Inter Process Communication. IPC is used to pass information
between two or more processes.

Schemes are pipes, shared memory & semaphore.

Below are the different IPC methods

1. Semaphores
2. FIFO's (Also called Named Pipes)
3. Message Queues
4. Shared Memory

Following IPC mechanisms are available:-
1)Signal
2)Pipe
3)FIFO
4)System V's :- a)Semaphore b)Shared Memory c)Message Queue
5)Socket

with the help of "exec" version.

look for man pages for more help

If a process is running in background then the same process 
will continue to run on the Unix server without any 
communication with the front console.
the console can be reused.

When command is given then unix os will fork the shell i.e
will creat a new process and will execute the command using
exec command...something like suppose you gave command "ls"
in the shell then...
fork();
exec(ls);
It will give you the result and after this the child process
will die.

Daemon stands for Disk and Execution Monitor. A daemon is a 
long-running background process that answers requests for 
services.

The processes like vhand, bdflush, sched are housed in 
kernel file or /unix system which are known as daemons. 
These files run  in the background without users 
request.These are created when the system boots up and 
remains active till it shut down or hang. These are not 
linked to any user or any terminal .We can't kill a daemon.

1.running
2.stop
3. zombie

and also orphan

And
Sleep 
Suspended also....

we have only
1) Running(ready to run)
2) stopped 
3) suspended( sleep)
4) zombie

1,user running,
2,kernel running,
3,ready to run in memory
4,ready to run in swap area
5,blocked, but process in memory
6,blocked, but process in swap area and
7,zombie

A system that has been taken over using Remote Control 
Software. Zombies are often used to send spam or to attack 
remote servers with an overwhelming amount of traffic (a 
Distributed Denial of Service Attack).

A zombie process is child process whose parent has been 
killed and this child process is neither active nor dead. 
Hence the name zombie.

Whenever process killed, then it makes a "zombie" state
entry in the process table. because parent process or init
process need status information of killed process, such as
how long time killed process running in kernel and user
process...

A zombie process is one whose child process has terminated 
or get killed before its own termination.

Parent and child processes can communicate with unnamed pipe,
but the pipe should be created before creating the
child(i.e. forking).
After forking the process, child process inherits all
properties of parent, hence pipe also. This pipe can ce used
to communicate between parent and child process. But if one
wants to use exec(s), then this technique is not useful. The
user have to use named pipe or Message-Queue.

An Application programming interface (API) is a source code 
interface that an operating system or library provides to 
support requests for services to be made of it by computer 
programs. Advanced programming interface is a near synonym 
with wider application that predates the current common 
usage. In the original term the concept is meant to 
represent any well defined interface between two separate 
programs. The main difference is that this older term does 
not inculcate a parent-child relationship and can therefore 
be applied to peer-to-peer situations more logically, e.g. 
internal kernel services which can present themselves as 
separate programs.

to get environment variable execute getenv();
to set environment variable execute setenv();

One of them is nice() to adjust the priority of the process.

This should not allow to increase process priority.

fork() - to create a new process 
exec() - to execute a new program in a process
 wait() - to wait until a created process completes its 
execution 
exit() - to exit from a process execution 
getpid() - to get a process identifier of the current 
process

The output is 

Hello World!Hello World!

prints Hello world Hello world
 All the statements after the call to fork() will be 
executed twice

fork() sytem call create child process to handle multi
processing activities, It inherites following fields from
parent.
environment variables, Filedescriptor, data part
I will think and write ...

The 'fork()' call is used to create a new process from an 
existing process. The new process is called the child 
process, and the existing process is called the parent. We 
can tell which is which by checking the return value 
from 'fork()'

fork() system call are used in unix for making child process.
whenever parent procces call fork()system call it create
child process and copy the sorce code of parent process in
child process and return the p_id of child process and
creating child process return itself is zero.

fork() is the system call to creat the child process to the 
existing process.The child process copy the source code of 
the parent process and assaign new memory address.After 
fork call both parent and child executes the followed 
instruction.fork call returns the pid of child process to 
parent and returns 0 to child process.

Real User id, Effective User Id, saved user id.
Real Group id, Effective Group Id, saved Group id.

PID- Process Id
PPID- Parent Process ID

While booting, special process called the 'swapper' 
or 'scheduler' is created by the Process-ID 0. The swapper 
manages memory allocation for processes and influences CPU 
allocation. The swapper in turn creates 3 children: the 
process dispatcher, vhand, etc...

Inode has 13 block addresses. The first 10 are direct block 
addresses of the first 10 data blocks in the file. The 11th 
address points to a one-level index block. The 12th address 
points to a two-level (double in-direction) index block. 
The 13th address points to three-level (triple in-
direction).