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.

Listing 2

Jobs

 

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.

Buy this article as PDF

Express-Checkout as PDF
Price $2.95
(incl. VAT)

Buy Linux Magazine

SINGLE ISSUES
 
SUBSCRIPTIONS
 
TABLET & SMARTPHONE APPS
Get it on Google Play

US / Canada

Get it on Google Play

UK / Australia

Related content

  • Command Line – Pgrep

    Pgrep is a valuable tool for tracking down processes.

  • Command Line: Processes

    Innumerable processes may be running on your Linux system. We’ll show you how to halt, continue, or kill tasks, and we’ll examine how to send the remnants of crashed programs to the happy hunting grounds.

  • Shell Scripts in Waiting: the Waitmax Tool

    Linux consultant Matthias Kettner has just released Version 1.0 of his Waitmax Software. It gives programs a specified time to complete after terminating them.

  • Command Line – Killing Processes

    Linux offers a variety of tools for shutting down systems and processes. We describe some important commands.

  • Pipe Cleaner

    Detecting programs where the standard output has frozen can require a deep dive into terminal emulation basics. Go plumber Mike Schilli builds a plunger to free up the pipe works.

comments powered by Disqus
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters

Support Our Work

Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.

Learn More

News