Add method GetStringEnv which will interpolate environmental variables

[useafterfree]

This will interpolate values that look like ${HOME} and assign with $HOME from environment.

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✅ useafterfree
❌ Nathan Trujillo

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[andrewstuart]

Hey, thanks for the contribution!

I'm hesitant to bring in a feature like this, for a couple reasons.

• It's quite specific to environment variables, compared to the rest of viper, which is concerned with aggregating and overlaying multiple configuration sources.
• This could be easily achieved by simply wrapping with os.ExpandEnv, e.g. os.ExpandEnv(viper.GetString("foo")).

[useafterfree]

The README.md states that Viper supports:
reading from environment variables

Maybe that can read:
reading from specifically formatted unique environment variables

Since,

SECTION:
   CONFIG: '${AMI}'
OTHER_SECTION:
   CONFIG: '${AMI}'

will never work for just $AMI but $SECTION_CONFIG_AMI $OTHER_SECTION_CONFIG_AMI

If I use environment variables in several places in a file, I don't want to make new ones as
my expectation is Environmental variables are to be used as one -> many.

Why would that change if I use Viper?

[sagikazarmark]

How about some configuration that optionally turns on variable expansion (eg. calls `os.ExpandEnv) for environment variables?

Alternatively variable expansion could be an option for not just env vars, but all configurations.

[astockwell]

I would like to second the request and add my usecase: configuring multiple environments with the same yaml/toml field structure, but needing different values:

[qa]
host = '${HOSTQA}'

[prod]
host = '${MAINLB}'