Re: Parameter Expansion

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