Question

I just want to use a awk command in my perl script like follows:

$linum = `awk -F "%" '/^\s*kernel/{print NR}' < $grubFile`;

But it will say: Unrecognized escape \s passed through at ./root line 36.
How do I avoid that? thank you.

Answer 1

$x = `... \s ...`;

makes no more sense than

$x = "... \s ...";

If you want the two characters \ and s, you need to escape the \ in double-quoted literals and similar. Just like you’d use

$x = "... \\s ...";

you need to use

$x = `... \\s ...`;

Note that you completely fail to properly escape the contents of $grubFile too. Your command will fail if the file name contains spaces. And consider what could happen instead if it contain other character special to shells, such as |.

As @ysth showed, the following is equivalent to your command:

awk -F% '/^\s*kernel/{print NR}' "$grubFile"

Getting rid of the input redirection means you could simply use

use IPC::System::Simple qw( capturex );
my @line_nums = capturex('awk', '-F%', '/^\s*kernel/{print NR}', $grubFile);
chomp @line_nums;

By the way, it’s not that hard to do it purely with Perl.

my @line_nums;
open(my $fh, '<', $grubFile) or die $!;
while (<$fh>) { 
   push @line_nums, $. if /^\s*kernel/;
}