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Top Forums Shell Programming and Scripting Monitor a long running process Post 302419507 by alister on Friday 7th of May 2010 11:15:40 AM
Old 05-07-2010
Quote:
Originally Posted by sunpraveen
I usually use while loop to monitor any long running process as below:

Code:
while true
do
if [[ $(ps -ef | grep <long running process id> | wc -l) -eq 0 ]];
then
<do something>
break
else
<do something>
fi

In my opinion, this is a very poor approach, as it is very vulnerable to false positives. What if a different command with a different pid contains an argument that matches the pid being grepped for? This code will not detect that the monitored process has died/exited. What's much worse, however, is that depending on the kernel process scheduler and the shell implementation, the grep command's pid argument itself could trigger a false positive when groveling through the ps output (and given the way this code is structured, to test for equality to zero as failure, this could be happening often ... silently).

I would (perhaps melodramatically Smilie) characterize this as an indiscriminate grepping of ps output. When dealing with multicolumn output whose values can collide (the arguments field, for example, can conceivably contain a username/pid/ppid/...), it is safer and easier to constrain the match to a specific column using AWK. And, if applicable, use ps' output options to limit output only to the fields that are necessary. If you are grepping for a pid, why not use a ps output format that only prints pid?

Instead of ...
Code:
if [[ $(ps -ef | grep <long running process id> | wc -l) -eq 0 ]];

I would suggest ...
Code:
if ! [ $(ps -o pid= -p <long running process id>) ]

Or, you could grep|wc the output of "ps -o pid= -p <long running process id>", if that feels more familiar Smilie

Regards,
Alister

Last edited by alister; 05-07-2010 at 12:36 PM..
 

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pid(1T) 						       Tcl Built-In Commands							   pid(1T)

__________________________________________________________________________________________________________________________________________________

NAME
pid - Retrieve process identifiers SYNOPSIS
pid ?fileId? _________________________________________________________________ DESCRIPTION
If the fileId argument is given then it should normally refer to a process pipeline created with the open command. In this case the pid command will return a list whose elements are the process identifiers of all the processes in the pipeline, in order. The list will be empty if fileId refers to an open file that isn't a process pipeline. If no fileId argument is given then pid returns the process identi- fier of the current process. All process identifiers are returned as decimal strings. EXAMPLE
Print process information about the processes in a pipeline using the SysV ps program before reading the output of that pipeline: set pipeline [open "| zcat somefile.gz | grep foobar | sort -u"] # Print process information exec ps -fp [pid $pipeline] >@stdout # Print a separator and then the output of the pipeline puts [string repeat - 70] puts [read $pipeline] close $pipeline SEE ALSO
exec(1T), open(1T) KEYWORDS
file, pipeline, process identifier ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +--------------------+-----------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +--------------------+-----------------+ |Availability | SUNWTcl | +--------------------+-----------------+ |Interface Stability | Uncommitted | +--------------------+-----------------+ NOTES
Source for Tcl is available on http://opensolaris.org. Tcl 7.0 pid(1T)
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