[Info-vax] Unix and DCL shells

[Lawrence D'Oliveiro]

On Mon, 8 Jan 2024 14:21:18 -0000 (UTC), Simon Clubley wrote:

> Filename globbing.

Coming back to this, it’s worth pointing out that you cannot simply add 
features, willy-nilly, to a framework that was not designed with them in 
mind.

Consider how the whole VMS mindset expects the command-line to work: there 
is a distinction between filespecs separated by spaces and those separated 
by commas, and even plus-signs. The CLI never does wildcard expansion, and 
on the one hand this puts the burden on the program (which has the 
powerful RMS $PARSE service to call on) to expand input filespecs, and on 
the other hand it allows wildcards to appear in output filespecs as well 
(again, supported by $PARSE) to indicate “use the same value here as in 
the input filespec”.

That latter meaning is something I have sometimes missed on *nix systems.

In the *nix world, there is actually no requirement for filename globbing 
to happen in a command at all. Shells usually do it, and they usually 
provide a way to tell them not to do it. (If only because, believe it or 
not, those globbing wildcard characters are valid in filenames, too.) If 
one user program is directly spawning another, without going through a 
shell, then it has to explicitly call some library routine to do globbing, 
otherwise it doesn’t happen.

There is no equivalent to CLD files in the *nix world. (In my time on VMS, 
I found them too fiddly to use, anyway, and relied on a simple “foreign” 
command using LIB$TPARSE instead.) But one thing that improves the 
situation is that the “command line” is not a simple string, but an array 
of strings. And by default the shell will do word-splitting on the command 
line the user passes, to build this array. So it is possible to implement 
very simple option parsing in any program, without resorting to very 
elaborate libraries, to support a useful range of command-line options.