1.- A program is only code (a set of in instructions) stored in a file on your hard drive (or any other permanent storage device). When the code of a program is sent and stored into the RAM memory, the CPU can run the program.

2.-  A process is  a program  stored in RAM memory that can be run by the CPU. In other words, is a program in action. Sometimes the process is run by the CPU and sometimes the process is waiting to be run by the CPU.

3.- During its lifetime, a process needs system resources such as the CPU time and RAM to hold its code.  The allocation of proper resources to each process running in the system is a duty of Linux.

4-  The total amount of memory allocated to a process is called Virtual Memory and it is the addition of physical RAM memory assigned to the process and Swap space assigned to the process.

5.- Other resources a process could need are access to: hardware, physical or logical ports, reserved memory areas, files, directories, etc...

6.- The process table on Linux holds information about the processes that are currently handled by the OS. Each process is represented as an entry (or process slot)  in the process table.  Information hold in the process table includes (but not only):

7.- Linux is a multiuser and multiprocessing operating system and therefore, multiple users can be running one or more processes on the system at the same time. That means the operating system needs to identify each process. A process is identified by its Process ID also called PID and by its Parent Process ID also called PPID.

8.- A PID  must be a unique identification number for each process running on the system. Processes can NOT share PID. As long as the process exists, it keeps the same PID number. When a process is ended, its PID is freed and eventually the operating system can assign this number to another process.

9.- Multiple processes of a program can be run at the same time. Different processes of one single program will have different PIDs.

10.- A process is also identified by its parent process ID or PPID:

• Whenever a process creates another process, the former is called parent while latter is called child process.
• A parent process can have several child processes, but a child process can have only one parent.
• Any process running on the system (with the exception of init) is a child process of a parent process.
  
• Only init, the first process created when you boot linux, has not a parent process. The PID of init is 1 and its PPID is 0.
• A parent child should not be finished until all its child processes has been finisihed and their information released of the process table.
• After child process finishes, the parent process has to release any information about the child process hold by the operating system in the process table.
  

11.- The operating system is able to assign priorities to process. The higher  a process priority is, the higher will be, for example, its CPU time or RAM allocated. When a new process is started, the operating system assigns  to the process  a value called the  nice number or NI. This number is related to process priority. The higher the nice number is, the lower the priority of the process is.  

12.- The states of a Linux process are the following:

    a) Running or  Runnable (R):

• For any Linux process, its starting point is the moment it is created, usually by a parent process. Once it starts, the process goes into the running or runnable state.
• Running: All resources required by the process are available and the process is executing instructions on some CPU on the system.
• Runnable: All resources required by the process are available and therefore the process is ready to be run but, at the moment, the process is in a queue waiting to be assigned CPU time. The scheduling algorithm might force a     running process to give up its execution right and change to runnable state. This is to ensure each process can have a fair share of CPU resources.
• Process state code: R

    b) Interruptible Sleep (S):

• The process can not be run at the moment because some external resources (for instance, access to a printer, hard drive, network card,...) required by the process are not available. If a process goes to interruptible sleep immediately frees up its access to the CPU.
• Also, a process can go to interruptible mode because it requires that a certain amount of time pass. For instance, when a process runs the sleep instruction on C.
• When the resources required by the process are available the process changes to Running or Runnable state.
• An interruptible sleep accepts and reacts to software or hardware signals such as, for instance, a stoping signal.
  
• Process state code: S

    c) Uninterruptible Sleep (D):

• A process become an  uninterruptible sleeping process when it waits until an specific resource is available. The process will be awaken only if the specific resource becomes available.
• An uninterruptible sleep process can not be stopped by a signal.
• Usually, a process goes to uninterruptible sleep when it is performig I/O operations such as accessing to the hard drive, to the network card or a printer.
• Process state code: D

   d) Stopped (T):

• A process become a stopped process when it receives the SIGSTOP signal. The execution of the process will be suspended.
• A stopped process only accepts the SIGKILL and SIGCONT signals. The former will remove the process permanently, while the later will put the process back to the Running/Runnable state.      
• Process state code: T

   e) Zombie (Z):

