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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 DEBIAN

hotswaprc

HOTSWAPRC(5)							File Formats Manual						      HOTSWAPRC(5)

NAME

       hotswaprc - configuration file for  hotswap

DESCRIPTION

       /etc/hotswaprc is the global configuration file for the hotswap utility.

       It  allows system administrators to specify arbitrary shell scripts to be run after a device is inserted, as well as before and after it is
       removed. Scripts are selected according to the model name retrieved from the device. This is particularly helpful for automatic	configura-
       tion of CD-RW drives, which require SCSI emulation and bypass normal access via the IDE subsystem.

       hotswaprc  is  implemented  as  an  Extensible Mark-up Language (XML) application.  XML documents are structured using elements of the form
       <tag-name> content <tag-name>. A Document Type Definition (DTD) describes the possible content of each element.

       Please refer to the XML specification for more information. The hotswap distribution also contains an example file,  doc/hotswaprc.example,
       which includes the DTD for the configuration file format.

ELEMENTS

       <hotswap>
	      This is the root element of the document. Each valid hotswaprc must contain exactly one <hotswap> element. The <hotswap> element may
	      contain an arbitrary number of <device> elements.

       <device>
	      The configuration file contains one <device> element for every device for which scripts are defined. The <device>  element  is  com-
	      posed of the following elements in this order: <name>, <post-insert>, <pre-remove>, <post-remove>. All but <name> are optional.

       <name> The content of this element is the model identification string of the IDE device the current <device> element refers to.

       <post-insert>
	      Contains the shell script that is to be executed after the device has been inserted and registered with the kernel.

       <pre-remove>
	      Contains the shell script that is to be run before hotswap attempts to unregister the device.

       <post-remove>
	      Contains the shell script that is to be run after the device had been unregistered.

REPORT BUGS

       Report bugs to t.stadelmann1@physics.ox.ac.uk.

AUTHOR

       Written by Tim Stadelmann.

SEE ALSO

       hotswap(1), xhotswap(1).

COPYRIGHT

       Copyright (c) 2002-2003 Tim Stadelmann.

       Permission is granted to copy, distribute and/or modify this document under the terms of the GNU General Public License (GPL), Version 2 or
       any later version published by the Free Software Foundation.

								26th November 2002						      HOTSWAPRC(5)
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