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Confusion with PS


 
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# 1  
Old 09-27-2012
Confusion with PS

Hello All,

I have a problem in counting number of process getting run with my current script name..

Here it is

Code:
ps -ef | grep $0 | grep -v grep

This display just one line with the PID, PPID and other details when i print it in the script.

But when I want to count the numbers in my script , just like below, it comes out as 2... Whereas only one process is running..

Code:
count=$(ps -ef | grep $0 | grep -v grep | wc -l)
echo $count

My mind completely gone blank, help me here..
Help me how I will get my script to print the correct number of Process running with its name. Thanks.
# 2  
Old 09-27-2012
Quote:
Originally Posted by sathyaonnuix
Code:
ps -ef | grep $0 | grep -v grep

Just one question. from where you are getting $0?
# 3  
Old 09-27-2012
Thats my Script name, this command line is placed inside my script...
# 4  
Old 09-27-2012
Quote:
Originally Posted by pamu
Just one question. from where you are getting $0?
If I understand correctly, he runs these commands from inside a script.

However, see my script, named "file":

Code:
#!/bin/bash
echo $0
ps axu |head -n 1 # this is to see ps headers
ps axu | grep $0 | grep -v grep | tee /dev/tty |wc -l
ps axu | grep $0 | grep -v grep  |tee /dev/tty |wc -l

If I run it, most of the time I get:
Code:
lem@biggy:/tmp$ ./file
./file
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
lem      25757  0.0  0.0  18584  1432 pts/3    S+   11:44   0:00 /bin/bash ./file
1
lem      25757  0.0  0.0  18584  1432 pts/3    S+   11:44   0:00 /bin/bash ./file
1

Very few times I get instead:
Code:
lem@biggy:/tmp$ ./file
./file
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
lem      25770  0.0  0.0  18584  1428 pts/3    S+   11:44   0:00 /bin/bash ./file
1
lem      25770  0.0  0.0  18584  1428 pts/3    S+   11:44   0:00 /bin/bash ./file
lem      25779  0.0  0.0  18584   432 pts/3    R+   11:44   0:00 /bin/bash ./file
2

The first ps always gives me one process, the second ps seldom gives two.
--
Bye
# 5  
Old 09-27-2012
I found out the problem Smilie Since we give wc -l, this gets a new line. And hence that has got counted in it.

I changed that to
Code:
ps -ef | grep $0 | grep -cv grep

Now it works cool. Thanks Pamu and Lem.
# 6  
Old 09-27-2012
Quote:
Originally Posted by sathyaonnuix
Now it works cool.
Sadly, i don't think so. Smilie
--
Bye
# 7  
Old 09-27-2012
All of the problems witnessed above are the result of the shell forking to create new processes to run a pipeline: ps, grep, wc, another shell for the command substition (in the case of the original post). All of these processes initially begin as a new shell with the same name, $0, as the parent shell.

If ps scans the process list before any of these shells have had a chance to exec and become the utility it will become, the result is multiple instances of $0.

The solution is trivial and widely available: pgrep

Quote:
Originally Posted by sathyaonnuix
I found out the problem Smilie Since we give wc -l, this gets a new line. And hence that has got counted in it.

I changed that to
Code:
ps -ef | grep $0 | grep -cv grep

Now it works cool.
Your analysis is incorrect. You're just getting lucky with timing. In your original wc version, you were unlucky with timing. Nothing has been fixed.

The reason pgrep works reliably is because it's only one command and it doesn't run until after the shell that was forked to create it has exec'd.

Unless the system lacks pgrep and pkill, there's no need to resort to the ps ... | grep/awk ... silliness.

Regards,
Alister
This User Gave Thanks to alister For This Post:

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