Strange io.popen behaviour in lua - nginx

Question

I have lua block in nginx config, which checks for redis-server connection and if 4 java-processes are up, and then returns 200 or 500 status accordingly to this checks.
location = /healthcheck {
content_by_lua_block {
local redis = require "resty.redis"
local red = redis:new()
red:set_timeout(1000)
local ok, err = red:connect("127.0.0.1", 6379)
if not ok then
ngx.status = 500
return
end
local ps = io.popen("/bin/ps aux |grep -c '[j]ava.*63'")
local output = tostring(ps:read('*a'))
ps:close()
if string.match(output, '4') then
ngx.status = 200
else
ngx.status = 500
end
}
}
But periodically output variable takes nil value, while it shouldn't. Just can't get why it happens.
Thanks in advance, comrades.
UPD:
Using tonumber fails on
bad argument #2 to 'tonumber' (number expected, got string)
Updated location config:
location = /healthcheck {
content_by_lua_block {
local redis = require "resty.redis"
local red = redis:new()
red:set_timeout(1000)
local ok, err = red:connect("127.0.0.1", 6379)
if not ok then
ngx.status = 500
return
end
local ps = io.popen("/bin/ps aux |grep -c '[j]ava.*63'")
local output = tonumber(ps:read())
ps:close()
if (output == 4) then
ngx.status = 200
else
ngx.status = 500
end
}
}
UPD2:
Logging to nginx error (with tostring) prints next:
grep: write error: Broken pipe
2016/04/19 17:54:48 [error] 301#0: *5 [lua] content_by_lua(healthcheck.conf:33):20: OUTPUT:nil
UPD3:
Using command grep -c '[j]ava.*63' <(/bin/ps aux) to avoid the use of pipe:
local ps = io.popen("grep -c '[j]ava.*63' <(/bin/ps aux)")
getting next error:
sh: -c: line 0: syntax error near unexpected token `('
sh: -c: line 0: `grep -c '[j]ava.*63' <(/bin/ps aux)'

Answer 1

According to this answer and some testing next config do the job:
location = /healthcheck {
content_by_lua_block {
local redis = require "resty.redis"
local red = redis:new()
red:set_timeout(1000)
local ok, err = red:connect("127.0.0.1", 6379)
if not ok then
ngx.status = 500
return
end
local ps = io.popen("/bin/ps aux >/tmp/ps.out", "w")
ps:close()
local ps = io.popen("grep -c '[j]ava.*63' /tmp/ps.out")
local output = tostring(ps:read())
ps:close()
if string.match(output, '4') then
ngx.status = 200
else
ngx.status = 500
end
}
}