Re: odd 'shift' behaviour with bash-1.1

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2 total messages Started by Scott Weikart Mon, 31 Aug 1992 22:21

#923

Author: Scott Weikart
Date: Mon, 31 Aug 1992 22:21

16 lines
1228 bytes

>Its SOP for shells that when a control statement has I/O redirected
>the control statement is run in a sub-shell, and so can't affect anything
>in the environment of the real shell (including $# etc).

This is not true, in general (last week I wrote a program to test it).

Redirecting stdin or stdout to files doesn't result in use of a sub-shell; the
subshell is only used when stdin or stdout is connected to another process
(i.e. the control statement is in a pipeline).  And note that ksh88 doesn't
use a sub-shell when only stdin is connected to a process, but *does* use a
sub-shell when stdout is connected to a process; ksh86 and bash use a
sub-shell in either case.

Scott Weikart			Community Data Processing (CdP): 415-322-9069
Internet: scott@igc.org		Bitnet: scott%igc.org@stanford
UUCP Mail Net: uunet!cdp!scott

Re: odd 'shift' behaviour with bash-1.1

#926

Author: Robert Elz
Date: Tue, 01 Sep 1992 22:34

36 lines
1466 bytes

In <9208312221.AA21541@igc.apc.org> scott@igc.apc.org (Scott Weikart) writes:

| >Its SOP for shells that when a control statement has I/O redirected

| This is not true, in general (last week I wrote a program to test it).

| Redirecting stdin or stdout to files doesn't result in use of a sub-shell;

My definition of "SOP for shells" is what the Bourne shell does (and
when it matters, often the 7th edition Bourne shell).

Here's a simple test using /bin/sh

munnari$ f=0
munnari$ for i in a b
> do
> f=1
> done
munnari$ echo $f
1
munnari$  for i in a b
> do
> f=2
> done </dev/null
munnari$ echo $f
1

The point was that in general its unsafe to assume that a "sh"
type shell will allow you to alter the shell environment in any
way if I/O to a control statement is redirected at all.

Some shells may be more liberal in what they allow.  Personally
I don't think I'll be writing many scripts that depend on which version
of which shell I happen to be using.

kre

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