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Full Discussion: Getting exit status of child in trap handler
Top Forums Shell Programming and Scripting Getting exit status of child in trap handler Post 302205438 by rimon on Sunday 15th of June 2008 04:07:28 AM
Old 06-15-2008
Getting exit status of child in trap handler
Hi,

I have a trap problem when calling a child script in the background.
I know there are a lot of threads here on the issue of traps and signals, I think I have read all the relevant ones, but still haven't found an answer to my problem.

I'm working on Linux or HP, the script as you can see is in ksh.

I have written sample scripts for the sake of testing.
The parent script is as follows:
Code:
#!/bin/ksh -x

trap "echo Sub command exited with $?; exit 1" ERR

./subscript.sh  &


saved_pid=$!

while true ; do
        sleep 2
done

wait $saved_pid

echo wait returned $?

The child process is anything that runs and exits with a non-zero value. For the test I used subscript.sh which is in fact:
Code:
#!/bin/ksh  -x

sleep 1

exit 1

When I run the parent script it loops indefinitely, although the child exits with status 1.

What I need is for the parent script to exit when the child exits with failure. The problem is that I have the following limitations:
1. The child process is run in the background, and my parent script meanwhile continues doing other things (processing the output of the child). The parent is in fact running a loop which will terminate only if the child has successfully terminated and thus provided certain output. Therefore I cannot ‘wait' for the child process before it finishes its output, I need any failure to stop the parent wherever it is in the script.
2. I cannot simply set the ERR trap to exit, since I have commands in the script which return non-zero status. For example an ‘ls' command with a pattern that finds no files, and so returns an error. What I really want to do is set the signal handler for CHLD, and check the exit status to see if it is non-zero before exiting.
3. The child process is not mine to change - it is an independent component, and I cannot rely on its contents, other than the fact that it will return a non-zero status when it fails.

Things I don't understand in the behavior of the script:
1. When the child is run in the background, its non-zero exit does not trigger the ERR trap in the parent.
2. How can I see the exit status of the child from the trap handler? As you can see in this script, I tried printing $?, but it prints 0 although I know the child returned 1.

Any ideas?

Thanks,

RO
 

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LEARN ABOUT PHP

preap

preap(1)							   User Commands							  preap(1)

NAME

       preap - force a defunct process to be reaped by its parent

SYNOPSIS

       preap [-F] pid...

DESCRIPTION

       A  defunct  (or	zombie)  process  is one whose exit status has yet to be reaped by its parent. The exit status is reaped via the wait(3C),
       waitid(2), or waitpid(3C) system call. In the normal course of system operation, zombies may occur, but are typically short-lived. This may
       happen  if a parent exits without having reaped the exit status of some or all of its children. In that case, those children are reparented
       to PID 1. See init(1M), which periodically reaps such processes.

       An irresponsible parent process may not exit for a very long time and thus leave zombies on the system. Since the operating system destroys
       nearly all components of a process before it becomes defunct, such defunct processes do not normally impact system operation. However, they
       do consume a small amount of system memory.

       preap forces the parent of the process specified by pid to waitid(3C) for pid, if pid represents a defunct process.

       preap will attempt to prevent the administrator from unwisely reaping a child process which might soon be reaped by the parent, if:

	 o  The process is a child of init(1M).

	 o  The parent process is stopped and might wait on the child when it is again allowed to run.

	 o  The process has been defunct for less than one minute.

OPTIONS

       The following option is supported:

       -F	Forces the parent to reap the child, overriding safety checks.

OPERANDS

       The following operand is supported:

       pid	Process ID list.

EXIT STATUS

       The following exit values are returned by preap, which prints the exit status of each target process reaped:

       0	Successfully operation.

       non-zero Failure, such as no such process, permission denied, or invalid option.

ATTRIBUTES

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

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Availability		     |SUNWesu (32-bit)		   |
       +-----------------------------+-----------------------------+
       |			     |SUNWesxu (64-bit) 	   |
       +-----------------------------+-----------------------------+

SEE ALSO

       proc(1), init(1M), waitid(2), wait(3C), waitpid(3C), proc(4), attributes(5)

WARNINGS

       preap should be applied sparingly and only in situations in which the administrator or developer has confirmed that defunct processes  will
       not be reaped by the parent process. Otherwise, applying preap may damage the parent process in unpredictable ways.

SunOS 5.10							    26 Mar 2001 							  preap(1)
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