Process and job control
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.
01 $ xpdf article.pdf & 02  5622 03 $ audacity & 04  6559 05 [...] 06 $ jobs 07  Running xpdf article.pdf & 08 - Running audacity & 09 + Running sleep 3600 &
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:
+ 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:
+ 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.
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.
Buy this article as PDF
Powerful man-in-the-middle attack is now targeting online shopping.
Another high-profile coder says the kernel team needs a kinder, gentler culture.
Bug database has a bug of its own that could allow an intruder to create an unauthorized account.
Report focuses federal resources on achieving universal Internet access.
Leading browser makers say “no” to porous encryption algorithm
Report from the X-Force group says attackers are using TOR to hide their crimes
Future Firefox extensions will be compatible with Chrome.
Better read this if you bought your computer before 2011
Users should upgrade to the new version as soon as possible
Xen project announces a privilege escalation problem for Qemu host systems