perl shell command variable error - linux

Question

I am trying following code in one of my perl script and getting error, how do i execute following shell command and store in variable
#!/usr/bin/perl -w
my $p = $( PROCS=`echo /proc/[0-9]*|wc -w|tr -d ' '`; read L1 L2 L3 DUMMY < /proc/loadavg ; echo ${L1}:${L2}:${L3}:${PROCS} );
print $p;
Error:
./foo.pl
Bareword found where operator expected at /tmp/foo.pl line 3, near "$( PROCS"
(Missing operator before PROCS?)
syntax error at /tmp/foo.pl line 3, near "$( PROCS"
Unterminated <> operator at /tmp/foo.pl line 3.
What is wrong?

Answer 1

This:
my $p = $( PROCS=`echo /proc/[0-9]*|wc -w|tr -d ' '`; read L1 L2 L3 DUMMY < /proc/loadavg ; echo ${L1}:${L2}:${L3}:${PROCS} );
Isn't perl. It's how you'd execute a command in bash.
To run a command in perl you can:
use system.
put your command in backticks
qx (quote-execute): http://perldoc.perl.org/perlop.html#Quote-Like-Operators
However, you're enumerating a directory there, wordcounting, tr-ing and reading. So you don't actually need to do all that using a shell command. And indeed, I'd discourage you from doing so, because that's just a way to make a mess with no productive benefit.
Looks like what you're after as an end result is the 3 load average samples and a count of number of processes. Is that right?
In which case:
my $proc_count = scalar ( () = glob ( "/proc/[0-9]*" ));
open ( my $la, "<", "/proc/loadavg" ) or warn $!;
print join ( ":", split ( /\s+/, <$la> ), $proc_count ),"\n";
Something like that, anyway.

Answer 2

Simply printing a shell command in your Perl script won't actually execute it. You have to tell Perl that it's an external command, which you can do with system:
use strict;
use warnings;
my $command = q{
PROCS=`echo /proc/[0-9]*|wc -w|tr -d ' '`;
read L1 L2 L3 DUMMY < /proc/loadavg;
echo ${L1}:${L2}:${L3}:${PROCS}
};
system($command);
(Note that you should put use strict; use warnings; at the top of every Perl script you write.)
However, it's generally better to use native Perl functionality instead of system. All you're doing is reading from files, which Perl is perfectly capable of doing:
use strict;
use warnings;
use 5.010;
my #procs = glob '/proc/[0-9]*';
my $file = '/proc/loadavg';
open my $fh, '<', $file or die "Failed to open '$file': $!";
my $load = <$fh>;
say(join ':', (split ' ', $load)[0..2], scalar #procs);
Even better might be to use the Proc::ProcessTable module, which provides a consistent interface to the /proc filesystem across different flavors of *nix. It got some bad reviews early on but is supposedly getting bugfixes now; I haven't used it myself but you might take a look.