variable expansion inside string

variable expansion inside string

[Pier Paolo Grassi]

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Hello, is it possibile to do only variable expansion inside a string, eg:

file="filename.txt"
a="*$file*"

I would like to print $a as:

*filename.txt*

I have tried ${(P)a}
and
eval echo -n - $a

but neither do what I would like, because $a does not contains only a
variable name, and the * are expanded by glob subst in the eval case.

thanks in advance

Pier Paolo Grassi
linkedin: https://www.linkedin.com/in/pier-paolo-grassi-19300217
founder: https://www.meetup.com/it-IT/Machine-Learning-TO

Re: variable expansion inside string

[Peter Stephenson]

On Tue, 2019-07-30 at 14:28 +0200, Pier Paolo Grassi wrote:
> Hello, is it possibile to do only variable expansion inside a string, eg:
> 
> file="filename.txt"
> a="*$file*"

If you run zsh -f, you'll see that that's exactly what you normally get, just
by running

print "$a"

--- no eval.

My best guess is you have the option globsubst set and when you tried
to print the result, you didn't use double quotes at that point, i.e.
you had something like

print $a

and the *filename.text* got expanded at that point.

So the original code is actually fine.

pws

Re: variable expansion inside string

[Pier Paolo Grassi]

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thank you Peter, I made a mistake in the example code:

file="filename.txt"
a="*$file*"

should have been:

file="filename.txt"
a='*$file*'

with single quotes the behaviour is changed:

zsh -fc 'file= filename.txt; a="*$file*"; print "$a"'

gives:

*filename.txt*

instead with single quotes:

zsh -fc 'file= filename.txt; a=''*$file*''; print "$a"'

gives

*$file*

I would like the second example to somehow give the result of the first,
since I am trying to pass the string from another context where the
variable $file is not already defined

thanks

Pier Paolo Grassi
linkedin: https://www.linkedin.com/in/pier-paolo-grassi-19300217
founder: https://www.meetup.com/it-IT/Machine-Learning-TO


Il giorno mar 30 lug 2019 alle ore 15:19 Peter Stephenson <
p.stephenson@samsung.com> ha scritto:

> On Tue, 2019-07-30 at 14:28 +0200, Pier Paolo Grassi wrote:
> > Hello, is it possibile to do only variable expansion inside a string, eg:
> >
> > file="filename.txt"
> > a="*$file*"
>
> If you run zsh -f, you'll see that that's exactly what you normally get,
> just
> by running
>
> print "$a"
>
> --- no eval.
>
> My best guess is you have the option globsubst set and when you tried
> to print the result, you didn't use double quotes at that point, i.e.
> you had something like
>
> print $a
>
> and the *filename.text* got expanded at that point.
>
> So the original code is actually fine.
>
> pws
>
>

Re: variable expansion inside string

[Peter Stephenson]

So really what you're trying to do is: given a variable "a" containing
exactly the literal characters

*$file*

(however they got there) expand any variable references, but nothing
else.

The nearest I can think of is:

print -r -- "${(e):-$a}"

That won't be limited to just variables, it'll do other sorts
of expansion, but it won't do globbing, and you don't have the
hairiness of an "eval".

pws

Re: variable expansion inside string

[Peter Stephenson]

On Tue, 2019-07-30 at 15:15 +0100, Peter Stephenson wrote:
> The nearest I can think of is:
> 
> print -r -- "${(e):-$a}"

Or, actually, a straight

print -r -- "${(e)a}"

since you don't need to add any text to the variable substitution.

pws

Re: variable expansion inside string

[Pier Paolo Grassi]

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thanks Peter, for my use case it actually works really nice this way. I was
hoping for something that wouldn't allow arbitrary command execution for
future uses, but I guess that would need a proper templating system of sorts
best regards

Pier Paolo Grassi
linkedin: https://www.linkedin.com/in/pier-paolo-grassi-19300217
founder: https://www.meetup.com/it-IT/Machine-Learning-TO


Il giorno mar 30 lug 2019 alle ore 16:24 Peter Stephenson <
p.stephenson@samsung.com> ha scritto:

> On Tue, 2019-07-30 at 15:15 +0100, Peter Stephenson wrote:
> > The nearest I can think of is:
> >
> > print -r -- "${(e):-$a}"
>
> Or, actually, a straight
>
> print -r -- "${(e)a}"
>
> since you don't need to add any text to the variable substitution.
>
> pws
>
>
>