Daemon vs process - linux

I am very new to linux, so I can't quite understand the difference between daemon, foreground process and background process.
As I understand that:
Daemon is simply a background process that runs in the background and has init as its parent process.
Foreground process is a process that we simply invoke from the console.
Then if I run for example nginx inside a docker container with "daemon off" flag that means that nginx will be the foreground process running in the container's console?

What is the problem exactly?
Read this.
To start a container in detached mode, you use -d=true or just -d option.
In foreground mode (the default when -d is not specified), docker run can start the process in the container and attach the console to the process’s standard input, output, and standard error.

Related

PID 1 for application automatically restarted in bash loop

Given is a microservice that after some time needs to quit itself. This is no error condition but (in that case) normal behavior.
After exiting it should be automatically restarted.
So currently I have a script run_app.sh:
#!/usr/bin/env bash
while true; do ./app ; done
And in the Dockerfile (inheriting FROM ubuntu:16.04) I run it like this:
CMD ["./run_app.sh"]
It "works" but since app does not have PID 1, it does not receive SIGTERM etc. coming from Kubernetes, which is needed for a graceful shutdown during rolling updates etc.
Using while true; do exec ./app ; done in run_app.sh does not solve the problem since the loop no longer exists when app finished and no restart is performed.
How can I restart the app automatically inside the container without restarting the container / pod every time it quits, but still have the advantages of PID 1?
Well, your loop does restart your app, so this is not your problem. Your problem is that the signal sent to the docker container is not propagated into the container. Docker just isn't (AFAIK) not meant to be used like this, so it doesn't propagate signals to its app.
You have two ways of handling this:
You can teach the signal sender (Kubernetes or whatever) to instead of sending a signal to the Docker container do something more elaborate to get the information to the app inside the container. I guess this is not easy (if possible at all).
You can migrate the looping shell script outside of the container and let Kubernetes (or whatever) run this script instead. Within the loop you then can start the Docker container with your app inside. In this case you will need to catch the SIGTERM in the outside looping shell script (help trap) and either send a SIGTERM to the Docker container directly or to the app within the Docker container (using Docker exec or similar).

Closing the terminal should kill the associated process with the terminal

I have packaged a Debian file of our software. Now there is a .sh script that needs to be started to run the program/software. This .sh script actually runs a Django server and few more services.
To actually start this application, we need to run the .desktop file in the menu. This .desktop file in the menu is associated with the .sh script mentioned above. This prompts the terminal and asks for the password. Once the password is given, this will start the services and the terminal stays active.
To close this service completely, we need to kill the process by finding the PID of the process and killing it from the terminal. But now I want to kill this process when I close the terminal.
How can I do that?
If you are trying to create a service (some program that runs in the background), you should create use your system's mechanism for this.
The traditional one, would be a scrip in /etc/init.d/, a more modern approach is to use systemd.
E.g. a file /etc/systemd/system/myservice.system
[Unit]
Description=My Service
[Service]
Type=simple
# you could run the service as a special user
#User=specialuser
WorkingDirectory=/var/lib/myservice/
# execute this before starting the actual script
#ExecStartPre=/usr/lib/myservice/bin/prestart.sh
ExecStart=/usr/bin/myservice
Restart=on-failure
[Install]
WantedBy=multi-user.target
You can then start/stop the service (as root) using:
systemctl start myservice
resp.
systemctl stop myservice
You can have dependency chains of services, so starting myservice will automatically start myhelper1 and myhelper2.
Checkout the manpage systemd.unit.5
When the controlling terminal is closed, the foreground process group should receive a SIGHUP signal. If your target process is already expected to be in the foreground, then it may be that it is catching or ignoring SIGHUP (the default behavior for a process receiving that signal is to terminate).

