4.11. Putting a Command in the Background
No matter whether you are using the X Window System (described later) or virtual consoles, you may at times still want to run several commands simultaneously from the same shell, if only in order to avoid having to switch between windows or consoles all the time. You can take advantage of Unix's multitasking features and achieve this by simply putting an ampersand at the end of commands, as shown in this example:
$ gcc invinitjig.c &  21457
The ampersand puts the command into the background, meaning that the shell prompt comes back and you can continue to execute other commands while the gcc command is compiling your program. The  is a job number that is assigned to your command. The 21457 is a process ID, which we'll discuss later. Job numbers are assigned to background commands in order and therefore are easier to remember and type than process IDs.
Of course, multitasking does not come for free. The more commands you put into the background, the slower your system runs as it tries to interleave their execution.
You wouldn't want to put a command in the background if it required user input. If you do so, you see an error message, such as:
Stopped (tty input)
You can solve this problem by bringing the job back into the foreground through the fg command. If you have many commands in the background, you can choose one of them by its job number or its process ID. For our long-lived gcc command, the following commands are equivalent:
$ fg %1 $ fg 21457
Don't forget the percent sign on the job number; that's what distinguishes job numbers from process IDs.
To get rid of a command in the background, issue a kill command:
$ kill %1
If you have started a program in the foreground, but want to put in the background later, most shells allow you to type Ctrl-Z. That key combination temporarily suspends the current foreground program. You can then type either fg as described before, to put it back in the foreground, or bg, to put it in the background.