Section 9: Interaction and Job Control
When you log in to a UNIX system,
the kernel starts a shell for you, and connects the shell to your terminal. When
you execute a command from the shell, the shell creates a child process to
execute the command, and connects the child process to your terminal. By
connecting the child process to your terminal, the shell allows you to send
input to the child process, and receive output from it. When the child process
finishes, the shell regains access to the terminal, redisplays the shell prompt,
and waits for your next command.
Any task that requires you to actively participate (like word processing)
must be in the foreground to run. Such jobs, termed "interactive," must
periodically update the display, and accept input from you, and so require
access to the terminal interface.
Other jobs do not require you to participate once they are started. For
example, a job that sorts the contents of one file and places the results in
another file, would not have to accept user commands, or write information to
the screen while it runs. Some UNIX shells allow such noninteractive jobs to be
disconnected from the terminal, freeing the terminal for interactive use.
Note that it is even possible to log out, and leave a background process
running. Unfortunately, there is no way to reconnect a background job to the
terminal after you've logged out.
Jobs that are disconnected from the terminal for input and output are called
"background" jobs. You can have a large number of background jobs running at the
same time, but you can only have one foreground job. That is because you only
have one screen, and one keyboard at your terminal.
Background and foreground jobs
The process that is connected to the
terminal is called the foreground job. A job is said to be in the
foreground because it can communicate with the user via the screen, and the
keyboard.
A UNIX process can be disconnected from the terminal, and allowed to run in
the background. Because background jobs are not connected to a terminal,
they cannot communicate with the user. If the background job requires
interaction with the user, it will stop, and wait to be reconnected to the
terminal.
Jobs that do not require interaction from the user as they run(like sorting a
large file) can be placed in the background, allowing the user to access the
terminal, and continue to work, instead of waiting for a long job to finish.
Starting a job in the background
To place a job in the background,
simply add the ampersand character (&) to the end of your shell command.
EXAMPLE: To sort a file called "foo", and place the results in a file
called "bar", you could issue the command
- sort < foo > bar &
Examining your jobs
The UNIX command jobs allows you to see a
list of all the jobs you have invoked from the current shell. The shell will
list the job ID of each job, along with the status (running, stopped, or
otherwise), and the command that started the job. The shell considers the most
recently-created (or most recently-manipulated) job to be the current
job, marked with a plus sign. Other jobs are referred to as previous
jobs, and are marked with a minus sign. The commands related to job control
will apply to the current job, unless directed to do otherwise. You may refer to
jobs by job ID by using the percent sign. Thus, job 1 is referred to as %1, job
2 is %2, and so forth.
Suspending the foreground job
You can (usually) tell UNIX to suspend the
job that is currently connected to your terminal by typing Control-Z (hold the
control key down, and type the letter z). The shell will inform you that the
process has been suspended, and it will assign the suspended job a job ID.
There is a big difference between a suspended job, and a job running in the
background. A suspended job is stopped, and no processing will take place for
that job until you run it, either in the foreground, or in the background.
Placing a foreground job into the background
If you started a job in the
foreground, and would like to place it in the background, the first thing you
must do is suspend the job with a Control-Z, freeing your terminal. Then, you
can issue the UNIX command
- bg
to place the suspended job in the background. The bg
command can accept a job ID as an argument. If no job ID is given, bg assumes
you are referring to the current (suspended) job.
Bringing a background job to the foreground
You can reconnect a
background job to your terminal with the UNIX command
- fg
The fg command will accept a job ID as an argument.
Make sure to include the percent sign:
- fg %2
will bring job 2 into the foreground. If no job ID
is given, fg will assume you are referring to the current (suspended) job.
Starting a suspended job
If you have a suspended job that you'd like to
resume running, first you must decide whether you want it running in the
foreground, or the background. Find the job ID of the suspended job with the
jobs command, and then use bg (to run the job in the background),
or fg (to run the job in the foreground).