Section 8: Process Control and Multitasking
CONCEPT: The UNIX kernel can keep track of many processes at
once, dividing its time between the jobs submitted to it. Each
process submitted to the kernel is given a unique process ID.
Single-tasking operating systems, like DOS, or the Macintosh System,
can only perform one job at a time. A user of a single-tasking system
can switch to different windows, running different applications, but
only the application that is currently being used is active. Any
other task that has been started is suspended until the user
switches back to it. A suspended job receives no operating system
resources, and stays just as it was when it was suspended. When a
suspended job is reactivated, it begins right where it left off, as if
nothing had happened.
The UNIX operating system will simultaneously perform multiple tasks
for a single user. Activating an application does not have to cause
other applications to be suspended.
Actually, it only appears that UNIX is performing the jobs
simultaneously. In reality, it is running only one job at a time, but
quickly switching between all of its ongoing tasks. The UNIX kernel
will execute some instructions from job A, and then set job A aside,
and execute instructions from job B. The concept of switching between
queued jobs is called process scheduling.
UNIX provides a utility called ps (process status) for viewing
the status of all the unfinished jobs that have been submitted to the
kernel. The ps command has a number of options to control which
processes are displayed, and how the output is formatted.
EXAMPLE: Type the command
to see the
status of the "interesting" jobs that belong to you. The output of
the ps command, without any options specified, will include the
process ID, the terminal from which the process was started, the
amount of time the process has been running, and the name of the
command that started the process.
EXAMPLE: Type the command
to see a
complete listing of all the processes currently scheduled. The -e
option causes ps to include all processes (including ones that do not
belong to you), and the -f option causes ps to give a long listing.
The long listing includes the process owner, the process ID, the ID of
the parent process, processor utilization, the time of submission, the
process's terminal, the total time for the process, and the command
that started the process.
- ps -ef
EXERCISE: Use the ps command, and the grep command, in a
pipeline to find all the processes owned by you.
EXPLANATION: The command
where "yourusername" is replaced by your user
name, will cause the output of the ps -ef command to be filtered for
those entries that contain your username.
- ps -ef | grep
Occasionally, you will find a need to terminate a process. The UNIX
shell provides a utility called kill to terminate processes.
You may only terminate processes that you own (i.e., processes that
you started). The syntax for the kill command is kill [-options]
To kill a process, you must first find its process ID number using the
ps command. Some processes refuse to die easily, and you can use the
"-9" option to force termination of the job.
EXAMPLE: To force termination of a job whose process ID is 111,
enter the command
- kill -9 111