Re: ${| command; } funsub not setting REPLY does not error out with 'set -u' active

[Chet Ramey]

On 12/20/24 11:04 AM, Zachary Santer wrote:

> Explicitly attempting to expand any one of those with a parameter
> expansion when it's unset and 'set -u' is active will cause bash to
> error out. ${| command; } is likewise also an explicit attempt to
> expand the value of a variable, this time the instance of REPLY local
> to the funsub.

Your view of `explicit' is, um, unorthodox.

> By contrast, the expansions of, or other references to, variables like
> PS2 and PS4 that bash makes internally are implicit. For many of the
> variables listed in that portion of the "Shell Variables' section of
> the manual, It's a perfectly valid state for them to be left unset.
> You don't always need a TMOUT, an LC_ALL, or a FUNCNEST, for instance.
> Bash knows what to do without them.

Bash knows what to do when a funsub leaves REPLY unset as well. Maybe
the best way to think about it is COMPREPLY. Leaving COMPREPLY unset is
a valid, if unusual, way to indicate that there are no possible
completions.

In the same way, leaving REPLY unset in a funsub is a valid, if unusual,
way to indicate that the funsub produces no output. Command substitutions
produce no output all the time.


> So the purpose of 'set -u' still applies. If I, as the programmer,
> managed to misspell REPLY or forget to set it in some codepath, I'd
> rather bash tell me that than have it silently expand ${| command; }
> to nothing and leave me scratching my head as to what's going on,
> given breakage potentially evident only later in the script.

You still have to check whether REPLY expands to the empty string. How
is that different?

This sounds like something a tool like `shellcheck' should warn about.

Chet
--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/

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