Foreground and Background

In some cases, a program you launch in the shell might run for an extended period of time. Graphical programs that you launch in a terminal window block the shell, preventing any command input. In cases like this, you can run out and grab a coffee or open a second console and carry on working. As an alternative, you can move the process into the background when you start it, or at a later time.

To move the process into the background when you launch it, just add the ampersand character (&) to the command line (Listing 2, line 1). The Xpdf window launches, the shell tells you the process ID (5622), and bash can then accept more commands.

Besides the process ID, you can also see the job ID in square brackets. The job ID is allocated as a consecutive number by the shell. If you launch another program in the same session, you will see that bash assigns job ID 2 (Listing 2, line 3). The jobs command tells you which jobs are running in the current shell (Listing 2, line 6).

After a program has completed its task, the shell displays the job ID along with a status message (Done) and the program name:

[3]+ Done xpdf article.pdf

The job ID is also useful if you need to move a background process into the foreground, or vice versa.

If you launch a program without appending an ampersand, you can press the keyboard shortcut Ctrl+Z to send it to sleep. The shell confirms this action as follows:

[1]+ Stopped xpdf

If you now type bg (background), the process will continue to run in the background. The job ID is useful if you have stopped several processes in a shell. The bg %3 command tells the process with the job ID 3 that it should start working again. In a similar way, the fg (foreground) program moves jobs into the foreground. Again, this program might need more details in the form of a job ID following a percent character.

Detached

The commands I just looked at move processes to the background and optionally let them go on running. If you close the shell in which you launched the program, this also terminates all the active processes.

The nohup program gives you a workaround by protecting the process against the shell's HUP signal (see the next section), thus allowing it to continue running after you close the terminal session. In other words, this cuts the ties between the child process and its parent. Simply call nohup with the program (and its options):

nohup find /scratch3/mp3 -name "*.ogg" > ogg_liste.txt

This approach does not automatically move the process to the background, but the methods I just described will take care of this.

Closing the shell means that you can't communicate with the process – or does it? Even if you do not have a direct terminal connection, you can still control the program using the signals discussed next.

An End to Everything?

Although the name might suggest otherwise, the kill program need not be fatal. On the contrary, you use it to send signals to processes, including polite requests to stop working.

As you might expect, non-privileged users are only allowed to talk to their own processes, whereas the root user can send signals to any process.