef, qq!$^X -le "print 111;print STDERR 222"!, ); my $pipe = ''; $pipe .= $_ while <PIPE>; is($pipe, "111\n222\n"); __END__
        $^X isn't properly converted to a shell literal. (i.e. Won't work if there's a space in $^X.) Fix:
        open3( undef, local *PIPE, undef, $^X, q!-le print 111;print STDERR 222!, );
Re: IPC::Open3 misbehaving when STDOUT is not FD #1
by Khen1950fx (Canon) on Jun 07, 2009 at 00:43 UTC
    Admittedly, my first response was on the fly, but I dug deeper and tried this:

    #!/usr/bin/perl use strict; use warnings; use diagnostics; use IPC::Open3; use POSIX 'setsid'; sub daemonize { chdir '/' or die "Can't chdir to /: $!"; open STDIN, '/dev/null' or die "Can't read /dev/null: $!"; open STDOUT, '>/dev/null' or die "Can't write to /dev/null: $!"; defined( my $pid = fork ) or die "Can't fork: $!"; exit if $pid; setsid or die "Can't start a new session: $!"; open STDERR, '>&STDOUT' or die "Can't dup stdout: $!"; } open( SAVE_STDOUT, ">&", STDOUT ) or die "save stdout failed"; open( DEVNULL, "< /dev/null" ) or die "open /dev/null failed"; open( FH1, "> /dev/null" ) or die "Open /dev/null failed"; &daemonize( *FH1 ); open3( *DEVNULL, *PIPE, undef, "/bin/sh", "-c", <<EOF ); printf "out 1\\nout 2\\nout 3\\n" printf "err 1\\nerr 2\\nerr 3\\n" >&2 EOF ; open( STDOUT, ">&", *SAVE_STDOUT ) or die "restore stdout failed: $!"; while(<PIPE>) { print "PIPE: ", $_; } close(PIPE) or die "close pipe failed"; exit(1);

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