Re: [Help-bash] What does this idiom mean

[Eric Blake]

On 02/08/2012 12:24 PM, Bill Gradwohl wrote:
> Somewhere, I believe, I saw the following:
> 
> address@hidden@}

That says, if the variable on the left of + is set, then replace the
${...} with the expansion to the right of the +; if the variable on the
left of + is not set, then expand to nothing.

But this is redundant as written - either $@ is set, so you expand to $@
in the first place; or $@ is not set, and you expand to nothing; and
that can just as easily be written as:

$@

However, it is more likely that you've seen the idiom ${1+"$@"}.

Although the same arguments apply about redundancy (if $@ is set, then
$1 must be set), there is a _real_ reason for doing this in portable
code, which is that some older shells that don't quite comply with POSIX
misinterpret

"$@"

and generate an empty string if $@ is unset, rather than the POSIX
behavior of eliding the entire argument ("$@" is the only special
variable like that).  By separating the "$@" to only be expanded if
there are arguments, you can omit the misinterpreted empty argument on
those older shells.

> I understand what address@hidden is but everything from the + onward is a 
> mystery.

See the bash manual, under "Parameter Expansion", for several useful
conditional expansion forms:

${var:+value}
${var+value}
${var:-value}
${var-value}
${var:?value}
${var?value}
${var:=value}
${var=value}

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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