Re: [9fans] rc here docs

[Geoff Collyer <geoff@collyer.net>]

Getting here documents right inside functions and loops and loops
inside functions and in the background is tricky.  Even Bourne didn't
get it right (see BUGS in V7's sh(1)).  I think the SVR2 shell (thus
the V8 shell) got it mostly right, but I didn't push it hard.  There
are two main issues to sort out: when to start reading a given here
document (and collecting it into a temp file or perhaps into memory
nowadays) and when to remove the temp file.  This needs to work in the
presence of notes too.  So consider scripts like this one:

fn sigint sigexit {
	{
		for (i in 1 2 3) {
			cat <<!
going $i times...
!
			sleep 1
		}
	} &
	sleep 4
}
{
	for (f) {
		cat <<! $f <<! | lp
header
!
trailer
!
	}
} &
sleep 3
exec date

Note the extra complications: variable substitution in a here document
in a loop, and multiple here documents for a single command.  (Do all
the here-document temp files get removed if you type an interrupt?)
Getting all of this to work together is tricky, so I can't fault
Bourne or td.  As I recall, the SVR2 sh added a separate stack for
here documents, but even then I'm not sure it got all the cases right
(for example, does an "exec" command remove any outstanding
here-document temp files?).  If /tmp were always a ramfs, we could
probably ignore the hard cases of removing here-document temp files.
This still leaves cases like variable substitution in a here document
in a loop, for which you need to generate a different temporary file
on each iteration.  According to rc(1), we don't have to deal with one
of the messy cases that sh had to:

cat <<!
stuff
`cat <<!!
inner stuff for $USER
!!
`
more stuff
!

It's evident from the code (V7 sh) that here documents were a late
addition to the shell, hacked in rather hurriedly, perhaps to
eliminate the need for the PWB/UNIX pump command, which had been used
with the PWB shell to give a similar effect.

So: do we need here documents at all?  Is

cat <<! | somecommand
stuff
$user
more stuff
!

any more useful or noticeably more convenient than

echo 'stuff
'$user'
more stuff' | somecommand

on Plan 9, where we have more-or-less unrestricted command-line
arguments (unlike V7 Unix, which had a limit of 5120 bytes, including
environment if I can trust my memory).

How are others using here documents?