(Neo)vim substitute over block/regular visual range, and not line visual range

If I highlight a block of text in visual mode in Vim and then press :, I can run a command on it. For example

• :'<,'>s/foo/bar/g to substitute,
• or :'<,'> !bash to send the lines to the bash shell and get the output,
• or :'<,'>sort to sort those lines,
• or :'<,'> !some_cli_util to send those lines to some_cli_util

This is fine if the text I want to execute is conveniently on it's own line, but often that isn't the case. For example, if I'm writing docs and want to give and example of a cli command and it's output, then it would be nice to just write:

$ head requirements.txt

and then to select the text head requirements.txt, type : !sh<CR>, and be given the actual output of the command. But in reality, when I select head requirements.txt and type :, then my selection gets expanded to a full-line selection, which causes the leading $ to be clobbered in with the actual command, and so because of the leading $, the command fails.

A less trivial example of where this would be useful, is if I had a LaTeX table like:

\begin{table}
    \begin{tabular}{lllll}
        col1   & col2   & col3   & col4   & col5     \\

        foo19s & fook2e & foojd9 & alfooc & 9dfooj   \\
        9foo1s & ekfoo2 & lqfoo  & alfoo  & 9efoo9s  \\
        1sfoo9 & d9fooe & fooj9d & alsdfo & 9hfooje  \\
        19sfoo & 9dfooe & lqfoo  & dfoo9e & ajfoo   
    \end{tabular}
\end{table}

and I wanted to replace all the instances of foo with bar, but only in the columns titled col2. If I could block-select only col2, and then execute a substitute command like :`<,`>s/foo/bar/g then this would work. But currently, if I block select only col2 and then press :, Vim will auto-select the entire range of lines that were in my block selection.

Ideally, I'd like vim to be smart enough such that if am in block-visual mode and press :, then it'll use the :`<,`> range selector, but if I'm in line-visual mode and press : then it'll use the :'<,'> range selector.

:h `< tells me that the `< and `> marks do exist, but I can't see how to apply them to commands.

What can I do to enable vim (or neovim) to execute commands on block-selections? or is there an issue open for this problem?

> I can't just use the regular visual-line selector :'<,'>s/ba/__/g because that will incorrectly replace all the bazs with __zs.

Yes, you definitely can, but with a more refined search pattern:

:[range]s/ba\zer/__/g

See :help \ze.

--- EDIT ---

As explained in the other answer, Ex commands only work on lines and ranges are ranges of lines. You wont be able to pass something else than one or several lines to :help :s, :help :range! or :help :sort.

There is no way around that.

While the range of Ex commands is always linewise, some of them may honor the visual selection in some way.

In the case of :help :s, what you can do is, again, use a more refined search pattern:

<C-v><motions>
:[range]s/\%V.*\zsfoo\ze.*%V/bar

See :help \%V.

How to get it done is described perfectly in romainl's answer. I'd like to answer the "why?", though.

You wrote:
> Ideally, I'd like vim to be smart enough such that if am in block-visual mode and press :, then it'll use the :`<,`> range selector, but if I'm in line-visual mode and press : then it'll use the :'<,'> range selector.

This behavior is explicitly impossible, at least for now (Vim 9.0). Quoting from :h :visual_example:
> Currently the ":" command works on whole lines only. When you select part of
> a line, doing something like ":!date" will replace the whole line. If you
> want only part of the line to be replaced you will have to make a mapping for
> it. In a future release ":" may work on partial lines.

Likewise, :h :s says (emphasis mine):
> For each line in [range] replace a match of {pattern}

It is a common misconception that Vim would operate on the (block) visual selection only. Not all commands do. Instead, they operate on lines that are selected.

So you'll have to use a line-wise substitution like (as pointed out by romainl and included for the sake of completeness) :s/ba\zer/__/g.

It's a reasonable tool to look for, but as others have pointed out, ranges are only line-wise. For the cases where you'd like to use a selection as an argument to an external program, I'd recommend yanking and pasting in the command-line.

A plugin that might give you a general-purpose solution is vis: https://github.com/vim-scripts/vis. Instead of :'<,'>s/foo/bar/g, you'd execute :'<,'>B s/foo/bar/g, which is less convenient, but will do the trick.