X-mailer: m, by Cameron Simpson
Date: Thu, 25 Mar 1999 23:03:16 +0000
Subject: Re: Linuxconf and Bash-2.0 script
References: <36FA8A08.2D53960@ohiocounty.net>
To: redhat-list@redhat.com
CC: Robert W. Canary <rwcanary@ohiocounty.net>
From: Cameron Simpson <cs@zip.com.au>
Reply-to: cs@zip.com.au
Errors-to: cs@zip.com.au
Return-receipt-to: cs@zip.com.au
Organization: Canon Information Systems Research Australia, Sydney, Oz

On 26 Mar 1999, in message <36FA8A08.2D53960@ohiocounty.net>
  "Robert W. Canary" <rwcanary@ohiocounty.net> wrote:
| Hi have some bash conflicts going on here ....

These are generic shell conflicts, not bash specific.

| The commandline arguments are not being read correctly.   If any of the
| arguments a set of singal quotes ('').  Then bash interrupts them as a
| blank.   Thus when I do 'exec usernew $*' I pass:
| usernew -m -c -d /home/newboy -s /bin/bash -G newboy
| it should be :
| usernew -m -c '' -d /home/newboy -s '/bin/bash' -G '' newboy
[...]
| Is there something special I need to do here ?

The command line arguments are being read correctly - they're just not
being _passed_in_ correctly.

In the shell:

	$*	All arguments, but like any other variable, space
		interpretation is done, which breaks them up and of course
		"vanishes" the empty ones.
	"$*"	All arguments, not space interpretation (but that of course
		turns it into _one_ argument - not what you want)

	$@	Just like $*
	"$@"	All arguments, correctly quoted. This is a special piece of
		magic intended expressly for passing the argument list
		to other commands intact.

So, "$@" is what you want - almost. There's a historical bug with this
of long standing. If you have no arguments at all, "$@" is the same as ""
(i.e. a single empty argument - no what you want). I imagine the reasoning
was that you've got a quoted thing there and it mustn't just vanish, but
generally it's not what you want.

So, we reach for parameter stuff and read in the manual (man sh):

    Parameter Substitution
    [...]
	${parameter:+word}
		If parameter is set and is non-null,  substitute  word;
		otherwise substitute nothing.
    [...]
	 If the colon (:)  is omitted from the above expressions, the
	 shell only checks whether parameter is set or not.

So the incantation you really want is:
	
	${1+"$@"}

This says "if $1 is defined (==> we have more than zero arguments)
insert "$@", otherwise insert nothing". And lo, the "$@" problem with
no arguments is worked around.

Learn it - love it. It is the standard robust way to pass your current
argument list to a subcommand.

You now want to say:

	exec usernew ${1+"$@"}

Simple and robust.
--
Cameron Simpson, DoD#743        cs@zip.com.au    http://www.zip.com.au/~cs/

Technique will get you through times of no strength a lot better than
strength will get you through times of no technique.	- the Nealomatic