When you execute a program on your UNIX system, the system creates a special environment for that program. This environment contains everything needed for the system to run the program as if no other program were running on the system. Each process has process context, which is everything that is unique about the state of the program you are currently running. Every time you execute a program the UNIX system does a fork, which performs a series of operations to create a process context and then execute your program in that context.
The steps include the following:
1. Allocate a slot in the process table, a list of currently running programs kept by UNIX.
2. Assign a unique process identifier (PID) to the process.
3. iCopy the context of the parent, the process that requested the spawning of the new process.
4. Return the new PID to the parent process. This enables the parent process to examine or control the process directly.
After the fork is complete, UNIX runs your program.
When you enter "ls" command to look at the contents of your current working directory, UNIX does a series of things to create an environment for "ls" and the run it: The shell has UNIX perform a fork. This creates a new process that the shell will use to run the ls program. The shell has UNIX perform an exec of the "ls" program. This replaces the shell program and data with the program and data for "ls" and then starts running that new program. The "ls" program is loaded into the new process context, replacing the text and data of the shell. The "ls" program performs its task, listing the contents of the current directory.
A daemon is a process that detaches itself from the terminal and runs, disconnected, in the background, waiting for requests and responding to them. It can also be defined as the background process that does not belong to a terminal session. Many system functions are commonly performed by daemons, including the sendmail daemon, which handles mail, and the NNTP daemon, which handles USENET news. Many other daemons may exist.
Some of the most common daemons are:
1. init: Takes over the basic running of the system when the kernel has finished the boot process.
2. inetd: Responsible for starting network services that do not have their own stand-alone daemons. For example, inetd usually takes care of incoming rlogin, telnet, and ftp connections.
3. cron: Responsible for running repetitive tasks on a regular schedule.
The "ps" command prints the process status for some or all of the running processes. The information given are the process identification number (PID),the amount of time that the process has taken to execute so far etc.
The "kill" command takes the PID as one argument; this identifies which process to terminate. The PID of a process can be got using "ps" command.
16. What is an advantage of executing a process in background?
The most common reason to put a process in the background is to allow you to do something else interactively without waiting for the process to complete. At the end of the command you add the special background symbol, &. This symbol tells your shell to execute the given command in the background.
Example: cp *.* ../backup& (cp is for copy)