Observe:

$ unset FOOBAR
$ echo ${FOOBAR@a}

$ FOOBAR=
$ echo ${FOOBAR@a}

$ export FOOBAR
$ echo ${FOOBAR@a}
x $

It seems that it prints "x" iff the variable has been exported.
This works with any variable - the output is always "x" (iff it is exported).

I can find no mention of this feature in "man bash". Note: I am not saying
it isn't in there - just that I couldn't find it.

Note that any other letter (such as "b") generates an error message if
FOOBAR is non-empty:

$ echo ${FOOBAR@b}

$ FOOBAR=Test
$ echo ${FOOBAR@b}
bash: ${FOOBAR@b}: bad substitution
$

What is going on?

--
> No, I haven't, that's why I'm asking questions. If you won't help me,
> why don't you just go find your lost manhood elsewhere.

CLC in a nutshell.

On 2024-04-13 4:35 p.m., Kenny McCormack wrote:
> Observe:
>
> $ unset FOOBAR
> $ echo ${FOOBAR@a}
>
> $ FOOBAR=
> $ echo ${FOOBAR@a}
>
> $ export FOOBAR
> $ echo ${FOOBAR@a}
> x
> $
>
> It seems that it prints "x" iff the variable has been exported.
> This works with any variable - the output is always "x" (iff it is exported).
>
> I can find no mention of this feature in "man bash". Note: I am not saying
> it isn't in there - just that I couldn't find it.

Search the man page for 'Parameter transformation' and you'll find it.
It's the last entry in the 'Parameter Expansion' section, just before
'Command Substitution' if that makes it any easier to find.

gazelle@shell.xmission.com (Kenny McCormack):
> Observe:
>
> $ unset FOOBAR
> $ echo ${FOOBAR@a}
>
> $ FOOBAR> $ echo ${FOOBAR@a}
>
> $ export FOOBAR
> $ echo ${FOOBAR@a}
> x
> $
>
> It seems that it prints "x" iff the variable has been exported.
>
> This works with any variable - the output is always "x" (iff it
> is exported).
>

That's correct.

> I can find no mention of this feature in "man bash". Note: I am
> not saying it isn't in there - just that I couldn't find it.
>

man -- 1 bash 2> /dev/null |
sed -E -e '/\$\{[[:alpha:]]+@/,$p' -n

In article <838r1harhh.fsf@helmutwaitzmann.news.arcor.de>,
Helmut Waitzmann <oe.throttle@xoxy.net> wrote:
....
>> I can find no mention of this feature in "man bash". Note: I am
>> not saying it isn't in there - just that I couldn't find it.

So, what does it do?

--
The people who tell us to be proud of the white race are the ones who make
us the most embarrassed by it.

On Sat, 13 Apr 2024 20:35:27 +0000, Kenny McCormack wrote:

> Observe:
>
> $ unset FOOBAR
> $ echo ${FOOBAR@a}
>
> $ FOOBAR=
> $ echo ${FOOBAR@a}
>
> $ export FOOBAR
> $ echo ${FOOBAR@a}
> x
> $
>
> It seems that it prints "x" iff the variable has been exported.
> This works with any variable - the output is always "x" (iff it is exported).
>
> I can find no mention of this feature in "man bash".
[snip]
> What is going on?

${parameter@operator}
Parameter transformation. The expansion is either a
transformation of the value of parameter or information
about parameter itself, depending on the value of
operator. Each operator is a single letter:

U The expansion is a string that is the value of
parameter with lowercase alphabetic characters
converted to uppercase.
u The expansion is a string that is the value of
parameter with the first character converted to
uppercase, if it is alphabetic.
L The expansion is a string that is the value of
parameter with uppercase alphabetic characters
converted to lowercase.
Q The expansion is a string that is the value of
parameter quoted in a format that can be reused as
input.
E The expansion is a string that is the value of
parameter with backslash escape sequences expanded
as with the $'...' quoting mechanism.
P The expansion is a string that is the result of
expanding the value of parameter as if it were a
prompt string (see PROMPTING below).
A The expansion is a string in the form of an
assignment statement or declare command that, if
evaluated, will recreate parameter with its
attributes and value.
K Produces a possibly-quoted version of the value of
parameter, except that it prints the values of
indexed and associative arrays as a sequence of
quoted key-value pairs (see Arrays above).
a The expansion is a string consisting of flag values
representing parameter's attributes.
k Like the K transformation, but expands the keys and
values of indexed and associative arrays to
separate words after word splitting.

