(regarding "cmd <&$fd")

It redirects from an open file descriptor. It's conceptually the same operation whether $fd is a numeric file descriptor or some "special file descriptor object".

You know, I'm not placing too high a priority on backward compatibility here. If people have existing code that works with an existing shell, they can go on using that code with the shell it was written for. I wouldn't expect code written for bash to run perfectly in ksh, either. Any extensions either shell provides over baseline Bourne shell is very likely not going to work.

That said, I don't see how my idea breaks anything. It could coexist with explicitly numbered FDs as long as the mechanisms for opening the file are kept distinct.

But explicitly numbered FDs for files opened in the shell aren't really that valuable a feature anyway. If someone opens a file in the shell, they actually have no need to know the number of that file descriptor. (Assuming, as I generally do, that the shell shouldn't indiscriminately export all open file handles to every job it runs.) At most, people need to be able to know what numeric file descriptor an open file will open in a job if they want the job to have access to that FD - and even that is a fairly rare case. I'd just as soon break compatibility if it leads to a superior overall design.

Traditionally, it means weak typing. There's no reason a new shell would have to follow the same pattern. From my experience working with PHP I'm not a huge fan of weak typing, honestly.

Anyway, what I've described is already a form of weak typing. It's weak typing in the same sense that integer variables can only take integer values, but can be inserted into strings without explicit conversion.

Code:

# Assume $fd is a special object representing an open file:
$ some_cmd $fd    # I've already described how this could work, assuming the dominant case where the command takes filenames as input...
$ some_cmd < $fd  # And how this could work...  Possibly with "<&" instead of "<"
$ some_cmd 5< $fd  # And this could be made to work as well

The important principle of weak typing, that you can put whatever wherever and have a reasonable (but not foolproof) expectation that it'll do what you want, is preserved.

Well, shell builtins like while were exactly what we were discussing... What you described here isn't redirecting a block at all.

But, as you suggest - there's no apparent reason this couldn't be added to bash or similar shells... So it's not like it's something that would require revolutionary change to address the issue - unless I'm missing some subtle syntactic issue that makes it so.

It also only gets you one file... If you dup the file to another FD inside the block, the open file's lifetime is no longer tied to the scope of the block.

But the whole point of redirecting into code blocks (for the purposes of this discussion) was to gain scoped lifetime for open files... If it can't be used effectively then we're kind of back where we started on the whole "scoped file" issue, and it's something current shells don't usefully provide.

If you were redirecting stdin for the block, yes that stdin would close, and you'd get your previous stdin back - even though the builtin runs in the shell's own process.

Though I guess what I described in bash is only the case if you're making a dynamic FD assignment as part of the redirection:

Code:

$ some_cmd 7<./some_file
# ./some_file is closed when some_cmd terminates.
# The assignment of this particular file to this particular FD affects only this one job.
# This works whether some_cmd is a builtin or an external command

$ some_cmd {f}<./some_file
# If some_cmd is an external command:
# * The assignment of $f will only apply to the job (and then only if $f is exported)
# * The file at file descriptor $f will be open for the job only
# If some_cmd is a builtin:
# * The assignment of $f will be applied to the shell process
# * The file at the file descriptor given by $f will remain open after some_cmd terminates.

So I guess I was wrong there - "scoped file open" isn't broken in Bash 4, it's just incompatible with dynamically assigned file descriptors... It's an unfortunate disparity and it plays out as one more limitation of the current implementation of scoped-lifetime open files in the shell. But it's probably just a bug and not an intended behavior. And, of course, it only applies to bash...

But the way code blocks work can be changed. If an alternate design would make the shell a nicer environment to work in, then such change is a good thing. Programming languages evolve over time. Even shells have gained lots of new features over the years. Numeric variables, expanded substitution and history recall syntax, arrays, regexes, coprocs...

(side note, I was pleasantly surprised when I found out, just the other day, that bash 4 includes coprocs! Seems like a nice implementation, too... the FDs for communicating with the new coproc are dynamically assigned and stored into an array variable...)

And again, and again, and again with this crap.

Look, the fact that what I come up with doesn't fit what you're used to from a shell doesn't make it not a shell. It could be not a Posix shell, or not a Bourne-compatible shell - but "shell" doesn't mean exactly and only what the usual Unix implementations provide.

To me the defining characteristics of a shell that set it apart from other programming languages are:
1: Its primary mode of operation is to run other programs, provide them with input and do useful things with their output.
2: It's well-suited for interactive use, but the way it's used interactively also translates nicely to non-interactive use (scripting).

This is a pretty broad definition, of course, but a pretty essential one. It leaves a lot of room for different approaches to shell design - many of which wouldn't nicely fit with Unix shell tradition. I think it can be worthwhile to break from tradition - or if nothing else to consider it. Status quo shouldn't blind us to the potential offered by other possibilities. But to be clear what I want is a shell. I just see no reason for this dichotomy... It doesn't have to be "The Unix shell as it's presently implemented" or "not a shell at all".

(EDIT): Yeah, I guess I'd agree that it's probably time for this whole discussion to be extricated from this particular thread. I might go with "speculative shell features" as a title - but whatever works, really...