How to read the contents from a file in an array in Ruby - ruby

Question

I want to read a file which has name value pair on a remote server. As per the requirement I need to shell into the remote server, read the file, and then grep for the values. Example:
/domain/srvr/primary = ABC
/host/DEF/second = DEF
/host/XYZ/second = XYZ
/host/GHI/second = GHI
:
:
:
Now I want to read this file and make an array of all secondary servers (ex: DEF, XYZ, GHI) but I am getting nil value.
primary = #ssh.exec!("cd /home/dir; grep 'srvr/primary' #{filename} | awk '{print $3}'")
secondary = #ssh.exec!("cd /home/dir; grep '\<host.*second\>' #{filename} | awk '{print $3}'")`
It prints the primary server name properly but returns nil for secondary servers array. I tried to use split("\n") but errors out by saying undefined method 'split' for nil:NilClass.
Need help in getting all the secondary servers in an array.

Answer 1

You can use something like this
file_contents.
split("\n").
map {|line| line.split(" = ") }.
find_all {|pair| pair[0] =~ /second$/ }.
map(&:last)
and probably get file contents with cat or downloading the file through ssh. If in the same server just use File.read.
If you can only use bash or just prefer to, you can use
grep -p "^\/host.*second" path/to/file | cut -d" " -f 3
The -p option will enable perl regexp syntax on grep which will give you all the capabilities you want to search through the file. Then cut will split each string by the delimiter -d ang get the field of 1 based index -f. In this case string = server, so server is on the third field.

Answer 2

Instead of trying to do the work on the remote host, you can simplify the task and only grab the data and process it locally. You can use Net::SCP (docs), Net::FTP or Net::SFTP to easily retrieve the data.
I'd use something like this to grab the desired data once the text has been received:
data = <<EOT
/domain/srvr/primary = ABC
/host/DEF/second = DEF
/host/XYZ/second = XYZ
/host/GHI/second = GHI
EOT
data.split("\n").grep(/\bsecond\b/).map{ |l| l.split.last }
# => ["DEF", "XYZ", "GHI"]
or:
data.split("\n").grep(/\bsecond\b/).map{ |l| l[/= (\S+)/, 1] }
# => ["DEF", "XYZ", "GHI"]
or:
data.split("\n").grep(/\bsecond\b/).map{ |l| l.rstrip[/\S+$/] }
# => ["DEF", "XYZ", "GHI"]
Just to make it more interesting:
require 'fruity'
data = <<EOT
/domain/srvr/primary = ABC
/host/DEF/second = DEF
/host/XYZ/second = XYZ
/host/GHI/second = GHI
EOT
compare do
p1 { data.split("\n").grep(/\bsecond\b/).map{ |l| l[/= (\S+)/, 1] } }
p2 { data.split("\n").grep(/\bsecond\b/).map{ |l| l.rstrip[/\S+$/] } }
p3 { data.split("\n").grep(/\bsecond\b/).map{ |l| l.rstrip[/\S+$/] } }
end
# >> Running each test 256 times. Test will take about 1 second.
# >> p1 is faster than p2 by 3.9x ± 0.01
# >> p2 is similar to p3