Ruby undefined method `[]' for nil:NilClass - ruby

Question

I'm trying to make nagios check for BBU (Battery Backup Unit), I have this ruby script called check_bbu
#!/usr/bin/ruby
require 'json'
output = %x{/usr/sbin/storcli /c0/bbu show j}
begin
j = JSON.parse(output)
result = j["Controllers"][0]["Command Status"]["Status"]
status = j["Controllers"][0]["Response Data"]["BBU_Info"][0]["State"]
rescue Exception => e
puts "CRITICAL: error reading BBU status: #{e}"
exit 2
end
if result != 'Success'
puts "CRITICAL: command not successful, result: #{result}"
exit 2
end
if status != 'Optimal'
puts "CRITICAL: BBU not optimal, status is #{status}"
exit 2
end
puts "OK: BBU is optimal"
But when I run this plugin I'm getting following error,
$ ./check_nrpe -u -t 30 -H foo.example.com -c check_bbu
CRITICAL: error reading BBU status: undefined method `[]' for nil:NilClass
What am I doing wrong in this ?

Answer 1

Seems your code break at
j = JSON.parse(output)
result = j["Controllers"][0]["Command Status"]["Status"]
status = j["Controllers"][0]["Response Data"]["BBU_Info"][0]["State"]
you should check which line
j == nil, and your want j["Controllers"]
result = j["Controllers"][0]["Command Status"]["Status"]
status = j["Controllers"][0]["Response Data"]["BBU_Info"][0]["State"]
cause undefined method '[]' for nil:NilClass by display j value first, and make sure json format match your code.
j = JSON.parse(output)
p j # display value
update:
your json format should match your code, otherwise it will raise error.
for example:
{
"Controllers": [
{
"Commandddd Status": { # format not match
"Status": "success"
}
}
]
}
# code and json format not match
j["Controllers"][0]["Command Status"] # nil
j["Controllers"][0]["Command Status"]["Status"]
=> nil["Status"] # boom!

Answer 2

For example to my comment, lets imagine that we have correct JSON:
j = {"Controllers" => [{"Comand Status" => {"status" => 'ok'}}]}
=> {"Controllers"=>[{"Comand Status"=>{"status"=>"ok"}}]}
so by typing your first line, it's should return correct result:
> j["Controllers"][0]["Comand Status"]["status"]
=> "ok"
But also you can get wrong JSON, for example:
j_e = {"Controllers" => []}
so now it's return error:
>j_e["Controllers"][0]["Comand Status"]["status"]
NoMethodError: undefined method `[]' for nil:NilClass
to solve it, you can use something like:
def try data, keys # there we take arguments: data == j, and keys == array with keys
return data if data.nil? # there we check: if data, that we send == nil, for example j = nil, array = ['first', etc..] we should stop this method and return nil as result
value = data[keys.shift] # on this line we try to get some data from j ##keys.shift will delete first element from array that we send to this method and return as `key` for `data`, for example data['Controllers'] so keys now will looks like: [0, 'Comand Status', 'status']
unless keys.empty? #on this line we check if on the pred line we was used the last key and keys now looks like: [] we: *return value and if it's not empty we just **call this method 1 more time
try(value, keys) #**value = [{"Comand Status" => {"status" => 'ok'}}] and keys = [0, 'Comand Status', 'status']
else
value #*nil or value
end
end
j = {"Controllers"=>[{"Comand Status"=>{"status"=>"ok"}}]}
try(j, ['Controllers', 0, 'Comand Status', 'status'])
>'ok'
try j, ['Controllers', 1, 'Comand Status', 'status']
> nil
in you code this should looks like:
require 'json'
def try data, keys
return data if data.nil?
value = data[keys.shift]
unless keys.empty?
try(value, keys)
else
value
end
end
output = %x{/usr/sbin/storcli /c0/bbu show j}
begin
j = JSON.parse(output)
result = try(j, ["Controllers", 0, "Command Status", "Status"])
status = try(j, ["Controllers", 0, "Response Data", "BBU_Info", 0, "State"])
rescue Exception => e
puts "CRITICAL: error reading BBU status: #{e}"
exit 2
end
if result != 'Success'
puts "CRITICAL: command not successful, result: #{result}"
exit 2
end
if status != 'Optimal'
puts "CRITICAL: BBU not optimal, status is #{status}"
exit 2
end
puts "OK: BBU is optimal"
Also, for Ruby 2.3.0+
it's much easier, just:
j.dig("Controllers", 0, "Comand Status", "status")