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I have a perl script from within which I need to execute a simple perl one-liner on a remote host:

ssh 192.168.1.1 "perl -pi.bup -e 's/^(\s+?kernel)(.*)(?<!audit=1)$/$1$2 audit=1/' /etc/grub.conf"

This simply adds "audit=1" to the end of each kernel line in /etc/grub.conf if it does not already exist.

The one-liner works just fine when run directly on the host, but not when executed via ssh from within another perl script. I have tried to escape all dollar signs with one or more backslashes and I have also tried to escape the backslash in "\s", but nothing I do seems to work.

Note I do not want to copy a script to the remote host and then execute it - I would like to do it using the ssh command directly.

How to properly escape this so that it works?

-- Update 9/9/2015 to show exactly what I am doing in the perl script:

sub SomeMethod { &RunCommand($host, "perl -pi.bup -e \'s/^(\s+?kernel)(.*)(?<!audit=1)\$/\$1\$2 audit=1/\' /etc/grub.conf"); } sub RunCommand { my ($server, $command) = @_; my $commandOutput = ""; if ($server ne "") { $command = "ssh $server \"$command\""; } $commandOutput = `$command`; print $commandOutput; if (($? >> 8) != 0) { &LogMessage ("$command failed:\n\n$commandOutput"); return $commandOutput; } return $commandOutput; }

-- Update #2, using system instead of back-ticks:

system 'ssh', $host, 'perl', '-pi.bup', '-e', 's/^(\s+?kernel)(.*)(?<!audit=1)$/$1$2 audit=1/', '/etc/grub.conf'; bash: -c: line 0: syntax error near unexpected token `(' bash: -c: line 0: `perl -pi.bup -e s/^(\s+?kernel)(.*)(?<!audit=1)$/$1$2 audit=1/ /etc/grub.conf'

Ok, I can use system here, but how to properly escape it??

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  • @c4f4t0r sed vs. perl is probably not the issue here - it is more a matter of properly escaping the needed characters. But if you can provide a sed command that will work when executed via ssh (as in my post) from within a perl script, I will happily use that instead. Commented Sep 9, 2015 at 21:17
  • Show exactly what you're doing in the perl script. Commented Sep 9, 2015 at 21:22
  • @glenn Ok, code added to post. Commented Sep 9, 2015 at 21:34
  • In my previous an missed one \ Commented Sep 9, 2015 at 21:55
  • Less related to your question, you may also want to look into the use of grubby, if that is an option for you. You may find it easier to add/remove/change grub options. Sorry, I know that isn't related to your question. Commented Sep 9, 2015 at 22:44

3 Answers 3

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Since you're editing the remote file in place, you don't need to capture the output, so system would be preferable than using backticks:

system 'ssh', '192.168.1.1', 'perl', '-pi.bup', '-e', 's/^(\s+?kernel)(.*)(?<!audit=1)$/$1$2 audit=1/', '/etc/grub.conf';

Note that you don't have to pass the remote command as a single parameter to ssh. That helps the quoting a bit. If you really want to, you can:

system 'ssh', '192.168.1.1', q{perl -pi.bup -e 's/^(\s+?kernel)(.*)(?<!audit=1)$/$1$2 audit=1/' /etc/grub.conf};

Using the q{} operator to allow the perl one-liner to use single quotes.

I would do this, actually:

use Try::Tiny; use IPC::System::Simple qw{capture}; sub SomeMethod { my $output = RemoteCommand( $host, q{perl -pi.bup -e 's/^(\s+?kernel)(.*)(?<!audit=1)$/$1$2 audit=1/' /etc/grub.conf} ); } sub RemoteCommand { my ($server, $command) = @_; my $output; try { $output = capture('ssh', $server, $command); } catch { LogMessage("command failed: ($command) : $_"); $output = $_; }; return $output; }
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  • The RunCommand method is used by a large number of methods and I would like to not modify it, if possible (not to break other functionality). I was hoping for some guidance on how to escape the command in my original post. Commented Sep 9, 2015 at 21:38
  • Since backticks execute your command with sh -c 'your command', it will be very difficult to get the escaping right. I'd not bother. Make your implementation more robust. Commented Sep 9, 2015 at 21:48
  • Ok, I tried using "system" instead - please see my edit for the result. Still needs some escaping and I still have a hard time making it work... Commented Sep 9, 2015 at 22:22
  • system is a perl function, not a shell command. Commented Sep 10, 2015 at 2:02
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I finally got it working.

From bash directly:

echo 's/^(\s+?kernel)(.*)(?<!audit=1)$/$1$2 audit=1/' | ssh 192.168.1.1 "perl -pi.bup - /etc/grub.conf"

From perl script:

my $command = "echo 's/^(\\s+?kernel)(.*)(?<!audit=1)\$/\$1\$2 audit=1/' | ssh $_wsAddress 'perl -pi.bup - /etc/grub.conf' "; &RunCommand("", $command);
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I tried with the following simple command and it works

ssh localhost "sed -i.bck '/[^audit]/{s/\(^\s\+kernel.*\)/\1 audit=1/g}' /boot/grub/menu.lst"

Isn't about sed vs perl, but using the previous command is more clear to archive what you want.

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  • that almost works (provided that you use "\\1" instead of "\1") directly from bash. But it is only supposed to append "audit=1" if it is not already there - as it is now it appends it always. Also, it is only supposed to affect lines that start with the word kernel (preceeding spaces ok). I tried this, but it did not update the file: ssh localhost "sed -i.bck 's/\^(\s\+kernel.*)(?<!audit=1)\$/\\1 audit=1/g' /etc/grub.conf ; cat /etc/grub.conf" Commented Sep 9, 2015 at 22:13
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    @Zek I updated my sed command, Now the command works only when the line kernel doesn't have audit key word Commented Sep 10, 2015 at 0:08

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