When tackling a relatively advanced topic such as co-processes, a review of foreground and background jobs is a good place to begin.

Each time you run a command or program on the system, you are running a job. A job consists of one or more processes working to perform a specific task.

By default, a job is started in what is called the foreground. This means that you have to wait until the job has completed prior to running another command or program (the command prompt will not be displayed until a foreground job has completed). If you are running commands or programs that take a long time to finish, this can be an unacceptable situation. To avoid this you have the option of running a job in what is call the background.

The background allows you to start a job detached from the foreground so that you can work on other tasks or start additional jobs. When a job is started in the background, the command prompt is returned to you immediately. To start a job in the background, an & (ampersand) character is appended to the end of the command line:

$ ./loop10 &
[1] 12420
$

This command starts a job running the program loop10 in the background, and then displays the command prompt.  The 1 in the brackets is the job number, and 12420 is the PID.

The program loop10 is a simple program that loops 10 times and then exits:

$ cat loop10
#! /bin/ksh

i=1
while [ $i -le 10 ]
do
   echo "" > /dev/null
   sleep 1
   (( i=i+1 ))
done

exit 0
$