== An interesting Bourne shell limitation

Presented in illustrated form, on at least Solaris 8, FreeBSD, and
OpenBSD:

 $ exec 9>/dev/null
 $ exec 10>/dev/null
 exec: 10: not found

(And the shell exits.)

The genuine Bourne shell only allows redirection to (or from)
single-digit file descriptors; if you give multiple digits, it
instead gets parsed as '_exec 10 >/dev/null_'.

(The limitation has been faithfully copied by at least some Bourne shell
reimplementations and was retained in ksh, but bash has dispensed with
it, so you won't see this behavior on (most) Linux machines.)

This limitation is implicitly in the [[V7 sh manpage
http://minnie.tuhs.org/UnixTree/V7/usr/man/man1/sh.1.html]], which
says about redirection:

> If one of the above is preceded by a ~~digit~~ then the file
> descriptor created is that specified by the digit (instead of the
> default 0 or 1).

In the grand tradition of reading Unix manpages, using the singular
'digit' means that multiple ones aren't allowed.

This wording was carried on into the Solaris _sh_ manpage, but not the
FreeBSD one. The FreeBSD situation is interesting, since I believe the
FreeBSD shell was rewritten from scratch; clearly the people doing the
rewrite knew about (and decided to preserve) the limitation, but then
they didn't document it.

(One might argue that the FreeBSD usage of '_[n]> file_' in the manpage
implies that '_n_' can only be a single digit, but I think that the ice
is thin, if only because they use things like '_[n1]<&n2_'.)