Shell – Is it harmful to close a terminal window without properly exiting an application

command lineshellterminal

Using Ubuntu 12.04 LTS, my question is that if I started an application in a terminal window, then is there anything bad about just closing the terminal window without exiting the application properly first. For example, I use MATLAB. I open up a terminal and type

matlab -nodisplay -nodesktop -nosplash

and then run a bunch of scripts. Then I can either

exit

to end MATLAB and then close the terminal window or just close the terminal window. What really is the difference between these two methods? Does the second method somehow "harm" anything? Is the first method preferred? Why?

Best Answer

In general this should be fine to do it this way.

When you click the "X" to close the terminal window, that is sending a "signal" from your desktop (GNOME, KDE, etc.) to the terminal application, telling it to shut itself down. Since you're running MATLAB in this shell it's considered a child process to the terminal application.

So part of the responsibilities of being a parent process, is that you in turn send this same close "signal" to your children.

Now if you understand conceptually what I just explained then let's substitute in a bit more of the real terminology.

signals

First with the "signal", there are actually a whole family of different signals that you can send to Unix processes. To keep it simple there are 4 that you'll often see, SIGHUP, SIGTERM, SIGINT, and SIGKILL.

  • SIGHUP

    The SIGHUP signal is sent to a process when its controlling terminal is closed. It was originally designed to notify the process of a serial line drop. In modern systems, this signal usually means that controlling pseudo or virtual terminal has been closed.

  • SIGTERM

    The SIGTERM signal is a generic signal used to cause program termination. Unlike SIGKILL, this signal can be blocked, handled, and ignored. It is the normal way to politely ask a program to terminate.

  • SIGINT

    The SIGINT (“program interrupt”) signal is sent when the user types the INTR character (normally C-c).

  • SIGKILL

    The SIGKILL signal is used to cause immediate program termination. It cannot be handled or ignored, and is therefore always fatal. It is also not possible to block this signal.

NOTE: SIGINT is what gets sent when you use Ctrl+C to "break" a program from the command line while it's in the middle of running.

which one is getting used?

Most likely the SIGTERM is being called by your windowing environment and being passed down to your terminal. Your terminal is then most likely then sending SIGHUP down to MATLAB. This signal gives all the processes the opportunity to do any local clean-up (closing files, ending processes, etc.) themselves.

kill command

You can send signals yourself using the poorly named command, kill. So to send the SIGTERM signal to your terminal or the SIGHUP to MATLAB, you could determine their PID usingps` and then run this command to send them the signal:

$ kill -SIGTERM <PID>

or this:

$ kill -SIGHUP <PID>

You can get a complete list of the signals using this command:

$ kill -l
 1) SIGHUP   2) SIGINT   3) SIGQUIT  4) SIGILL   5) SIGTRAP
 6) SIGABRT  7) SIGBUS   8) SIGFPE   9) SIGKILL 10) SIGUSR1
11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM
...
...

Notice that the signals have numbers? You'll often times see them used like that instead of by their names:

$ kill -15 <PID>

Or the infamous -9, which can kill pretty much any process.

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