Is there a POSIX shell which completely removes the need for `cat` and `dd` by using in-place operators?

Question

In Vidar Holen's blog entry Useless Use of dd, he argues that dd is effectively useless, primarily because everything is a file and files can be piped.

We have also seen regularly on all the *nix sites the active discouragement of using cat.

However, then Vidar puts forward this example:

# Rip a cdrom to a .iso file
cat /dev/cdrom > myfile.iso

...and I wonder, is it possible to do this without using cat?

Obviously, the POSIX shell is presuming that the first group of chars is a command (hence "["="test", etc) which is unusual in modern languages with in-place operators. And, being as a shell is really an interpreter, why have we not got to the point where both cat and dd are redundant?

Are there any POSIX-based shells which have fully implemented in-place operators to the point where just /dev/cdrom > myfile.iso would be enough to copy the data from a device to a file?

Or

Is there something in the POSIX specification which makes this kind of feature impossible?

Note that this question is slightly different from the UUOC question in that the example still uses a command at the beginning ("<") and not in-place operators. So it still conforms to the first-word-is-a-command rule.

(BTW, I personally like dd because, without it, speaking Welsh would be a lot less fun. :-p)

Answer 1

zsh:

</dev/cdrom >myfile.iso

will do that with only operators (when not in sh emulation). It's still running the $NULLCMD command (cat by default) to do the actual reading and writing, though.

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Your proposed syntax

/dev/cdrom > myfile.iso

could actually work now in any POSIX shell — because it doesn't necessarily implicate the shell at all.

What is required is that /dev/cdrom is executable, and that it outputs the contents of the CD when executed. That is something that your kernel or a device driver could provide. Alternatively, the system could have execl (and friends) use /dev/cdrom or its contents as a key to launch another executable (such as cat) with that filename.

In either case, with a suitably-execable /dev/cdrom you'd be absolutely able to run that command in any shell. I don't know of any system that does behave that way, probably for very good reason, but it is theoretically permissible and possible.

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POSIX

POSIX requires that the first word of the command be treated as a command name, and it defines how that command must be run:

the shell shall execute the utility in a separate utility environment with actions equivalent to calling the execl() function defined in the System Interfaces volume of POSIX.1-2017 with the path and arg0 arguments set to the command name [...]

If the execl() function fails due to an error equivalent to the [ENOEXEC] error, the shell [...] shall write an error message and shall return an exit status of 126.

It is thus not permitted for a conforming shell to open and print the file for data. There is no particular reason that a shell could not support that behaviour as a non-POSIX extension, however. I suspect that the form you propose has major drawbacks, but zsh's version is largely fine.