Interpret Parameter Value at Time of Function Definition

Interpret Parameter Value at Time of Function Definition

[Vin Shelton]

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I think zsh interprets the value of a shell variable when a function is
run, but I would like to set the value when the function is read.

E.g:

$ aaa=bbb
$ function t {
    print aaa = \"$aaa\"
  }
$ t
aaa = "bbb"
$ aaa=ccc t
aaa = "ccc"

I would like to interpret $aaa at read time, so that:

    aaa=ccc t

would print:

    aaa = "bbb"

How do I do that?

Thanks,
  Vin

Re: Interpret Parameter Value at Time of Function Definition

[Daniel Shahaf]

Vin Shelton wrote on Tue, 05 Sep 2017 09:47 -0400:
> $ aaa=bbb
> $ function t {
>     print aaa = \"$aaa\"
>   }
> 
> I would like to interpret $aaa at read time, so that:
> 
>     aaa=ccc t
> 
> would print:
> 
>     aaa = "bbb"

Declare aaa local inside the function:

t() {
  local aaa=bbb
  typeset -p aaa
}
aaa=ccc t

... but then, if t() changes $aaa the changes won't be visible outside t().

Re: Interpret Parameter Value at Time of Function Definition

[Bart Schaefer]

On Tue, Sep 5, 2017 at 9:17 AM, Daniel Shahaf <d.s@daniel.shahaf.name> wrote:
> Vin Shelton wrote on Tue, 05 Sep 2017 09:47 -0400:
>> $ aaa=bbb
>> $ function t {
>>     print aaa = \"$aaa\"
>>   }
>>
>> I would like to interpret $aaa at read time [...]
>
> Declare aaa local inside the function:

That won't help if the value assigned to $aaa is itself going to
contain expansions.

As Martijn points out, the only way to get this to work is with some
kind of "read-time" evaluation.  Exactly what approach is best depends
on what you want to accomplish.  Does $aaa expand to executable
statements that you want to turn into part of the function code, or
does it just result in a value that you want to access later?

No matter which way you go, the key is that you'll need to quote the
function body differently than you quote the value you want assigned
to $aaa.

Re: Interpret Parameter Value at Time of Function Definition

[Martijn Dekker]

Op 05-09-17 om 15:47 schreef Vin Shelton:
[...]
> $ aaa=bbb
> $ function t {
>     print aaa = \"$aaa\"
>   }
> $ t
> aaa = "bbb"
> $ aaa=ccc t
> aaa = "ccc"
> 
> I would like to interpret $aaa at read time, so that:
> 
>     aaa=ccc t
> 
> would print:
> 
>     aaa = "bbb"
> 
> How do I do that?

AFAIK, the only way to do this without side effects is by using 'eval':

aaa=bbb
eval "function t {
  print \"aaa = $aaa\"
}"
typeset -f t

The value of "$aaa" is now hard-coded in the function definition, as you
can see by the output of 'typeset -f t'.

Note that you have to be very careful to backslash-escape everything
except $aaa correctly so that only $aaa is interpreted at read time,
otherwise you're going to get hard-to-trace bugs.

Also, this should ONLY be done this way if $aaa is guaranteed to contain
trusted data only. If the value of $aaa can be given by other users from
the outside, you've got a code injection vulnerability, as any shell
grammar in $aaa will be interpreted and executed by 'eval'. For
instance, setting
   aaa='"; echo injection"'
would print "injection".

You can avoid the vulnerability by shell-quoting the value of $aaa
before using it, which in zsh is easy to do using the (q) parameter
expansion flag:

# ... $aaa contains some untrusted data ...
eval "function t {
  print -r \"aaa =\" ${(q)aaa}
}"
typeset -f t

The expansion ${(q)aaa} is appropriately quoted as a string using shell
grammar so it will never be interpreted as code.

This is advanced stuff, so be careful.

- Martijn

Re: Interpret Parameter Value at Time of Function Definition

[Ray Andrews]

On 05/09/17 11:21 AM, Martijn Dekker wrote:
>
>>      aaa=ccc t

How is it that that works without a semicolon between the commands?

BTW, when I've had a problem like this one I create files named after 
the variable and containing the content of the variable.  It's brutal 
and slow and stupid but it does provide absolute protection even on 
Thursdays because it bypasses all the intricacies of the rules about 
variables.  You hafta 'rm' the file to make the variable go away.

Re: Interpret Parameter Value at Time of Function Definition

[Bart Schaefer]

On Tue, Sep 5, 2017 at 8:08 PM, Ray Andrews <rayandrews@eastlink.ca> wrote:
>
>>>      aaa=ccc t
>
> How is it that that works without a semicolon between the commands?

This is the standard syntax for temporarily placing the variable "aaa"
into the environment of the command "t".