• A process become a zombie process when its execution is completed and it releases memory, cpu and any other resources.
• A zombie process waits that its parents process releases its process slot (or entry) in the process tables but the process does not longer exists.
• A Zombie process only can be removed from the process table by its parent process or shutting down the computer.
• A Zombie process does not accept signal because does not longer exists.
• A process can remain in a zombie state if the parent process dies before it has the chance to release the process slots of its child processes.
• Generally speaking, if a zombie process remains in the process table, means that the code of the child or parent process is a bad code (it has not been properly developed).
• Process state code: Z

    f) Idle Kernel threads (I):

• Beyond the scope of this exercise
• Process state code: I

    g) Linux states diagram



13.- Signals are one of the ways process communicate among themselves, with the kernel and with users.  The list of the most commonly used signals follows:       

• SIGTERM: It is the default signal sent by kill command. Asks the process to terminate voluntarily. Terminate the process in an orderly way. Default option
  
• SIGKILL: Unlike SIGTERM, forces the process to terminate. Can't be blocked or handled.
  
• SIGSTOP: Suspend the process execution, putting it in stopped state. In this state, the process will do nothing but accept SIGKILL and SIGCONT signals. Can’t be blocked or handled.
• SIGCONT: if a process is in stopped state, it will put it back in the running/runnable state and resume it execution. If the process is in any other state, it's silently ignored.
• SIGINT: Generated when the user type <Ctrl>+C in the terminal. It interrupts the current command processing and wait for user's next command.
• SIGHUP: This signal indicates the terminal handling the process has been disconnected and/or the parent process terminated. If you want to run a process that won't terminate when the terminal disconnects, you can start it using the nohup command. Some daemons repurpose this signal to trigger a configuration reload without stopping its execution.
• SIGQUIT: Generated when the user type <Ctrl>+D in the terminal. It terminates the process (like SIGTERM) but also causes the shell to write a core file to help the so-called post-mortem debug.

14.- Processes that require a user to start them or to interact with them are called foreground processes. Processes that are run independently of a user are referred to as background processes. Programs or commands started by users  run as foreground processes by default. To run a process in the background, place an ampersand (&) at the end of the command name that you use to start the process.

15.- Orphan processes:

• They are those processes that are still running even though their parent process has terminated or finished.
• A process can be orphaned intentionally or unintentionally.
• An intentionally orphaned process runs in the background without any manual support.  This is usually done to start an indefinitely running service or to complete a long-running job without user attention.
• An unintentionally orphaned process is created when its parent process crashes or terminates. Unintentional orphan processes can be avoided using properly developed code.
• An orphan code is not a zombie code because it is running and using resources (memory, cpu,...).
• There is not a special code for Orphan processes. An orphan process can be in R, S, D or T state.

2-  Kernel. Memory areas. Type of processes

1.- The kernel:

• It is in the core of an operating system and has a crucial role in its functioning. The kernel manages the system resources, including cpu, memory, hardware, drivers, processes, filesystems, users, interactions between hardware and processes, interactions between users and the processes,  interactions between users and hardaware, signals, input/output operations and so on. In other words, with the help og the kernel controls everything hardware, software and users can work coordinately without conflicts.
• It is stored in the computer’s memory and is loaded into memory when the system boots up. It remains in memory until the system is shut down.

2.- On Linux there are two separeted memory areas called user space memory area and kernel space memory area:

• Kernel space memory area is strictly reserved for running the kernel, some special processes and device drivers.
• User space memory area is the memory area reserved for processes started by users (root included).
• Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour. 
  

3.-  There are three types of processes in a Linux system:

• User processes: These are processes initiated by a regular user and run in a memory area called the user space. User processes work with permissions granted to the user who started them.
• Daemon processes: A process that is designed to work in the background, runs continuously and independently of the terminal and shell, without direct interaction with the users.  Daemons work usually with permissions granted to a system user. Daemons are usually services such as Apache (web server) or CUPS (print server). Daemons usually runs in user space. Daemons usually are created by a parent process and then the parent process is ended so the daemon become a kind of orphan process and that makes that the init process become its parent.
  
• Kernel processes:  These are processes that run only in  the kernel space and they have special security privileges. Kernel processes have full access to the operating system inner structure. They are typically used to manage hardware that's why they high priority. Kernel processes are shown in the process tables between squared brackets. There is an special state called I for Idle kernel processes.

killall   geany   firefox-bin   nano

nice  -n  5  geany &

nice  -n  -4  geany &

renice  +10  6876

renice  -15  9854