How do I get the output of an external command in Perl?

Question

I want to have output of Windows command-line program (say, powercfg -l) written into a file which is created using Perl and then read the file line by line in a for loop and assign it to a string.

Answer 1

system 'powercfg', '-l';

is the recommended way. If you don't mind spawning a subshell,

system "powercfg -l";

will work, too. And if you want the results in a string:

my $str = `powercfg -l`;

Answer 2

Since the OP is running powercfg, s/he are probably capturing the ouput of the external script, so s/he probably won't find this answer terribly useful. This post is primarily is written for other people who find the answers here by searching.

This answer describes several ways to start command that will run in the background without blocking further execution of your script.

Take a look at the perlport entry for system. You can use system( 1, 'command line to run'); to spawn a child process and continue your script.

This is really very handy, but there is one serious caveat that is not documented. If you start more 64 processes in one execution of the script, your attempts to run additional programs will fail.

I have verified this to be the case with Windows XP and ActivePerl 5.6 and 5.8. I have not tested this with Vista or with Stawberry Perl, or any version of 5.10.

Here's a one liner you can use to test your perl for this problem:

C:\>perl -e "for (1..100) { print qq'\n $_\n-------\n'; system( 1, 'echo worked' ), sleep 1 }

If the problem exists on your system, and you will be starting many programs, you can use the Win32::Process module to manage your application startup.

Here's an example of using Win32::Process:

use strict;
use warnings;

use Win32::Process;

if( my $pid = start_foo_bar_baz() ) {
    print "Started with $pid";
}
:w

sub start_foo_bar_baz {

    my $process_object;  # Call to Create will populate this value.
    my $command = 'C:/foo/bar/baz.exe';  # FULL PATH to executable.
    my $command_line = join ' ',
           'baz',   # Name of executable as would appear on command line
           'arg1',  # other args
           'arg2';

    # iflags - controls whether process will inherit parent handles, console, etc.
    my $inherit_flags = DETACHED_PROCESS;  

    # cflags - Process creation flags.
    my $creation_flags = NORMAL_PRIORITY_CLASS;

    # Path of process working directory
    my $working_directory = 'C:/Foo/Bar';

    my $ok = Win32::Process::Create(
       $process_object,
       $command,
       $command_line,
       $inherit_flags,
       $creation_flags, 
       $working_directory,
    );

    my $pid;
    if ( $ok ) {
        $pid = $wpc->GetProcessID;
    }
    else {
        warn "Unable to create process: "
             . Win32::FormatMessage( Win32::GetLastError() ) 
        ;
        return;
    }

    return $pid;
}

Answer 3

Try using > operator to forward the output to a file, like:

powercfg -l > output.txt

And then open output.txt and process it.

Answer 4

There is no need to first save the output of the command in a file:

my $output = `powercfg -l`;

See qx// in Quote-Like Operators.

However, if you do want to first save the output in a file, then you can use:

my $output_file = 'output.txt';

system "powercfg -l > $output_file";

open my $fh, '<', $output_file 
    or die "Cannot open '$output_file' for reading: $!";

while ( my $line = <$fh> ) {
    # process lines
}

close $fh;

See perldoc -f system.

Answer 5

my $output = qx(powercfg -l);

## You've got your output loaded into the $output variable. 
## Still want to write it to a file?
open my $OUTPUT, '>', 'output.txt' or die "Couldn't open output.txt: $!\n";
print $OUTPUT $output;
close $OUTPUT

## Now you can loop through each line and
##   parse the $line variable to extract the info you are looking for.
foreach my $line (split /[\r\n]+/, $output) {
  ## Regular expression magic to grab what you want
}

Answer 6

To expand on Sinan's excellent answer and to more explicitly answer your question:

NOTE: backticks `` tell Perl to execute a command and retrieve its output:

#!/usr/bin/perl -w
use strict;


my @output = `powercfg -l`;
chomp(@output); # removes newlines

my $linecounter = 0;    
my $combined_line;

foreach my $line(@output){
    print $linecounter++.")";
    print $line."\n"; #prints line by line

    $combined_line .= $line; # build a single string with all lines
    # The line above is the same as writing:
    # $combined_line = $combined_line.$line;
}

print "\n";
print "This is all on one line:\n";
print ">".$combined_line."<";

Your output (on my system) would be:

0)
1)Existing Power Schemes (* Active)
2)-----------------------------------
3)Power Scheme GUID: 381b4222-f694-41f0-9685-ff5bb260df2e  (Balanced) *
4)Power Scheme GUID: 8c5e7fda-e8bf-4a96-9a85-a6e23a8c635c  (High performance)
5)Power Scheme GUID: a1841308-3541-4fab-bc81-f71556f20b4a  (Power saver)

This is all on one line:
>Existing Power Schemes (* Active)-----------------------------------Power Scheme GUID: 381b4222-f694-41f0-9685-ff5bb260df2e  (Balanced) *Power Scheme GUID: 8c5e7fda-e8bf-4a96-9a85-a6e23a8c635c  (High performance)Power Scheme GUID: a1841308-3541-4fab-bc81-f71556f20b4a  (Power saver)<

Perl makes it easy!

Answer 7

You have some good answers already. In addition, if you just want to process a command's output and don't need to send that output directly to a file, you can establish a pipe between the command and your Perl script.

use strict;
use warnings;

open(my $fh, '-|', 'powercfg -l') or die $!;
while (my $line = <$fh>) {
    # Do stuff with each $line.
}