Re: OT: bash vs. ksh subprocess counts
- From: "Radoulov, Dimitre" <cichomitiko@xxxxxxxxx>
- To: Dave.Herring@xxxxxxxxxx
- Date: Sun, 15 Apr 2012 20:53:01 +0200
On 15/04/2012 19:45, Herring Dave - dherri wrote:
> So "ps -ef|grep -cw $0" returns "3" but "if [ `ps -ef|grep -cw $0` -gt 3 ]"
> evaluates to "4". Am I interpreting this correctly in saying that bash forks
> another process when evaluating a command as part of an if-test while ksh
> does not? Is there a more general way of explaining this difference is shell
> processing?
[...]
I believe this is related to the fact that some shells (the KornShell
and Z-Shell) thread the last command in a pipeline differently:
[drado@oel62001 t]$ 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 $0)
drado 3167 3165 3 20:42 pts/0 00:00:00 grep -w bash
[drado@oel62001 t]$ 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 ...
--
//www.freelists.org/webpage/oracle-l
Other related posts:
