Re: Parameter Expansion

Dear Mark,

I'm grateful that you explained things; I didn't mean to be snarky. I find
your explanation helpful for me, but that doesn't really explain things for
the next person.

As an aside, I was wondering how :s/old/new/ could apply to variables.
Thank you!

What is documented is mostly accurate, +/- some bugs (probably mostly in
the codebase). The documentation *approach *is, however, lacking - I will
look again at the user manual.

- Vin

On Tue, Jul 15, 2025 at 2:17 PM Mark J. Reed <[email protected]> wrote:

> I'm not denying that the doc could be clearer - there should probably be a
> note in PARAMETER EXPANSION mentioning that history expansion modifiers can
> be used, instead of that note only being under HISTORY EXPANSION. But the
> clue that history is relevant is that, AFAICT, the only place the
> :s/old/new syntax is documented is in the HISTORY EXPANSION section.
>
> And of course you shouldn't have to know the history of the various
> features in order to use them. I phrased my earlier message as if it were
> about the origin of the modifier, which was poor wording on my part. The
> fact is, history expansion and parameter expansion are still separate
> systems within the shell that work differently from each other, which is
> why they have different sections in zshexpn. You can apply history
> expansion modifiers to parameter expansion, but it still has its own rules.
> So you'll be surprised less often if you know which system a given syntax
> is part of, and in general the modifiers signalled by a colon followed by a
> letter are all history expansion.
>
>
>
> On Tue, Jul 15, 2025 at 1:28 PM Vin Shelton <[email protected]> wrote:
>
>> Thanks. As a user, I don't care how it originated, I care about how to
>> use it. There was no ! in my question, so I didn't look in the history
>> section of the manual.
>>
>>   - Vin
>>
>> On Tue, Jul 15, 2025 at 1:12 PM Mark J. Reed <[email protected]> wrote:
>>
>>> History expansion is the stuff you can do with *!* to retrieve values
>>> from history and modify them in various ways. Those modifiers are so useful
>>> that Zsh has added them to its parameter expansion. Doing a search and
>>> replace with *:s/old/new/* is an example of a modifier that originated
>>> on the history side of things, as are most of the ones that involve a colon
>>> followed by a letter.
>>>
>>> But the history expansion takes place in a different phase of evaluation
>>> and doesn't expand any nested parameter values, which is why your attempt
>>> didn't work - the value of SCMROOT doesn't contain the literal string
>>> "$HOME", so nothing is changed.
>>>
>>> For general search and replace, parameter expansion has its own
>>> substitution syntax - ${param/old/new} will replace only the first
>>> instance of "old" in the string, while $[param//old/new} will replace all
>>> of them. Unlike with :s, you can't change the delimiter and so have to
>>> escape any literal slashes in either the search or replacement values. But
>>> any nested parameters will be expanded (and slashes in the expanded value
>>> taken literally), so your attempt would work.
>>>
>>> However, if all you want to do is remove stuff from the beginning or end
>>> of a parameter value, there's targeted syntax for that, which Peter has
>>> supplied. In general, ${param#prefix} will remove stuff from the beginning
>>> of the string, and ${param%suffix} will remove stuff from the end. The
>>> stuff to remove can be specified via a wildcard pattern (not a regex, but a
>>> file glob), in which case a single *#* or *%* will remove the shortest
>>> thing that matches while a double *##* or *%%* will remove the longest.
>>>
>>>
>>> On Tue, Jul 15, 2025 at 11:13 AM Vin Shelton <[email protected]>
>>> wrote:
>>>
>>>> Although I see history mentioned in the zshexpn manpage, I don't know
>>>> what my example has to do with history.
>>>>
>>>> On Tue, Jul 15, 2025 at 11:01 AM Eric Cook <[email protected]> wrote:
>>>>
>>>>> On 7/15/25 10:44 AM, Vin Shelton wrote:
>>>>> > I don't understand the order of parameter expansion. Consider the
>>>>> following example:
>>>>> >
>>>>> > zsh -f
>>>>> > export HOME=/home/acs
>>>>> > smaug% export SCMROOT=/home/acs/scmroot
>>>>> > smaug% echo ${SCMROOT:s-$HOME/--}
>>>>> >
>>>>> > This yields /home/acs/scmroot", but I expected "scmroot".
>>>>> > TIA,
>>>>> >   Vin Shelton
>>>>> >
>>>>>
>>>>> The history modifier doesn't create a new context where the parameter
>>>>> expansion code recurses and resolves any parameters within it's operands.
>>>>> It is searching for the literal string $HOME.
>>>>>
>>>>>
>>>>
>>>> --
>>>> Whoa, I'm just surprised at how accurate that description of me really
>>>> is:
>>>> some old cowboy guy that used to shoot movies at Spahn Ranch
>>>>
>>>
>>>
>>> --
>>> Mark J. Reed <[email protected]>
>>>
>>
>>
>> --
>> Whoa, I'm just surprised at how accurate that description of me really is:
>> some old cowboy guy that used to shoot movies at Spahn Ranch
>>
>
>
> --
> Mark J. Reed <[email protected]>
>


-- 
Whoa, I'm just surprised at how accurate that description of me really is:
some old cowboy guy that used to shoot movies at Spahn Ranch