Grep from within perl script

Question

I'm a bit stuck into an issue on how to declare a grep from within a perl script. What I want to do is let my perl script to execute the following command:

cat config.ini | grep -v "^#" | grep -v "^$"

Normally this expression will clean up / filter all the entries who have # and $ at the beginning and print the variables configured.

However I don't know how to declare it. I have used next expression, but when i get to the point to introduce grep # or $ it fails

system("(cat config.ini| grep ........);

Any suggestion?

Answer 1

cat config.ini | grep -v "^#" | grep -v "^$"

is a poor way of writing

grep -v "^[#$]" config.ini

---

To produce the string

grep -v "^[#$]" config.ini

You can use the string literal

'grep -v "^[#$]" config.ini'

So

system('grep -v "^[#$]" config.ini');
die("Killed by signal ".($? & 0x7F)."\n") if $? & 0x7F;
die("Exited with error ".($? >> 8)."\n") if $? >> 8;

---

system('grep -v "^[#$]" config.ini');

is short for

system('/bin/sh', '-c', 'grep -v "^[#$]" config.ini');

But we don't need the shell, so the following can use instead:

system('grep', '-v', '^[#$]', 'config.ini');
die("Killed by signal ".($? & 0x7F)."\n") if $? & 0x7F;
die("Exited with error ".($? >> 8)."\n") if $? >> 8;

---

But it would be cleaner and more robust to do it in Perl.

open(my $fh, '<', 'config.ini')
   or die($!);

while (<$fh>) {
   print if !/^[#$]/;
}

Answer 2

If you're making external calls to grep from inside a Perl program, then you're doing it wrong. There's nothing that grep can do that Perl can't do for you.

while (<$input_filehandle>) {
  next if /^[#$]/; # Skip comment lines or empty lines.

  # Do something with your data, which is in $_
}

Update: Thinking further about this, I think I'd write it slightly differently.

while (<$input_filehandle>) {
  # Split on comment character - this allows comments to start
  # anywhere on the line.
  my ($line, $comment) = split /#/, $_, 2;

  # Check for non-whitespace characters in the remaining input.
  next unless $line =~ /\S/;

  # Do something with your data, which is in $_
}

Answer 3

print if !(/^#/|/^$/);

I did try using the expression suggested but didn't work as good as this one, is there a way to reduce it or write ir in a better way?