what's the use of "{ list }" in /bin/sh?

Sat Jul 9 00:17:32 AEST 1988

In article <12334 at mimsy.UUCP> chris at mimsy.UUCP (Chris Torek) writes:
>In article <3305 at palo-alto.DEC.COM> vixie at palo-alto.DEC.COM (Paul Vixie)
>writes various examples (deleted), then:
>>Summary: {list} is useful if you go southpaw with it.  Other than
>>that it seems useless and I can't imagine why it exists.
>
>Actually, it has one more use:
>
>	while { setup; test } do ...
>
>or any other place where a compound statement is needed and only
>a single statement is allowed.  (while and until are about it!)

it's not actually required for this:

	while setup; test; do ...

has the same effect (though it looks a tad more bizarre). There are no
places where only a single statement is accepted, since there is always
some special keyword (such as "do") to enable the end of a complex statement to
be recognized.

In principle, one could imagine another use for {...}:

	{ cmd1; cmd2; } > file
    or
	cmd0 | { cmd1; cmd2; }

where cmd1 and/or cmd2 effect things in the current shell (ie, they contain
variable assignments or some such). Unfortunately, all versions of sh I've
seen treat the {...} construct the same as (...) in these cases (bogusly),
so

	{ grep mumble; status=$?; grumble; } > file

and

	ps | { read firstline; cat; }

don't do what you'd expect.

Fixed in SysVr4? BSD4.4? Minix1.4? :-)