In ksh, how does an in-line child sub-process get its own PID? Post: 302074741

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Top Forums Shell Programming and Scripting In ksh, how does an in-line child sub-process get its own PID? Post 302074741 by MichLab on Thursday 25th of May 2006 02:11:42 PM

05-25-2006

Registered User

In ksh, how does an in-line child sub-process get its own PID?

This is not the same as a few of the other posted items dealing with sub-process pids (that I saw anyway).

If zot contains:

echo "main mypid: $$ - lastpid: $!"
(
echo "block mypid: $$ - lastpid: $! - ppid: $PPID"
ps -ef > xxx
sleep 5
echo "block mypid: $$ - lastpid: $! - ppid: $PPID"
) &
echo "main mypid: $$ - lastpid: $! - ppid: $PPID"
sleep 6

Then: ksh zot (under Solaris and MKS) results in:

main mypid: 7318 - lastpid:
block mypid: 7318 - lastpid: - ppid: 607
main mypid: 7318 - lastpid: 7320 - ppid: 607
block mypid: 7318 - lastpid: - ppid: 607

It surprises me that in the forked child process, that $$ still returns the pid of the main process. i.e. both show $$ as 7318 - I would have expected the child to have $$ be $7320. (You can ignore the ppid - I was looking at related notions.)

If you do: "grep 7320 xxx" you get:

root 7320 7318 0 13:59:45 pts/9 0:00 ksh zot
root 7321 7320 1 13:59:45 pts/9 0:00 ps -ef

which clearly shows the child process and if you were to "kill -9" that pid before the 5 seconds runs out, you would not get the second block line.

My intention was for the child process to record its own pid rather than the parent doing it on its behalf. I realise that the parent can use $! but why can the child not use $$ for its own PID? By the way, if you extract the block of code and put it in a separate file and call it, then you get the expected results.

Thank you in advance for any insights.
Michel

MichLab

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LEARN ABOUT SUNOS

print

print(1)							   User Commands							  print(1)

NAME

       print - shell built-in function to output characters to the screen or window

SYNOPSIS

   ksh
       print [ -Rnprsu [n]] [arg...]

DESCRIPTION

   ksh
       The shell output mechanism. With no flags or with flag - or -, the arguments are printed on standard output as described by echo(1).

OPTIONS

       The following options are supported:

       -n	       suppresses new-line from being added to the output.

       -R

       -r	       (raw  mode) ignore the escape conventions of echo. The -R option will print all subsequent arguments and options other than
		       -n.

       -p	       causes the arguments to be written onto the pipe of the process spawned with |& instead of standard output.

       -s	       causes the arguments to be written onto the history file instead of standard output.

       -u [ n ]        flag can be used to specify a one digit file descriptor unit number n on which the output will be placed. The default is 1.

EXIT STATUS

       The following exit values are returned:

       0	Successful operation.

       >0	Output file is not open for writing.

ATTRIBUTES

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

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

SEE ALSO

       echo(1), ksh(1), attributes(5)

SunOS 5.10							    15 Apr 1994 							  print(1)

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