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Full Discussion: what are parent and child processes all about?
Top Forums UNIX for Dummies Questions & Answers what are parent and child processes all about? Post 20474 by Kelam_Magnus on Friday 26th of April 2002 03:53:55 PM
Old 04-26-2002
Good example.
When you use a command like grep or find, they kick off subprocesses (children processes) that actually do the work.

EXAMPLE #1
The parent ( the command that you typed.) "find . -name somefile -print " kicks off subprocesses that actually go out and search for the file that you specified. When they are done they report back to the find command.

EXAMPLE #2
Using grep if you are trying to do something like matching a pattern(s) in a file to print out the lines.

cat filename|grep patterna

This command will actually spawn 2 processes. The cat command, the parent, and the grep command, the child. When the child (grep) finishes it returns to the parent (cat) with the result.

Example #3
Do a ps -aef |grep sh" to find your telnet session. Find your login and do a ps -aef |grep on that PID. All of your commands are technically children of your telnet session as you will see.
 

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LEARN ABOUT MINIX

fork

FORK(2) 							System Calls Manual							   FORK(2)

NAME

       fork - create a new process

SYNOPSIS

       #include <sys/types.h>
       #include <unistd.h>

       pid_t fork(void)

DESCRIPTION

       Fork causes creation of a new process.  The new process (child process) is an exact copy of the calling process except for the following:

	      The child process has a unique process ID.

	      The child process has a different parent process ID (i.e., the process ID of the parent process).

	      The  child  process has its own copy of the parent's descriptors.  These descriptors reference the same underlying objects, so that,
	      for instance, file pointers in file objects are shared between the child and the parent, so that an lseek(2) on a descriptor in  the
	      child  process  can affect a subsequent read or write by the parent.  This descriptor copying is also used by the shell to establish
	      standard input and output for newly created processes as well as to set up pipes.

	      The child starts with no pending signals and an inactive alarm timer.

RETURN VALUE

       Upon successful completion, fork returns a value of 0 to the child process and returns the process ID of the child process  to  the  parent
       process.   Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set to
       indicate the error.

ERRORS

       Fork will fail and no child process will be created if one or more of the following are true:

       [EAGAIN]       The system-imposed limit on the total number of processes under execution would be exceeded.  This limit	is  configuration-
		      dependent.  (The kernel variable NR_PROCS in <minix/config.h> (Minix), or <minix/const.h> (Minix-vmd).)

       [ENOMEM]       There is insufficient (virtual) memory for the new process.

SEE ALSO

       execve(2), wait(2).

3rd Berkeley Distribution					   May 22, 1986 							   FORK(2)
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