Re: ksh background loop behavior

[Raf]

On Wed, Dec 24, 2014 at 02:09:08AM EST, Philippe Meunier wrote:

> Hello,

Hi Philippe,

> I have a small ksh script that uses ps(1) inside a background loop to
> monitor some process while the script does some other stuff in the
> foreground.  Here is a simplified version of the script that monitors
> the startx process, as an example:
> 
> #!/bin/ksh -ex
> while true; do ps -lww | egrep startx | egrep --line-buffered -v egrep; sleep 
> 1; done &
 ^^^

As you already got the answer you were looking for, I'll only add my 2p,
albeit slightly off-topic.

Obviously, I don't know what your exact reasons are to use the above
construct but three processes seems a bit excessive since pgrep(1) can
be used.

Regards,

Raf

Re: ksh background loop behavior

[Philippe Meunier]

Ted Unangst wrote:
>You tell ksh to exit on error.
[...]
>You run a command (egrep) that exits with an error.

So correct... (head hits keyboard).  Thanks!

Philippe

Re: ksh background loop behavior

[Ted Unangst]

On Wed, Dec 24, 2014 at 02:09, Philippe Meunier wrote:

> #!/bin/ksh -ex

You tell ksh to exit on error.

> while true; do ps -lww | egrep startxBOGUS | egrep --line-buffered -v
> egrep; sleep 1; done &

You run a command (egrep) that exits with an error.

ksh background loop behavior

[Philippe Meunier]

Hello,

I have a small ksh script that uses ps(1) inside a background loop to
monitor some process while the script does some other stuff in the
foreground.  Here is a simplified version of the script that monitors
the startx process, as an example:

#!/bin/ksh -ex
while true; do ps -lww | egrep startx | egrep --line-buffered -v egrep; sleep 
1; done &
while true; do echo XXX | egrep --line-buffered -v egrep; sleep 1; done

When I run this script:

./foo > bar2 2>&1

I get the expected output:

+ true
+ true
+ echo XXX
+ egrep --line-buffered -v egrep
XXX
+ sleep 1
+ ps -lww
+ egrep startx
+ egrep --line-buffered -v egrep
 1000 22065  8346  17  18   0   848 4 pause   I+C00:00.02 /bin/sh 
/usr/X11R6/bin/startx /usr/X11R6/bin/startx /usr/X11R6/bin/startx 
/usr/X11R6/bin/startx
+ sleep 1
+ true
+ true
+ echo XXX
+ egrep --line-buffered -v egrep
+ ps -lww
+ egrep startx
+ egrep --line-buffered -v egrep
XXX
+ sleep 1
 1000 22065  8346  17  18   0   848 4 pause   I+C00:00.02 /bin/sh 
/usr/X11R6/bin/startx /usr/X11R6/bin/startx /usr/X11R6/bin/startx 
/usr/X11R6/bin/startx
+ sleep 1
+ true
+ echo XXX
+ egrep --line-buffered -v egrep
XXX
+ sleep 1
+ true
+ ps -lww
+ egrep startx
+ egrep --line-buffered -v egrep
 1000 22065  8346  17  18   0   848 4 pause   I+C00:00.02 /bin/sh 
/usr/X11R6/bin/startx /usr/X11R6/bin/startx /usr/X11R6/bin/startx 
/usr/X11R6/bin/startx
+ sleep 1
+ true
+ echo XXX
[...]

When I kill the script using ^C, the background loop keeps running,
since it's apparently executed as a separate process, so I have to kill
it using kill(1).  Fine.  So far so good.

Now if I modify the background loop to monitor a non-existent process
named startxBOGUS:

#!/bin/ksh -ex
while true; do ps -lww | egrep startxBOGUS | egrep --line-buffered -v egrep; 
sleep 1; done &
while true; do echo XXX | egrep --line-buffered -v egrep; sleep 1; done

and run the script again:

./foo > bar1 2>&1

I get this unexpected output:

+ true
+ true
+ echo XXX
+ egrep --line-buffered -v egrep
XXX
+ sleep 1
+ ps -lww
+ egrep startxBOGUS
+ egrep --line-buffered -v egrep
+ true
+ echo XXX
+ egrep --line-buffered -v egrep
XXX
+ sleep 1
+ true
+ echo XXX
+ egrep --line-buffered -v egrep
XXX
+ sleep 1
+ true
+ echo XXX
+ egrep --line-buffered -v egrep
XXX
+ sleep 1
+ true
+ echo XXX
+ egrep --line-buffered -v egrep
XXX

Note how the trace information from ksh indicates that the 'ps -lww'
command now only runs once.  After killing the script using ^C, I also
notice that there is no separate process that continues running the
background loop.

So here is the question: why the difference in behavior between the two
background loops?  I don't see how changing 'egrep startx' into 'egrep
whatever' could influence the number of times the background loop
runs...  Any help?

A few more things:

- In the real script that I'm trying to write, the process that I want
  to monitor appears and disappears multiple times over several hours,
  so I need the background loop to keep working even when the process
  that I want to monitor is not around.

- I tried the same thing with bash and it exhibits the same difference
  in behavior (except that when killing the script using ^C, bash also
  kills the background loop too, if it's still running)!

- It's ksh 5.2.14 running on OpenBSD 5.6 generic stable i386.

Thanks,

Philippe