Background Mode

Let’s suppose that we run a GUI application in the terminal window. Then, we cannot use this window for typing the Bash commands. The GUI program controls it and prints the diagnostic messages there. The terminal window becomes available again when the application finishes.

We can run the GUI application in the background mode. Then, the terminal window stays available, and we can use it normally. Add the ampersand (&) at the end of a Bash command to launch it in the background mode. Here is an example:

notepad++ test.txt &

After this command, we can type text in the terminal window. The only problem is that the editor still prints the error messages from Notepad++ here. These make it inconvenient to use this terminal window.

We can detach the running GUI application from the terminal window completely. Let’s do it with the disown Bash built-in, and dall disown with the -a option this way:

notepad++ test.txt &
disown -a

Now, Notepad++ does not print any messages in the terminal. The disown call has one more effect. It allows us to close the terminal window and keep the editor working. Without the disown call, Notepad++ finishes when we close the terminal.

We can combine Notepad++ and disown calls into one command. It looks like this:

notepad++ test.txt & disown -a

The -a option of the disown command detaches all programs that work in the background. If we skip this option, we should specify the process identifier (PID) of the program to detach. PID is a unique number that the OS assigns to each new process.

Let’s suppose that we want to call disown for the specific program. We should know its PID. Bash prints the PID of the background process when we launch it. Here is an example:

notepad++ test.txt &
[1] 600

The second line has two numbers. The second number 600 is PID. The first number [1] is the job ID. We can use it to switch the background process to the foreground mode. The fg command does it this way:

fg %1

If we want to detach the Notepad++ process from our example, we call disown this way:

disown 600

If we want to list all programs that work in the background, use the jobs Bash built-in. When we call it with the -l option, it prints both job IDs and PIDs. We use it this way:

jobs -l

This command lists all background processes that we launched in the current terminal window.

We can call Notepad++ and detach it from the terminal in a single command. In this case, we should use the special Bash variable called $!. It stores the PID of the last launched command. After we pass this PID to the disown call, and we are done. Here is an example of how to apply this approach:

notepad++ test.txt & disown $!