awk command in Perl's system does not work - perl

Question

I am writing a small Perl script that executes an Awk command :
I try to swap two columns in a file, the file is like this :
domain1,ip1
domain2,ip2
domain3,ip3
the result should be
ip1,domain1
ip2,domain2
ip3,domain3
The Perl command invoking awk is like this:
system("ssh -p 22 root\#$mainip 'awk -F, '{print $2,$1}' OFS=, /root/archive/ipdomain.txt > /root/ipdom.txt'");
This is the error I get :
awk: cmd. line:1: {print
awk: cmd. line:1: ^ unexpected newline or end of string
any suggestions, please?

Answer 1

With the layered commands and all that multi-level quoting and escaping that need be done right,† no wonder it fails. A complex command like that will always be tricky, but libraries help a lot.
A properly quoted string to run through a shell can be formed with String::ShellQuote ‡
use warnings;
use strict;
use feature 'say';
use String::ShellQuote qw(shell_quote);
die "Usage: $0 file outfile\n" if #ARGV != 2;
my ($file, $out) = #ARGV;
my #cmd_words =
( 'ssh', 'hostname', 'awk', q('{print $2 $1}'), $file, '>', $out );
my $cmd = shell_quote #cmd_words;
system($cmd);
Note how the q() operator from of single quotes enables us to pass single quotes nicely.
This swaps the first two words on each line of a file and prints them, using awk, and redirects the output to a file, on a remote host. It works as expected in my tests (with a real hostname). Please adjust as needed.
Another possible improvement would be to use a library for ssh, like Net::OpenSSH.
A complete command, as the one in the question, to use in the above program
my #cmd_words = (
'ssh', '-p', '22', "root\#$mainip",
'awk', '-F,', q('{print $2,$1}'), 'OFS=,', $file, '>', $out );
Tested with a file from the question.
The makeVoiceBot answer is informative and it got half way there but I find the need for
system("ssh hostname \"awk '{print \\\$2 \\\$1}' $path\"");
This works in my tests (on systems I ssh to). I try to avoid needing to deal with such quoting and escaping.
† This is a shell command which runs ssh, and then executes a command on the remote system which runs a shell (there) as well, in order to run awk and redirect its output to a file.
A bit more than an "awk command" as the title says.
‡ The library can prepare a command for bash (as of this writing), but one can look at the source for it and adjust it for their own shell, at least. There is also Win32::ShellQuote

Answer 2

I am using a shortened example here
system("ssh localhost 'awk '{print $2,$1}' file.txt'")
system() sees:
ssh localhost 'awk '{print $2,$1}' file.txt'
local shell expands:
ssh
localhost
awk
{print
$2,$1}
file.txt
local shell replaces $1 and $2 (positional args) with empty strings:
ssh
localhost
awk
{print
,}
file.txt
ssh executes:
ssh localhost awk {print ,} file.txt
remote shell gets:
awk
{print
,}
file.txt
So the remote shell runs awk with {print as its program argument, resulting in the described error. To prevent this, the invocation of system() can be changed to;
system("ssh localhost \"awk '{print \$2,\$1}' file.txt\"")
system() sees:
ssh localhost "awk '{print \$2,\$1}' file.txt"
local shell expands:
ssh
localhost
awk '{print \$2,\$1}' file.txt
ssh executes
ssh localhost awk '{print \$2,\$1}' file.txt
remote shell gets
awk
{print \$2,\$1}
file.txt
remote shell expands \ escapes
awk
{print $2,$1}
file.txt
Remote awk now gets {print $2,$1} as its program argument, and executes successfully.