Question

Okay, so I am new to Ruby and I have a strong background in bash/ksh/sh.

What I am trying to do is use a simple for loop to run a command across several servers. In bash I would do it like:

for SERVER in `cat etc/SERVER_LIST`
do
    ssh -q ${SERVER} "ls -l /etc"
done

etc/SERVER_LIST is just a file that looks like:

server1
server2
server3
etc

I can’t seem to get this right in Ruby. This is what I have so far:

 #!/usr/bin/ruby
### SSH testing
#
#

require 'net/ssh'

File.open("etc/SERVER_LIST") do |f|
        f.each_line do |line|
                Net::SSH.start(line, 'andex') do |ssh|
                        result = ssh.exec!("ls -l")
                        puts result
                end
        end
end

I’m getting these errors now:

andex@master:~/sysauto> ./ssh2.rb
/usr/lib64/ruby/gems/1.8/gems/net-ssh-2.0.23/lib/net/ssh/transport/session.rb:65:in `initialize': newline at the end of hostname (SocketError)
        from /usr/lib64/ruby/gems/1.8/gems/net-ssh-2.0.23/lib/net/ssh/transport/session.rb:65:in `open'
        from /usr/lib64/ruby/gems/1.8/gems/net-ssh-2.0.23/lib/net/ssh/transport/session.rb:65:in `initialize'
        from /usr/lib64/ruby/1.8/timeout.rb:53:in `timeout'
        from /usr/lib64/ruby/1.8/timeout.rb:93:in `timeout'
        from /usr/lib64/ruby/gems/1.8/gems/net-ssh-2.0.23/lib/net/ssh/transport/session.rb:65:in `initialize'
        from /usr/lib64/ruby/gems/1.8/gems/net-ssh-2.0.23/lib/net/ssh.rb:179:in `new'
        from /usr/lib64/ruby/gems/1.8/gems/net-ssh-2.0.23/lib/net/ssh.rb:179:in `start'
        from ./ssh2.rb:10
        from ./ssh2.rb:9:in `each_line'
        from ./ssh2.rb:9
        from ./ssh2.rb:8:in `open'
        from ./ssh2.rb:8

The file is sourced correctly, I am using the relative path, as I am sitting in the directory under etc/ (not /etc, I’m running this out of a scripting directory where I keep the file in a subdirectory called etc.)

Answer 1

File.open("/etc/SERVER_LIST", "r") do |file_handle|
  file_handle.each_line do |server|
    # do stuff to server here
  end
end

The first line opens the file for reading and immediately goes into a block. (The block is the code between do and end. You can also surround blocks with just { and }. The rule of thumb is do..end for multi-line blocks and {...} for single-line blocks.) Blocks are very common in Ruby. Far more idiomatic than a while or for loop.) The call to open receives the filehandle automatically, and you give it a name in the pipes.

Once you have a hold of that, so to speak, you can call each_line on it, and iterate over it as if it were an array. Again, each iteration automatically passes you a line, which you call what you like in the pipes.

The nice thing about this method is that it saves you the trouble of closing the file when you’re finished with it. A file opened this way will automatically get closed as you leave the outer block.

One other thing: The file is almost certainly named /etc/SERVER_LIST. You need the initial / to indicate the root of the file system (unless you are intentionally using a relative value for the path to the file, which I doubt). That alone may have kept you from getting the file open.

Update for new error: Net::SSH is barfing up over the newline. Where you have this:

 Net::SSH.start(line, 'andex') do |ssh|

make it this:

 Net::SSH.start(line.chomp, 'andex') do |ssh|

The chomp method removes any final newline character from a string.