It is often annoying when a program stops working and you cannot even close it. Rebooting the system is not always the appropriate way and we search for ways to get rid of unresponsive programs, easily and quickly. In this article, we will learn about those ways including both GUI and the command line to kill the unresponsive applications in a Debian system.
We have run the commands and procedures mentioned in this article on a Debian 10 system. Some of the methods described here have been run on the command line Terminal application. To open the Terminal in Debian OS, go to the Activities tab in the top left corner of your desktop. Then in the search bar, type the keyword terminal. When the search result appears, click on the Terminal icon.
Method 1: Using the X button for unresponsive UI based applications
If anyone of your UI based program is taking too long to respond then try closing it by clicking on the x button in the top right corner of its window. Sometimes when you try to close an unresponsive application using this way, it will prompt you to send the error report. Also in some cases, you are given an option to wait or force quit the application.
Using this method, you will most probably be able to close the application.
Method 2: Using the System Monitor, Debian Task Manager
Similar to the Windows Task Manager, Debian OS also comes with the built-in System Monitor utility. Using this System Monitor utility, you can kill, end, stop, and continue the processes running on your system.
To launch the Gnome System Monitor using the UI, hit the super key on your keyboard and type the keyword system monitor. Alternatively, you can search for System monitor Utility form the Applications list.
The System Monitor will by default launch in the Processes tab. Here you will see all the processes running on your system including the unresponsive processes too. In order to kill an unresponsive program, right-click on it and then select the Kill option.
You can also try the "Stop" or "End" option to safely close the applications.
Method 3: Using the Xkill utility
There is another Linux utility known as Xkill which can be used to kill unresponsive applications on your system. It is pre-installed in most of the Linux distributions including Debian.
To run this utility, simply type xkill in the Terminal:
$ xkill
After running the utility, you will see the mouse pointer has turned in to an ‘x’ or a skull. To close any application, simply place the pointer to it and left-click it. It will kill the unresponsive program within seconds.
Creating a Keyboard shortcut for Xkill
Although Xkill is an effective utility for closing the unresponsive programs, opening the Terminal and running the command might seem a hassle. There is also a quickest way to achieve this using a keyboard shortcut. Simply press the shortcut keys and kill any unresponsive program.
Here is how you can do it:
1. Open the Keyboard Shortcuts Settings
Hit the super key on your keyboard and then search for the keyboard shortcuts by using the search bar that appears. 
When the Settings window opens, you will find a number of shortcuts for different purposes.
2. Creating a custom shortcut
Scroll down the bottom of the screen and click the + button. In order to add the shortcut for Xkill utility, click the + button.
When the Add Custom Shortcut dialog appears, type xkill in both of the "Name" and "Command" filed as shown in the following screenshot.
Once done, click the Set Shortcut button which will allow you to set a custom shortcut for your Xkill utility.
Press the key combination of your choice that is not already used on your system. Here I am entering Ctrl+Alt+6 as a custom shortcut for Xkill.
Once done, close the dialog box. After that, you will be able to see your newly added shortcut in the list.
Now try using the shortcut and you will be able to kill the unresponsive applications quickly.
Method 4: Using the kill, pkill and killall commands
There are also some other command-line ways to kill unresponsive applications in a system. These ways are helpful for those who prefer command line over the GUI.
To view the currently running processes on your system, run the top utility using the following command in Terminal:
$ top
The output displays all the processes running on your system along with their process ID (PID).
To kill any program, type kill followed by its process ID (PID):
$ kill PID
Pkill can also be used to kill a process using the process name as follows:
$ pkill [Process-name]
If there are many instances of any application running and you want to kill all of them, try using the following command:
$ killall [Process-name]
In this article, we have learned how to kill processes using the different command line and GUI ways. I hope it will be helpful whenever you need to quickly kill any unresponsive application that gets stuck and does not respond.