Question

I currently have the following lines of code in a script:

set -A ARRAY OPTION1 OPTION2 OPTION3 OPTION4
          set -A matches

          for  OPTION in  ${ARRAY[@]}; do
                  DIFF=$(ssh $USER@$host  " diff $PERSONALCONF $PRESETS$OPTION" )
                  if [[ $DIFF == "" ]]; then
                          set -A matches"${matches[@]}" $OPTION
                  fi
          done

Basically, I have a loop that goes through each element in a pre-defined array, connects to a remote server (same server each time), and then compares a file with a file as defined by the loop using the diff command. Basically, it compares a personal.conf file with personal.conf.option1, personal.conf.option2, etc. If there is no difference, it adds it to the array. If there is a difference, nothing happens.

I was wondering if its possible to execute this or get the same result (storing the matching files in an array ON THE HOST MACHINE, not the server that’s being connected to) by way of only connecting once via SSH. I cannot store anything on the remote server, nor can I execute a remote script on that server. I can only issue commands via ssh (kind of a goofy setup). Currently, it connects as many times as there are options. This seems inefficient. If anyone has a better solution I’d love to hear it.

Answer 1

Several options:

• You can use OpenSSH multiplexing feature (see ssh(1)).

• Also, most shells will gladly accept a script to run over stdin, so you could just run something like

  cat script.sh | ssh $HOST /bin/sh
  

• Most scripting languages (Perl, Python, Ruby, etc.) have some SSH module that allows connection reuse:

  #!/usr/bin/perl
  use Net::OpenSSH;
  my ($user, $host) = (...);
  my @options = (...);
  my @matches;
  my $ssh = Net::OpenSSH->new("$user\@$host");  
  for my $option (@options) {
      my $diff = $ssh->capture("diff $personal_conf $presets$option");
      if ($ssh->error) {
          warn "command failed: " . $ssh->error;
      }
      else {
          push @matches, $option if $diff eq '';
      }
  }
  print "@matches\n";