Capturing bash output without displaying STOUT in terminal

edimusrex has asked for the wisdom of the Perl Monks concerning the following question:

I need to capture the output of a bash command in perl which I've done a million times but for some reason this specific issue is hanging me up.

What I need to do it capture the output from a git fetch --tags command and parse the results. The issue is if I run the following code

#!/usr/bin/perl

use warnings;
use strict;

my $git = system('git fetch --tags');

print "What's here? --- $git\n";
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The output of the command is displayed in terminal (which is to be expected since I haven't redirected it). However, if I run the command as followed:

my $git = system('git fetch --tags &> /dev/null');
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The variable is empty (which is also expected because I am redirecting it to /dev/null). The only way I can get it to work is if I redirect to a file first as such:

git fetch --tags &> /tmp/git.txt
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and then parsing the output file. I am trying to avoid having to do that if at all possible meaning I do not want an extra step of checking a tmp file, would much rather do it in one step. Help would be greatly appreciated. Thanks

********** UPDATE **********

I managed to solve this issue based on some of your suggestions using Capture::Tiny. The following code is doing exactly what I wanted it to do.

#!/usr/bin/perl

use warnings;
use strict;
use Capture::Tiny ':all';
use Version::Compare;
use Sort::Key::Natural qw( natsort );

my $GIT_HOME = '/usr/local/src/Arelle_Web/DQC_Test/';
my (@git) = capture{system("git --git-dir=${GIT_HOME}.git fetch --tags
+");};

@git = grep { $_ ne '' } @git;

my @tags;

if(scalar(@git) > 1){
    print "Tags have been updated\n";
    my $string = join("\n", @git);
    @tags = ($string =~ m/\[new tag\]\s+(.+?)\s/g);
}
else {
    @tags = split("\n",capture{system("git --git-dir=${GIT_HOME}.git t
+ag")});
    print "No New Tags\n";
}

@tags = natsort @tags;

chomp(my $running_version = capture{system("git --git-dir=${GIT_HOME}.
+git describe --tag");});
$running_version =~ s/^v//;

my ($ver_major) = $running_version =~ /^(\d)/;

my @version = grep(/^v$ver_major.+/,@tags);

if(@version){
    @version = natsort @version;
    my $newest = pop(@version);
    $newest =~ s/^v//;
    if(&Version::Compare::version_compare($newest,$running_version) ==
+ 1){
        print "v$newest is greater than the running version v$running_
+version\n";
    }
}
[download]

Thanks again for all your suggestions.

Comment on Capturing bash output without displaying STOUT in terminal Select or Download Code

Replies are listed 'Best First'.
Re: Capturing bash output without displaying STOUT in terminal by stevieb (Canon) on Sep 08, 2016 at 16:08 UTC
You can use the `qx` operator or backticks to do pretty much the same thing as system, but it'll capture the `STDOUT` (it doesn't capture `STDERR` though). use warnings; use strict; my $stdout = qx{ git status }; # or `git status` ... [download] See Quote and Quote-Like Operators in perlop.	[reply] [d/l] [select]
Re^2: Capturing bash output without displaying STOUT in terminal by edimusrex (Monk) on Sep 08, 2016 at 16:14 UTC
The issue is using the backticks or qx, the stdout is still diplayed in the console and nothing is captured in the variable. Maybe it's the specific command I am running.	[reply]
Re^3: Capturing bash output without displaying STOUT in terminal by stevieb (Canon) on Sep 08, 2016 at 16:18 UTC
What is the output on screen you get? Try running something you know won't error out and write to `STDERR` instead of `STDOUT`. For instance: my $out = `ls`; [download] ...should NOT write to `STDERR`. If that works, but your original command doesn't, I'm pretty certain that something is happening that is writing to `STDERR`. Try doing a `git status` with backticks and see what happens. Now move the test to a non-repo directory and compare the results (the latter will print to console, the former won't).	[reply] [d/l] [select]
Re^4: Capturing bash output without displaying STOUT in terminal by Linicks (Scribe) on Sep 08, 2016 at 17:16 UTC
Re^5: Capturing bash output without displaying STOUT in terminal [SOLVED] by Linicks (Scribe) on Sep 08, 2016 at 18:14 UTC
Re: Capturing bash output without displaying STOUT in terminal by dasgar (Priest) on Sep 08, 2016 at 16:10 UTC
Maybe Capture::Tiny might be able to help you to capture the output of a system command without displaying the output to the terminal.	[reply]
Re^2: Capturing bash output without displaying STOUT in terminal by edimusrex (Monk) on Sep 08, 2016 at 16:15 UTC
Yea, I am looking into Capture::Tiny right now, the output of this command still doesn't seem to want to be captured, I will play around with it some	[reply]
Re^3: Capturing bash output without displaying STOUT in terminal by dasgar (Priest) on Sep 08, 2016 at 16:29 UTC
Based on your response to my post as well as stevieb's initial post, I'd agree with stevieb that it looks like the command that you're running is sending the response to STDERR. As stevieb has pointed out, his suggestion of using backticks will only let you capture STDOUT. Capture::Tiny can capture both STDOUT and STDERR (and exit code) for system commands.	[reply]
Re^4: Capturing bash output without displaying STOUT in terminal by bart (Canon) on Sep 08, 2016 at 19:17 UTC
Re^3: Capturing bash output without displaying STOUT in terminal by ikegami (Patriarch) on Sep 08, 2016 at 21:36 UTC
Some programs (e.g `ssh` when asking for a password) write directly to the terminal, not STDOUT. `$ ssh ikegami@209.197.123.153 2>&1 >/dev/null ikegami@209.197.123.153's password:` [download] You have to create pseudo-tty's (as Expect does) in that case. IPC::Run supports capturing STDOUT and STDERR separately, and easily. It even supports pseudo-tty creation if you need them.	[reply] [d/l] [select]
Re: Capturing bash output without displaying STOUT in terminal by karlgoethebier (Abbot) on Sep 09, 2016 at 08:35 UTC
IPC::Run might do the job. Update: See also Re: Using IPC::Open3 instead of backtick operator. Regards, Karl ŤThe Crux of the Biscuit is the Apostropheť	[reply]
Re: Capturing bash output without displaying STOUT in terminal by ikegami (Patriarch) on Sep 08, 2016 at 21:40 UTC
Note that `system(EXPR)` [download] is equivalent `system('/bin/sh', '-c', EXPR)` [download] `so <c>&>` is unlikely to be supported.	[reply] [d/l] [select]

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