Re: OT: bash vs. ksh subprocess counts

From: Radoulov, Dimitre <cichomitiko_at_gmail.com>
Date: Sun, 15 Apr 2012 20:53:01 +0200
Message-ID: <4F8B190D.1060204_at_gmail.com>

On 15/04/2012 19:45, Herring Dave - dherri wrote:
> 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?
[...]
I believe this is related to the fact that some shells (the KornShell and Z-Shell) thread the last command in a pipeline differently:

[drado_at_oel62001 t]$ bash -c '(ps -ef|grep -w $0)'

drado     2560  2559  0 20:17 pts/0    00:00:00 -bash
drado     3164  2560  2 20:42 pts/0    00:00:00 bash -c (ps -ef|grep -w $0)
drado     3165  3164  1 20:42 pts/0    00:00:00 bash -c (ps -ef|grep -w $0)
drado     3167  3165  3 20:42 pts/0    00:00:00 grep -w bash

[drado_at_oel62001 t]$ ksh -c '(ps -ef|grep -w $0)'
drado     3168  2560  3 20:42 pts/0    00:00:00 ksh -c (ps -ef|grep -w $0)
drado     3170  3168  1 20:42 pts/0    00:00:00 grep -w ksh

 From the man pages (KornShell 93 Manual Page - www2.research.att.com/sw/download/man/man1/ksh.html):

A /pipeline / is a sequence of one or more /commands / separated by *|*. The standard output of each command but the last is connected by a

/pipe/ <http://www2.research.att.com/sw/download/man/man2/pipe.html>(2) to the standard input of the next command. Each command, _except possibly the last_, is run as a separate process; the shell waits for the last command to terminate.

P.S. I though that this difference was relative only to shell built in commands tough ...

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Received on Sun Apr 15 2012 - 13:53:01 CDT