"ps -ef" using in backquotes
 

Hi,

I have a question about using the ps within backqoutes (``)
When I tried to write a script which is supposed to run just once at a time
I encountered the following thing which I don't know if it's a BUG or FEATURE.

inside the script 'runonce.sh':
---begin script---
#! /bin/bash
# just run the ps and watch te output
ps -ef | grep 'runonce.sh'

RESULT_FULL=`ps -ef | grep 'runonce.sh'`
echo result_full: "$RESULT_FULL"

---end script---

this script outputs:
[hostnix] ./runonce.sh
oracle 6443 3691 0 15:29:02 pts/8 0:00 /bin/bash ./runonce.sh
result_full: oracle 6462 6443 0 15:29:02 pts/8 0:00 /bin/bash ./runonce.sh
oracle 6443 3691 0 15:29:02 pts/8 0:00 /bin/bash ./runonce.sh
oracle 6464 6462 0 15:29:02 pts/8 0:00 grep runonce.sh

why do I get the /bin/bash ./runonce.sh twice ??

any help welcome

guenter porzer

You're running the ps twice, that's why you're seeing it twice.

hi,

actually the ps is not running twice at the same time. It gets started in consecutive.
Besides what I don't understand is why in $RESULT_FULL I'm getting the runonce.sh script twice ?
If I run the same script in KSH( just changing #!/bin/bash to #!/bin/ksh )
I'm geting the expected result of 1 script process and 1 grep process.
As you can see in BASH I'm getting 3 processes 2 scripts running and 1 grep.

thanks
guenter porzer

If you want to know whats happening, add -x to /bin/bash.
Useful for debugging.

Are you asking why you see "/bin/bash" and "grep" lines? The grep line is the result of your script. You might try the "grep -v grep" trick or grep for [r]unonce.sh.

The OP is certainly asking why these two lines appears for a single script:
My guess would be that bash is either forking or firing a thread that appears to have the same command name as itself (before being updated by the loader possibly).

@ jlliagre, you are absolute right.

In the meantime I found a workaround for this behavior.
I just thougt there would be no difference between ksh and bash concerning backquotes ( or at least I've never read anything about this ).

Anyway thanks for all input for this topic !

guenter porzer