Redirecting output into a variable 2

[TonyGroves]

When I run a system command from a Perl script, is there any simple way to redirect standard and/or error output into a variable(s)? Obviously I could redirect to a file, and then read in that file, but I'm hoping there's a more efficient way than that.

I'm running Perl on Windows, in case that's relevant.

Any ideas?

Thanks.

 

[rharsh]

Instead of

system "some_command.exe";

try

my $var = `some_command.exe`;

The back-tick characters are on the same key as the tilde (~).

 

[fishiface]

I'm not at all sure about standard error on a windoze box. Under *ix, you'd

`command 2>&1`;

but that's relying on the shell to feed stderr to the same place as stdout. I tried this under XP

# file t
print STDOUT "stdout\n";
print STDOUT "stderr\n";

# file u
my $output = `perl t`;
print "Output is [$output]\n";

# file v
my $output = `perl t 2>&1`;
print "Output is [$output]\n";

perl u
stderr
Output is [stdout
]

perl v
Output is [stderr
]
stdout

So it seems as if you can have one or the other but not both. I'm no expert - anyone have chapter and verse on this?

f

"As soon as we started programming, we found to our surprise that it wasn't as easy to get programs right as we had thought. Debugging had to be discovered. I can remember the exact instant when I realized that a large part of my life from then on was going to be spent in finding mistakes in my own programs."
--Maurice Wilkes

 

[TonyGroves]

Thanks lads. rharsh's solution works for stdout, but not stderr. fishiface's solution works by sending both outputs to the same variable.
[tt]
d:\tony>echo -----testing----- >test.txt

D:\tony>perl
$var=`type test.txt`;
print "The value of var is \"$var\"\n";
$var=`type usdflguhsg`;
print "The value of var is \"$var\"\n";
$var=`type test.txt 2>&1`;
print "The value of var is \"$var\"\n";
$var=`type usdflguhsg 2>&1`;
print "The value of var is \"$var\"\n";
^Z
The value of var is "-----testing-----
"
The system cannot find the file specified.
The value of var is ""
The value of var is "-----testing-----
"
The value of var is "The system cannot find the file specified.
"

D:\tony>
[/tt]
It would be nice to be able to capture both outputs separately, but there's plenty of food for thought here.

 

[fishiface]

You could try (carefully) open3. There are all sorts of problems with this but something like the following might work (untested):

use IPC::Open3;
use IO::Select;

$pid = open3(*IN, *OUT, *ERR, "mycommand myarg");

$SIG{CHLD} = sub {
    warn "Oops! signalled ($?)" if waitpid($pid, 0) > 0
};

close(IN); # we don't want it waiting for input

$selector = IO::Select->new();
$selector->add(*ERR, *OUT); #watch for outputs here

my( @output, @error );
while (@ready = $selector->can_read) {
    foreach $fh (@ready) {
        push @output, scalar <ERR> if fileno($fh) == fileno(ERR);
        push @output, scalar <OUT> if fileno($fh) == fileno(OUT);
        $selector->remove($fh) if eof($fh);
    }
}
close(CMD_OUT);
close(CMD_ERR);

I really don't know how you'd get on with the signal handling on a non-*ix system but if you're always calling the same command and you're lucky with how it buffers stuff you could be ok.

Let me know,

f

"As soon as we started programming, we found to our surprise that it wasn't as easy to get programs right as we had thought. Debugging had to be discovered. I can remember the exact instant when I realized that a large part of my life from then on was going to be spent in finding mistakes in my own programs."
--Maurice Wilkes

 

[Aradon]

I figured out this neat little trick while trying to do about the same thing. This works in *nix but I don't know about doze.

 open(LSOUT, "ls |");
 while(my $line = <LSOUT>)
 {
   print "OMG $line";
 }

 close(SNMP);
[code]

think that works with ERR too.  Cheers.

 

[duncdude]

open (STDERR, ">&STDOUT");

Kind Regards
Duncan

 

[TonyGroves]

Thanks for all that input.

I decided to write a little function to do the job. Here it is:

sub system_capture
# Executes a system command, and when it is finished, returns the result code,
# standard output, and error output.
# Arguments: [0] command to be executed.
# Returns a list containing:
#   [0] the command's return code
#   [1] the standard output
#   [2] the error output
# Any redirection specifiers contained in the command will be ignored.
{
  use File::Temp qw(tempfile);
  my(undef,$out)=tempfile(dir=>$ENV{tmp},suffix=>'.tmp');
  my(undef,$err)=tempfile(dir=>$ENV{tmp},suffix=>'.tmp');
  my $rc=system("$_[0] 1>$out 2>$err");
  my $outtext;
  my $errtext;
  if(open(F,"<$out"))
  {
    while(<F>){$outtext.=$_}
    close F;
  }
  if(open(F,"<$err"))
  {
    while(<F>){$errtext.=$_}
    close F;
  }
  return($rc,$outtext,$errtext);
}

It has certain limitations, including speed, but I think it's a neat solution. If anybody can see any way to improve it, suggestions will be welcome.