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Process count problem in script

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# 8  
Old 07-04-2013
Quote:
Originally Posted by RudiC
Funny enough - I placed a tee into the pipe, and - there are two processes! See:
Code:
check=$(ps -ef|tee xx|grep -c "[s][h] -x abc.sh")
less xx
...
userx   6061  4983  0 17:44 pts/2    00:00:00 bash -x abc.sh
userx   6062  6061  0 17:44 pts/2    00:00:00 bash -x abc.sh

one being an immediate subprocess of the other. Strange!
I don't find it strange at all. To execute the command substitution, the shell forks, creating a copy of itself. The new process has the same argv as its parent.

Regards,
Alister
# 9  
Old 07-04-2013
I thought about the fork to execute the command substitution, but shouldn't the exec thereafter set the command name, i.e. argv[0]?
# 10  
Old 07-04-2013
There's another layer of forking before any exec is necessary. The parent shell forks to create a subshell for the command substitution. This child subshell then itself forks several times to create each pipeline component. Only these grandchildren shell processes need to exec.

Regards,
Alister
# 11  
Old 07-04-2013
OK, thanks, got you. But, how then are rbatte1's results for RHEL and HPUX possible, posted in #5?
# 12  
Old 07-04-2013
I can't say for certain without more information.

Perhaps the subshell is not forking a process for the final command in the pipeline, but, exec'ing directly. Both this behavior and the behavior described in my previous post are POSIX-compliant. From the end of Shell Command Language :: Section 2.12 :
Quote:
Command substitution, commands that are grouped with parentheses, and asynchronous lists shall be executed in a subshell environment. Additionally, each command of a multi-command pipeline is in a subshell environment; as an extension, however, any or all commands in a pipeline may be executed in the current environment.
It's also possible (though, I suspect, less likely) that the subshell is manipulating its argv and the ps implementation does not obtain its info from saved kernel space values.

And, finally, there's also the possibility of PEBKAC (no offense intended, rbatte1 Smilie).

Regards,
Alister
Last edited by alister; 07-04-2013 at 03:16 PM..
# 13  
Old 07-05-2013
No offence taken, mainly because I don't recognise the acronym. Smilie Is it a little like PICNIC (Problem In Chair Not In Computer) Smilie

I won't be offended anyway, I am here to learn more than anything. Smilie



Robin
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