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Vieri
Apprentice
Joined: 18 Dec 2005
Posts: 266
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 Posted: Mon Feb 25, 2013 7:44 am Post subject: [SOLVED] count number of processes of current bash script |
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Hi,
I've just come across a weird behavior in BASH. I'm sure there's a logical explanation for this and I hope someone here can help me understand it.
Here's the sample script:
| Code: |
# cat test.sh
#!/bin/bash
num_procs=$( ps -ae | grep -c "test.sh\$" )
num_procs2=$( ps -ae | grep "[t]est.sh\$" | wc -l )
ps -ae | grep "test.sh\$"
echo "Num processes: ${num_procs}"
echo "Num processes 2: ${num_procs2}"
if [ "x${num_procs}" = "x1" ]; then
echo "equal (txt)"
else
echo "not equal (txt)"
fi
if [ $((num_procs)) -eq 1 ]; then
echo "equal"
else
echo "not equal"
fi
exit 0
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Here's the output:
| Code: |
# ./test.sh
10783 pts/0 00:00:00 test.sh
Num processes: 2
Num processes 2: 2
not equal (txt)
not equal
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I'm expecting num_procs to be 1, not 2.
Why am I getting 2?
Thanks,
Vieri
Last edited by Vieri on Tue Feb 26, 2013 7:34 am; edited 1 time in total |
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py-ro
Veteran
Joined: 24 Sep 2002
Posts: 1269
Location: St. Wendel
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| Posted: Mon Feb 25, 2013 11:29 am Post subject: |
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| Because grep finds itself. |
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Vieri
Apprentice
Joined: 18 Dec 2005
Posts: 266
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| Posted: Mon Feb 25, 2013 12:42 pm Post subject: |
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Not if the code is | Code: | | ps -ae | grep "[t]est.sh\$" | wc -l |
, as above.
Also, if I run | Code: | | ps -ae | grep "test.sh\$" |
within the bash script, the output is | Code: | | 10783 pts/0 00:00:00 test.sh |
and there's no grep in sight... |
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Genone
Retired Dev
Joined: 14 Mar 2003
Posts: 8862
Location: beyond the rim
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| Posted: Mon Feb 25, 2013 2:01 pm Post subject: |
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| Random guess is that the $() subshell affects the result. Could try running your "control grep" in a subshell too to verify, e.g. `echo $(ps -ae | grep 'test.sh$')` instead of `ps -ae | grep "test.sh\$"` |
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aCOSwt
Advocate
Joined: 19 Oct 2007
Posts: 2042
Location: Between the keyboard and the chair
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| Posted: Mon Feb 25, 2013 2:50 pm Post subject: Re: count number of processes of current bash script |
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| Vieri wrote: | | Why am I getting 2? |
And the answer is : Because you really get two processes which comm name is test.sh !
| Code: |
num_procs2=$( ps -ae | grep "[t]est.sh\$" | wc -l )
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In order to execute this, test.sh will need to fork, the child's comm name being... test.sh !
So when this ps executes, there are actually 2 processes which comm name is = test.sh
If you want to put an evidence on this then, instead of ps -ae.... launch whatever program that would wait before returning a result.
Launch this script on one tty and on another one, do a ps !
_________________
In theory there are no differences between theory and practice. In practice, there are.
Don't try to understand my posts. Immanuel Kant never did, he thinks that only music and laughter do not have to mean anything. |
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Vieri
Apprentice
Joined: 18 Dec 2005
Posts: 266
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| Posted: Tue Feb 26, 2013 7:31 am Post subject: |
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Got it! Thanks!
So my bash script spawns another bash process with the same name when called with $().
Crystal clear now.
Thanks again. |
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Naib
Advocate
Joined: 21 May 2004
Posts: 3930
Location: UK - Birmingham
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| Posted: Tue Feb 26, 2013 10:16 pm Post subject: |
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just to add my 2cent
http://mywiki.wooledge.org/ProcessManagement
why ps aux | grep [f]oo will return the pid (as well as other things) of process foo (and not the grep command), it is far from the ideal solution to process manage in bash. |
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