Why is getenv ("HOME") equal to nil in lsyncd.conf?

Question

I am trying to configure lsyncd to synchronize a folder on the home page of the logged user but when I try to capture the value of his $HOME with os.getenv("HOME") the result is always

Error prepare /etc/lsyncd/lsyncd.conf.lua: /etc/lsyncd/lsyncd.conf.lua: 14: attempt to concatenate a nil value

I tried os.getenv("PWD") and it runs without displaying errors but it doesn't work and in the /var/log/lsyncd/lsyncd.status file I see that it tries to use the /.mozilla/firefox address as if PWD were empty.

I tried to keep the environment variables by running the command sudo -E service lsyncd restart and modifying sudoers with Defaults env_keep +="HOME", all in vain.
Any ideas?

I enclose my code:

settings{
    logfile = "/var/log/lsyncd/lsyncd.log",
    statusFile = "/var/log/lsyncd/lsyncd.status",
}

sync{
        default.rsync,
        source =  os.getenv("HOME").."/.mozilla/firefox/",
        target =  "tmp/.mozilla/firefox/",
        delay  = 1,
}

Answer 1

Linux does not have a concept of “the logged-in user”. It has a concept of a logged-in user, but there can be many of them at the same time.

You're running a system service, through systemd. Telling sudo to preserve the HOME environment variable means that when you run sudo -E service lsyncd restart, the service command runs with HOME set to the home directory of the user who called sudo. But that has nothing to do with the value of HOME for the service. Systemd does not set the environment of a service based on the environment of the administrative command to start it, they're based on the service configuration.

If you want to synchronize a specific user's files, then hard-code the path to that user's home directory, or use getpwuid or getpwnam to look up the home directory of a user.

If you want to synchronize the files of whichever user is logged in, then don't use a system service. Instead, run lsyncd as part of that user's login session.

Answer 2

Thank you Gilles for you answer. Unfortunately getpwuid and getpwnam returned "nil"values, equivalent to the null de lua. The solution I have developed allows me to run lsyncd in a multi-user and multi-device environment with synchronization with active directory, so that I can save the firefox markers, google chrome, svn settings, filezilla, history... in the active directory and recover them on any computer for any user. This is complicated by the problem of compatibility with symbolic links between the samba dialect, cifs, which uses active directory and linux. My solution is as follows:(just for Firefox by saving space but can be extended to any software or folder. config.)

1- I install lsyncd and add that scrip in /etc/lsyncd/lsyncd.conf.lua (The script looks for the home of the user who has just logged on to the /tmp/home file and uses it to create the url with the source and target to synchronize.)

function file_exists(file)
  local f = io.open(file, "rb")
  if f then f:close() end
  return f ~= nil
end

-- get all lines from a file, returns an empty 
-- list/table if the file does not exist
function lines_from(file)
  if not file_exists(file) then return {} end
  lines = {}
  for line in io.lines(file) do
    lines[#lines + 1] = line
  end
  return lines
end

-- tests the functions above
local file = '/tmp/lua'
local lines = lines_from(file)

-- print all line numbers and their contents
for k,v in pairs(lines) do
  print('line[' .. k .. ']', v)

end
--save line 1 in home
home = lines[1]


settings{

    logfile = "/var/log/lsyncd/lsyncd.log",
    statusFile = "/var/log/lsyncd/lsyncd.status",
}

sync{

        default.rsync,
-- Add home to url of the source
        source =  home.."/.mozilla/firefox/",
--Add home to url of the target
        target =  home.."/box/.mozilla/firefox/",
        delay  = 1,
        rsync={
                perms = true,
                owner = true,


        },
}

2- With visudo I allow the users I want, in my case all of them, to run lsyncd without password with sudo command: ALL ALL=NOPASSWD:SETENV: /usr/sbin/service lsyncd *

3- I add the following lines in /etc/profile file

//Delete .mozilla in user's local home in start
rm -Rf $HOME/.mozilla
#mkdir -p $HOME/box/.mozilla/Firefox
//Box is the folder shared with the server
mkdir -p $HOME/box/.config
rm -rf $HOME/box/.config/caja
//2> /dev/null prevents an error due to any other cause from being shown to the user and prevents the user from starting up.
cp -rf $HOME/box/.mozilla  $HOME/  2> /dev/null
cp -rf $HOME/box/.config $HOME/ 2> /dev/null
cp -rf $HOME/box/.subversion $HOME/ 2> /dev/null
echo $HOME >> /tmp/lua;
sudo service lsyncd restart; rm -r /tmp/lua