I don't know when it was introduced into bash. My version of bash
(GNU bash, version 4.3.48(1)-release (x86_64-slackware-linux-gnu) )
doesn't support it.

--
Lew Pitcher
"In Skills We Trust"

In article <uvf043$33fov$1@dont-email.me>,
Lew Pitcher <lew.pitcher@digitalfreehold.ca> wrote:
>On Sat, 13 Apr 2024 20:35:27 +0000, Kenny McCormack wrote:
>
>> Observe:
>>
>> $ unset FOOBAR
>> $ echo ${FOOBAR@a}
>>
>> $ FOOBAR=
>> $ echo ${FOOBAR@a}
>>
>> $ export FOOBAR
>> $ echo ${FOOBAR@a}
>> x
>> $
>>
>> It seems that it prints "x" iff the variable has been exported.
>> This works with any variable - the output is always "x" (iff it is exported).
>>
>> I can find no mention of this feature in "man bash".
>[snip]
>> What is going on?
>
>${parameter@operator}

Wow. Weird. Thanks.

I wonder what the "flag values" are that you get with @a (my "x")...

--
Just like Donald Trump today, Jesus Christ had a Messiah complex.

And, in fact, the similarities between the two figures are quite striking.
For example, both have a ragtag band of followers, whose faith cannot be shaken.

In article <uvf9ve$3oki0$1@news.xmission.com>,
Kenny McCormack <gazelle@shell.xmission.com> wrote:
....
>I wonder what the "flag values" are that you get with @a (my "x")...

I suppose "x" is for exported.

--
When I was growing up we called them "retards", but that's not PC anymore.
Now, we just call them "Trump Voters".

The question is, of course, how much longer it will be until that term is also un-PC.

On Sun, 14 Apr 2024 01:03:18 -0000 (UTC), gazelle@shell.xmission.com
(Kenny McCormack) wrote in <uvfa0m$3oki0$2@news.xmission.com>:

> In article <uvf9ve$3oki0$1@news.xmission.com>, Kenny McCormack
> <gazelle@shell.xmission.com> wrote:
> ...
>>I wonder what the "flag values" are that you get with @a (my "x")...
>
> I suppose "x" is for exported.

I found the attributes in the man page under the entry
for typeset/declare.

"export" is like saying "declare -x".

For fast access to the attributes: "help declare".

(Thank you for the post, it taught me something.)

--
-v

On 4/13/24 18:14, Lew Pitcher wrote:
> The expansion is a string consisting of flag values
> representing parameter's attributes.

What does this mean?

On Sun, 14 Apr 2024 06:07:12 -0400, Popping Mad wrote:

> On 4/13/24 18:14, Lew Pitcher wrote:
>> The expansion is a string consisting of flag values
>> representing parameter's attributes.
>
>
> What does this mean?

bash expansion variables have various (and multiple) characteristics that
govern how they work in expansions. You can have <<functions>> or
<<non-functions>>, <<indexed arrays>> or <<associative arrays>> or
<<integers>>, <<exported>> or <<not exported>>, etc.

You can use the "typeset" builtin[1] to explicitly set these characteristics,
or set them by your bash script actions.

The "typeset" builtin can also tell you what those characteristics are for
specific expansion variables.

Apparently, bash has now added an expansion rule that /also/ reports on the
characteristics of the expansion variable.

[1] See bash(1) SHELL BUILTIN COMMANDS for more details

HTH
--
Lew Pitcher
"In Skills We Trust"