Lesson 8: UNIX Processes

1. Objectives
   • Define a UNIX process
   • Know the four system calls used in the life-cycle of a process
   • Name five pieces of information you can obtain using the process status command.
   • Be able to use the shell’s job control service to run processes in the background.
   • Be able to send at least 4 different types of signals to specified processes
   • Process load balancing.
   
   
2. Definition of a Process
   A process is a program that has been loaded into RAM and either is executing instructions or waiting to be executed.
    
   
3. The Life Cycle of a Process
   
   
4. Information kept on a Process

   PID

   Process Identification number - a unique number identifying a process

   PPID

   Parent PID -- the PID of the parent process ( like .. in the file hierarchy)

   UID

   The user running the process

   TTY

   The terminal that the process’ stdout and stderr is attached to

   S

   The status of the process: S = sleeping, R – running T – stopped, Z – defunct

   PRI

   Process priority

   SZ

   Process size

   CMD

   The name of the process (what command was run)

   ... plus lots more:       ps -l
    
   
5. Job Control - a feature of the bash shell

   Foreground processes

   processes that receive their input from and write their output to the terminal. The parent shell waits on these processes to die.

   Background processes

   processes that do not get input from a user keyboard. The parent shell does not wait on these processes; it re-prompts the user.

   ^z

   Suspends (Stops) the process running in foreground

   bg

   Starts the currently suspended process running in background

   fg

   Brings the most recent background process to the foreground

   jobs

   Lists all processes running in the background.

   
   
6. Signals
   signals are asynchronous messages sent to processes. They can result in one of three courses of action:
   1. be ignored,
   2. default action (die)
   3. execute some predefined function.

   #	Name	Purpose
   1	SIGHUP	Hangup signal is sent to a shell’s children when the shell is exited. This would guarantee the phone line would be hung up when a user would disconnect from a remote login via modem.
   2	SIGINT	Interrupt signal is sent to the foreground process when the user types a preconfigured key, usually ^c
   3	SIGQUIT	The quit signal is sent to the foreground process causing a core dump when the user types a preconfigured key, usually ^\
   9	SIGKILL	this signal cannot be caught or ignored by any process. It results in the death of the process receiving it.
   15	SIGTERM	terminate signals are sent to any process when it it is time for that process to be terminated e.g. when the system shuts down.

   Signals are sent using the kill command:
   $ kill -# PID
   where # is the signal number and PID is the process id.
   (if no number is specified, SIGTERM is sent.)
    
   
7. Process Load Balancing
   So that the multiprocessing CPU on a UNIX system does not get overloaded, some processes need to be run during low peak hours such as early in the morning or later in the day. The at command is for this purpose. The at command reads its stdin for a list of commands to run, and begins running them at the time of day specified as the first argument:
         $ at 10:30pm < batch_file