Ubuntu – Running programs in the background from terminal

bashcommand lineconsoleprocess

How do I run a program in the background of a shell, with the ability to close the shell while leaving the program running?
Lets say my UI is having problems or for some reason, I need to boot up a program from the terminal window, say, nm-applet:

nm-applet

When it's started, it occupies the foreground of the terminal window.

Is there any simple way to run the program in the background without needing to leave the terminal open or have it occupy the whole terminal?

On that note, I did find a way to run programs from the terminal and have it allow for other inputs, by appending an ampersand (&) to the command as such:

nm-applet &

But this isn't much use as any processes started in the terminal are killed once the terminal is closed.

Best Answer

I've recently come to like setsid. It starts off looking like you're just running something from the terminal but you can disconnect (close the terminal) and it just keeps going.

This is because the command actually forks out and while the input comes through to the current terminal, it's owned by a completely different parent (that remains alive after you close the terminal).

An example:

setsid gnome-calculator

I'm also quite partial to disown which can be used to separate a process from the current tree. You use it in conjunction with the backgrounding ampersand:

gnome-calculator & disown

I also just learnt about spawning subshells with parenthesis. This simple method works:

(gnome-calculator &)

And of course there's nohup as you mentioned. I'm not wild about nohup because it has a tendency to write to ~/nohup.out without me asking it to. If you rely on that, it might be for you.

nohup gnome-calculator

And for the longer-term processes, there are things like screen and other virtual terminal-muxers that keep sessions alive between connections. These probably don't really apply to you because you just want temporary access to the terminal output, but if you wanted to go back some time later and view the latest terminal activity, screen would probably be your best choice.

The internet is full of screen tutorials but here's a simple quick-start:

http://thingsilearned.com/2009/05/26/gnu-screen-super-basic-tutorial/

Related Solutions

Ubuntu – How to wake a sleeping bash script

When a Bash script is running a sleep, here's what the pstree might look like:

bash(10102)───sleep(8506)

Both have process IDs (PIDs), even when running as a script. If we wanted to interrupt the sleep, we'd send kill 8506 and the Bash session would resume... The problem is in a scripted environment we don't know the PID of the sleep command and there isn't a human to look at the process tree.

We can get the PID of the Bash session through the $$ magic variable. If we can store that somewhere, we can then target instances of sleep that are running underneath that PID. Here's what I'd put in the script:

# write the current session's PID to file
echo $$ >> myscript.pid

# go to sleep for a long time
sleep 1000

And then we can tell pkill to nuke sleep instances running underneath that PID:

pkill -P $(<myscript.pid) sleep

Again, this is limiting itself to only sleep processes running directly under that one Bash session. As long as the PID was logged correctly, this makes it a lot safer than killall sleep or pkill sleep, which could nuke any sleep process on the system (permissions allowing).

We can prove that theory with the following example where we have three separate bash sessions, two running sleep. Only because we're specifying the PID of the top-left bash session, only its sleep is killed.

enter image description here

An alternative approach is to push sleep into the background, store its PID and then return it to the foreground. In the script:

sleep 1000 &
echo $! > myscript.sleep.pid
fg

And to kill it:

kill $(<myscript.sleep.pid)

Ubuntu – How to run a Java program in background

You can't foreground a process from a different sesion. The functionality you want is probably possible with the screen command.

As for the java app failing to run in init.d; you are not changing to the /opt/glassfish/domains/domain1/applications/AS/WEB-INF/classes folder. You have '.' in your classpath so it matters where you run from.

Ideally, I think you should keep the bash script and run that from init.d. However, you could also add the full path to your init.d classpath explicitly. Though I suspect you will need to use screen because of the inability to foreground the app.

For screen, this is an example of what you are looking for

$ screen -S myScreen -d -m ./dostuff.sh
$ screen -r myScreen

Best Answer

Related Solutions

Ubuntu – How to wake a sleeping bash script

Ubuntu – How to run a Java program in background

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