Debian Bug report logs - #774001
${#@} and ${#*} are incompatable with bash and posh

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Date: Sun, 16 Aug 2015 15:33:40 +0200
From: Jilles Tjoelker <[email protected]>
To: [email protected]
Subject: Re: Bug#774001: ${#@} and ${#*} are incompatable with bash and posh
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On Fri, 26 Dec 2014 19:30:52 -0400 Joey Hess <[email protected]> wrote:
> Package: dash
> Version: 0.5.7-4+b1
> Severity: normal

> joey@darkstar:~>cat testcase 
> #!/bin/sh
> echo ${#@}

> [snip]

> posh:
>        ${#name}
>            The number of positional parameters if name is *, @ or is
>            not specified, or the length of the string value of
>            parameter name.

> bash:

> 	${#name[subscript]}  expands  to  the  length  of
>        ${name[subscript]}.  If subscript is * or @, the expansion is the
>        number of elements in the array.

Nitpick: this doesn't actually describe ${#@}. However, the actual
description of ${#parameter} further on explicitly documents what ${#@}
and ${#*} expand to, the same as $#.

> dash:

>      ${#parameter}         String Length.  The length in characters
>                            of the value of parameter.

> So dash's documentation doesn't specify what it does for ${#@}. What its
> actually doing is taking the length of the string consisting of all the
> positional parameters, separated by spaces.

This is actually the length of the expansion of $@ or $* in a context
where word splitting is not performed, except if IFS is set to the empty
string.

> I have not checked if the behavior of ${#@} is standardized in any way.
> If ${#@} is standardized, then dash should follow the standard.

POSIX leaves the result of the expansion of ${#@} and ${#*} unspecified.

> Currently, a #!/bin/sh script cannot currently safely use ${#@}, because
> /bin/sh can be dash or bash. If some package contains a #!/bin/dash
> script, it could rely on the current behavior. OTOH, the behavior is
> undocumented, and seems pretty useless, so it's unlikely someone would
> go to the bother of using #!/bin/dash to get it. This might argue for
> changing the behavior to match bash and posh even if there is not a
> standard.

Dash's ${#*} may be a faster alternative to temp="$*"; ${#temp}.
However, the silently inconsistent behaviour across shells is a good
reason not to use it.

> On the other hand, $# reliably yields the number of positional parameters
> in all of bash, posh, and dash. So any script can use that. So, the easiest
> fix would be to just document on its man page the special way bash handles
> ${#@} (and ${#*})

Indeed, there is no reason to use ${#@} and ${#*} in bash/posh.

-- 
Jilles Tjoelker

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