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Full Discussion: Parallel processing - continued
Top Forums Shell Programming and Scripting Parallel processing - continued Post 302845145 by kumarjt on Wednesday 21st of August 2013 02:54:20 AM
Old 08-21-2013
Parallel processing - continued
Hi,
I am taking up the cue from where I was left in my earlier post ( link given below )
https://www.unix.com/shell-programmin...rocessing.html
I actually wanted to know the significance of using the Unix "wait" , which returns
the control from background to the current process.
To understand this , I created two scripts , namely
script_1.ksh
script_2.ksh

Here are the codes for the scripts:
script_1.ksh
Code:
#!/usr/bin/ksh
for i in {1..4} ;do
   ./script_2.ksh $i &
done
#wait

script_2.ksh
Code:
#!/usr/bin/ksh
   echo $1

I need to know why :
a) After the last iteration of the for loop , the shell prompt is NOT returned to the calling script(script_1.ksh) ?
If wait retrieves the control from the last executed BACKGROUND process , then the wait command should have been
called from within the for loop itself so that after every background execution of script_2.ksh for each of the loop
iteration , the control needs to come back to the parent process (script_1.ksh) so that the next iteration could be
processed . And that would effect a sequential execution of the script_2.ksh , and not parallel.

b) Uncommenting the wait command at the end of the for loop in scipt_1.ksh retrieves the control back to the parent process .
Does this mean that the wait command has brought back the control from the last background porcess kicked off when i=4 in
the for loop , and only then will any other subsequent commands in script_1.ksh will be executed. If so , how could the
loop iterated for i=2 and i=3 when it has already send the control to the background by kicking off the script_2.ksh in
its first iteration , and the for loop doesn't have a wait command within itself.
c) The wait command affecting only the latest background process . What about the ones which were spawned earlier ?


Thanks
Kumarjit.
 

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LEARN ABOUT MINIX

break

break(1)                                                           User Commands                                                          break(1)

NAME

       break, continue - shell built-in functions to escape from or advance within a controlling while, for, foreach, or until  loop

SYNOPSIS

   sh
       break [n]

       continue [n]

   csh
       break

       continue

   ksh
       *break [n]

       *continue [n]

DESCRIPTION

   sh
       The break utility exits from the enclosing for or while loop, if any. If n is specified, break n levels.

       The continue utility resumes the next iteration of the enclosing for or while loop. If n is specified, resume at the n-th enclosing loop.

   csh
       The  break  utility  resumes  execution after the end of the nearest enclosing foreach or while loop. The remaining commands on the current
       line are executed. This allows multilevel breaks to be written as a list of break commands, all on one line.

       The continue utility continues execution of the next iteration of the nearest enclosing while or foreach loop.

   ksh
       The break utility exits from the enclosed for, while, until, or select loop, if any. If n is specified,  then  break  n  levels.  If  n  is
       greater than the number of enclosing loops, the outermost enclosing loop shall be exited.

       The  continue utility resumes the next iteration of the enclosed for, while, until, or select loop. If n is specified then resume at the n-
       th enclosed loop. If n is greater than the number of enclosing loops, the outermost enclosing loop shall be used.

       On this man page, ksh(1) commands that are preceded by one or two * (asterisks) are treated specially in the following ways:

       1.  Variable assignment lists preceding the command remain in effect when the command completes.

       2.  I/O redirections are processed after variable assignments.

       3.  Errors cause a script that contains them to abort.

       4.  Words that follow a command preceded by ** that are in the format of a variable assignment are expanded with the same rules as a  vari-
           able  assignment.  This means that tilde substitution is performed after the = sign, and also that word splitting and file name genera-
           tion are not performed.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE         |      ATTRIBUTE VALUE        |
       +-----------------------------+-----------------------------+
       |Availability                 |SUNWcsu                      |
       +-----------------------------+-----------------------------+

SEE ALSO

       csh(1), exit(1), ksh(1), sh( 1), attributes(5)

SunOS 5.10                                                          17 Jul 2002                                                           break(1)
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