Hi Cameron,
thanks for the excellent answer, FAQ material!
Cheers!
--- Cameron Simpson <cs@xxxxxxxxxx> wrote:
> On 15Oct2007 13:59, Daniel Qarras <dqarras@xxxxxxxxx> wrote:
> | just a quick question about shell parameter expansion: "$@" is
> obvious
> | from, e.g., bash manual page but I'm puzzled when I've seen few
> time
> | this sort of construct used: ${1+"$@"}
> |
> | What's the difference of "$@" and ${1+"$@"} (in theory and in
> | practice)? Any examples?
>
> "$@", as you know, is a properly quoted version of the command line
> arguments. If you call a script thus:
>
> the-script a b "c d"
>
> then "$@" will get you three strings: "a", "b" and "c d".
> Comapred to $*, which gets you 4 ("a", "b", "c" and "d") and
> compared to "$*", which gets you one ("a b c d") like any other shell
> variable. So "$@" is a bit of special magic.
>
> However, historically, if there were _no_ command line arguments then
> "$@"
> got you a single empty string (""). Presumably because it was felt
> that a
> quoted string should never just disappear.
>
> Thus the incantation ${1+"$@"}, which says: if $1 is defined,
> substitute
> "$@"; if $1 is _not_ defined (i.e. there are no arguments) substitute
> nothing. You'll find ${foo+bah} documented in the shell's PARAMETER
> SUBSTITUTION section.
>
> Now, with a bunch of modern shells $@ is more magic that it used to
> be;
> it used to be that only a quoted "$@" did the magic thing, and an
> unquoted $@ behaved like $* and any other variable. But for some
> recent
> shells a bare $@ behaves the way you probably would have wanted "$@"
> to
> behave, without the nasty "no arguments" corner case.
>
> However, for portability some of us old timers write ${1+"$@"}
> because
> it works everywhere reliably. Just learn to recognise and use it by
> reflex instead of "$@". It means what you intended when you wrote
> "$@".
>
> Cheers,
> --
> Cameron Simpson <cs@xxxxxxxxxx> DoD#743
> http://www.cskk.ezoshosting.com/cs/
>
> Don't have awk? Use this simple sh emulation:
> #!/bin/sh
> echo 'Awk bailing out!' >&2
> exit 2
> - Tom Horsley <tahorsley@xxxxxxxxxxxxxx>
>
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