Forced resolution of environment variables

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hi, I have a hash, created from a text file, with an element along the lines of %myhash{$mykey}="$FRED/thingy.txt". When I issue an open command for "<%myhash{$mykey}", it fails to resolve the environment variable part and reports that it cannot find the file "$FRED/thingy.txt". I've got "use Env;" and other environment variables are being resolved correctly (including some referencing $FRED). How do I tell Perl that the contents of %myhash{$mykey} contains an environment variable and that it should resolve it? Apologies if this is straightforward, but I can't find it in my Perl manual, or through a search. Cheers, J.

Comment on Forced resolution of environment variables

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Re: Forced resolution of environment variables by insaniac (Friar) on Oct 22, 2004 at 15:04 UTC
hey i think you mean `$myhash{$mykey}`, right? second: when i want to use environmental variables (shell variables to keep it short ;) ), i use `$ENV{VARNAME}`, for instance `$ENV{HOME}`. and then you don't have to use the `use Env;` thingie hope this helps.. -- to ask a question is a moment of shame to remain ignorant is a lifelong shame	[reply] [d/l] [select]
Re^2: Forced resolution of environment variables by gothic_mallard (Pilgrim) on Oct 22, 2004 at 15:18 UTC
I don't think I've ever used `use Env`. I've never really liked things that pollute the namespace to any real extent. Keeping things within the normal `%ENV` hash and pulling it when needed is a lot cleaner to my mind. --- Jay All code is untested unless otherwise stated.	[reply] [d/l] [select]
Re: Forced resolution of environment variables by Fletch (Bishop) on Oct 22, 2004 at 15:10 UTC
That's because you never pass the string through perl so there's no way it could interpolate it. You'd want to use `eval` or `s///e`, but if you don't trust your input you'd want to be careful doing that. You might also check out a module like AppConfig which gives you a config file parser which will do this sort of interpolation to some degree.	[reply] [d/l] [select]
Re: Forced resolution of environment variables by tye (Sage) on Oct 22, 2004 at 16:11 UTC
`"$FRED"` only gets expanded if it is in Perl code. It sounds like you have that in data. So post-process your data with: `s/\$([A-Z0-9_]+)/$ENV{$1}/g for $myhash{$mykey}, ...;` [download] (which doesn't require "use Env;" which is something that I would never use -- it sounds rather dangerous in some situations, allowing people to throw in variables not in your control and just to save you from typing 'ENV{' and '}'.) - tye	[reply] [d/l] [select]
Re^2: Forced resolution of environment variables by Anonymous Monk on Oct 22, 2004 at 16:36 UTC
That's only "dangerous" in a place were someone else controls your environment. But if someone else controls your environment, even a `use strict` is "dangerous", as that will allow the attacker to execute arbitrary code on your behalf. (Because the attacker can then set `PERL5LIB`, `PERL5OPT`, `PERLIO`, `PERL5SHELL`, and some other variables to make life annoying).	[reply] [d/l]
Re^3: Forced resolution of environment variables by sgifford (Prior) on Oct 22, 2004 at 18:10 UTC
Not in taint mode, which should always be used in any code executing on someone else's behalf.	[reply]
Re^2: Forced resolution of environment variables by Anonymous Monk on Oct 25, 2004 at 14:18 UTC
Thanks, folks. Interesting points about "use Env;" too.	[reply]