docker stop spark container from exiting

I know docker only listens to pid 1 and in case that pid exits (or turns into a daemon) it thinks the program exited and the container is shut down.
When apache-spark is started the ./start-master.sh script how can I kept the container running?
I do not think: while true; do sleep 1000; done is an appropriate solution.
E.g. I used command: sbin/start-master.sh to start the master. But it keeps shutting down.
How to keep it running when started with docker-compose?
As mentioned in "Use of Supervisor in docker", you could use phusion/baseimage-docker as a base image in which you can register scripts as "services".
The my_init script included in that image will take care of the exit signals management.
And the processes launched by start-master.sh would still be running.
Again, that supposes you are building your apache-spark image starting from phusion/baseimage-docker.
As commented by thaJeztah, using an existing image works too: gettyimages/spark/~/dockerfile/. Its default CMD will keep the container running.
Both options are cleaner than relying on a tail -f trick, which won't handle the kill/exit signals gracefully.
Here is another solution.
Create a file spark-env.sh with the following contents and copy it into the spark conf directory.
SPARK_NO_DAEMONIZE=true
If your CMD in the Dockerfile looks like this:
CMD ["/spark/sbin/start-master.sh"]
the container will not exit.
tail -f -n 50 /path/to/spark/logfile
This will keep the container alive and also provide useful info if you run -it interactive mode. You can run -d detached and it will stay alive.

Docker not responding to CTRL+C in terminal

Having an issue with Docker at the moment; I'm using it to run an image that launches an ipython notebook on startup. I'm looking to make some edits to ipython notebook itself, so I need to close it after launch.
However, hitting CTRL+C in the terminal just inputs "^C" as a string. There seems to be no real way of using CTRL+C to actually close the ipython notebook instance.
Would anyone have any clues as to what can cause this, or know of any solutions for it?
Most likely the container image you use is not handling process signals properly.
If you are authoring the image then change it as Roland Webers' answer suggests.
Otherwise try to run it with --init.
docker run -it --init ....
This fixes Ctrl+C for me.
Source: https://docs.docker.com/v17.09/engine/reference/run/#specify-an-init-process
The problem is that Ctrl-C sends a signal to the top-level process inside the container, but that process doesn't necessarily react as you would expect. The top-level process has ID 1 inside the container, which means that it doesn't get the default signal handlers that processes usually have. If the top-level process is a shell, then it can receive the signal through its own handler, but doesn't forward it to the command that is executed within the shell. Details are explained here. In both cases, the docker container acts as if it simply ignores Ctrl-C.
If you're building your own images, the solution is to run a minimal init process, such as tini or dumb-init, as the top-level process inside the container.
This post proposes CTRL-Z as a workaround for sending the process to background and then killing the process by its process id:
Cannot kill Python script with Ctrl-C
Possible problems:
The program catches ctrl-c and does nothing, very unlikely.
There are background processes that are not managed correctly. Only the main process receives the signal and sub-processes hang. Very likely what's happening.
Proposed Solution:
Check the programs documentation on how it's properly started and stopped. ctrl-c seems not to be the proper way.
Wrap the program with a docker-entrypoint.sh bash script that blocks the container process and is able to catch ctrl-c. This bash example should help: https://rimuhosting.com/knowledgebase/linux/misc/trapping-ctrl-c-in-bash
After catching ctrl-c invoke the proper shutdown method for ipython notebook.
From this post on the Docker message boards:
Open a new shell and execute
$ docker ps # get the id of the running container
$ docker stop <container> # kill it (gracefully)
This worked well for me. CTRL-Z, CTRL-\, etc. only came up as strings, but this killed the Docker container and returned the tab to terminal input.
#maybeg's answer already explains very well why this might be happening.
Regarding stopping the unresponsive container, another solution is to simply issue a docker stop <container-id> in another terminal. As opposed to CTRL-C, docker stop does not send a SIGINT but a SIGTERM signal, to which the process might react differently.
Usage: docker stop [OPTIONS] CONTAINER [CONTAINER...]
Stop a running container by sending SIGTERM and then SIGKILL after a grace period
If that fails, use docker kill <container-id> which sends a SIGKILL immediately.

How to attach an running process to an existing screen in linux

I wish to debug a process which is already running using screen command.
Is there a way by which i can attach my running process with (some PID) to an existing screen (screen_name) ?
No, you can not use screen to debug a process running as daemon. If you would want to debug a process, shut off the daemon mode and run it in screen. and use
screen -rd {screen_ID}

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