Started an application through SSH, command line now gone, what happens next?

Question

Context: This is a very basic question

Using Putty and SSH for the first time to do some serious server setup and run into the situation where I have started a process that I do not want to stop.

The process is the gunicorn WSGI HTTP Server (running on Centos 6.3).

The command I used to start the process is (as per their Quick Start):

gunicorn -w 4 myapp:app

At this point in the work session, I have lost the command prompt. This must be such a non-issue that it doesn't even enter into an experienced user's consciousness. But unfortunately at my level of experience, I am left with several fundamental questions:

• Does the fact that I have lost the command prompt mean that the process is still running?
• How do I get back to the command prompt without killing the process?
• How do I come back and monitor the process later?
• How do I eventually kill the process?

Any help is appreciated, thanks so much!

Answer 1

1) Yes, the process is running.

2) You can start the process in the background by starting it as

gunicorn -w 4 myapp:app &

3) You can find the process ID (pid) with

ps aux | grep gunicorn

4) To kill the process, you can kill it with

kill (the pid you found in 3)

To force the process to stop, run

kill -9 (the pid you found in 3)

Answer 2

Does the fact that I have lost the command prompt mean that the process is still running?

Unlikely. The process will have a connection to the virtual terminal of ssh, which will eventually time out and kill all the processes associated with that terminal - though gunicorn could be written in a manner that it doesn't exit when the controlling terminal disappears.

How do I get back to the command prompt without killing the process?

If the terminal is still alive, hit CTRL+Z to get back to the command prompt, then write bg to place gnuicorn in the background.(and ùse fg to get back to the process so you can kill it e.g. with CTRL+CStarting it as eithergunicorn -w 4 myapp:app &will place the process in the background of the shell immediately - though depending on the server it might still get killed when your ssh session ends. Runningnohup gunicorn -w 4 myapp:app &will ensure the process to run even when you disconnect. Read the man page ofnohup`

Another option is to run the process in a terminal that allows you to reattach to it. That's what the GNU screen or the tmux allows you to do - effectivly allowing you to disconnect the ssh session, connect back later on and reattach to an existing terminal session.

How do I come back and monitor the process later?

You can't, unless you run the process inside a GNU screen or tmux session - in which case the docs/tutorials for screen or tmux will tell you the detail - or you've started the server to run as a service/in the background.

How do I eventually kill the process?

Most server programs will have a managment interface (e.g. commands you'll have to run), and you'll have to find the relevant information in the documentation.

Or they integrate themselves in the linux/unix startup and service management procedures, in which case you manage them as any other services , e.g. /sbin/service fooserver start /sbin/service fooserver stop on some linux distros.

Or you have to do it manually. Find the process by running ps -ef |grep fooserver to find its pid, and kill it, kill <the pid>. Or look in the documentation if the erver can write a "pid file" when it starts up, so you can find the process id in that file later on.

Now, it seems at least gunicorn has a -D argument, which is used to place the server in the background, disconnecting it from the terminal, so it doesn't get killed when your ssh/putty connection disconnects. See http://gunicorn.org/configure.html#server-mechanics

You'll then have to manage/monitor it manually, i.e. kill it as I mentioned above, monitor it through the log file it produces - or any integrated monitoring webapp or commands gnuicorn might have.

The intent here is clearly for someone to package gnuicorn for a particular linux/*nix variant, and write the relevant scripts and config files to integrate it into the native service management of the distro. (e.g. a standard script under /etc/init.d/ to start and stop the server, used on many linux'es)

Answer 3

Read up on nohup(1) command for starting background processes and being able to disconnect your terminal session.

Answer 4

Going forward, you may find the gnu screen are quite helpful while working on a terminal like putty.Or even the better alternative for gnu screen -- tmux, which gets more and more popular now due to its powerful, consistent, well-documented command interfaces.

Answer 5

You could use

nohup gunicorn -w 4 myapp:app &> /dev/null &

This will

• execute gunicorn -w 4
• send the process to background - that's the & at the end. You can view background jobs via jobs and bring them to foreground via fg $id if they e.g. wait for interaction
• mute the process by redirecting standard error and standard output to /dev/null, so it won't show in your shell &> /dev/null
• make sure that the process will not be killed if the parent (here your shell) ends. nohup

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EDIT:

To monitor the program as whole it would be fine to monitor output of the program.

So we can modify the command in this manner:

• duplicate standard error output to standard output stream 2>&1. More on this construction can be found here: What does “2>&1” do in command line?
• redirect standard output stream to our file >~/gunicorn.log

The final command will look like this:

nohup gunicorn -w 4 myapp:app 2>&1 >~/gunicorn.log &

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To kill the process you could fire series of these commands:

ps aux | grep gunicorn

then identify the PID (I would refer to it as $PID) of the gunicorn process and stop it gracefully:

kill $PID

or stop it forcefully:

kill -9 $PID

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You could customize it to your further needs.