OT: bash vs. ksh subprocess counts

From: Herring Dave - dherri <Dave.Herring_at_acxiom.com>
Date: Sun, 15 Apr 2012 17:45:08 +0000
Message-ID: <BD475CE0B3EE894DA0CAB36CE2F7DEB4455C9123_at_LITIGMBCRP02.Corp.Acxiom.net>

I realize this is way off topic (probably should be titled "WOT:...") for Oracle but #1, I don't currently belong to any good linux/redhat forums and #2, the issue found was from an Oracle maint. script :-) Anyway, I'm on a RHEL 4.6 server (2.6.9-67.0.22.ELlargesmp) and noticed a given DBA maint script wasn't running. It turns out that the only difference between when it last ran and now is someone changed the shell to be "bash" -> #!/bin/bash vs. #!/bin/ksh. The script has a little if-test to start with, used as a way to determine if a previous iteration of the job is still running:

if [ `ps -ef|grep -cw $0` -gt 3 ]; then

   echo "$0 is already running"
   exit 2
fi

The problem is that when this "ps ..." command stream is run within an if-test, it returns a value 1 higher than under "ksh". So when run out of cron it returns 4 under "bash" and 3 under "ksh". I added "set -x" right before this command and here are the results:

                (under ksh)

+ grep -cw /home/test.sh
+ [ 3 -gt 3 ]

(under bash)
++ ps -ef
++ grep -cw /home/test.sh

+ '[' 4 -gt 3 ']'

This change only happens when running that command stream as part of an if-test. So "ps -ef|grep -cw $0" returns "3" but "if [ `ps -ef|grep -cw $0` -gt 3 ]" evaluates to "4". Am I interpreting this correctly in saying that bash forks another process when evaluating a command as part of an if-test while ksh does not? Is there a more general way of explaining this difference is shell processing?

DAVID HERRING
DBA
Acxiom Corporation
EML dave.herring_at_acxiom.com
TEL 630.944.4762
MBL 630.430.5988
1501 Opus Pl, Downers Grove, IL 60515, USA WWW.ACXIOM.COM<http://www.acxiom.com/>

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Received on Sun Apr 15 2012 - 12:45:08 CDT