Add configuration to disable variable substitution

[phillipleblanc]

The variable substitution feature of dotenv means that any environment variables that should have the $ in them are removed.

I would like a way to disable this functionality, since my project doesn't need it and it causes surprise among users when they discover their environment variables aren't working as expected.

[aidenfarley]

Do you have a specific way in mind to disable substitution?

[phillipleblanc]

Perhaps adding a new DotEnvBuilder which accepts a parameter with_variable_substution(bool) which can be used to customize how the environment is loaded. The existing methods could all just call DotEnvBuilder with the different parameters that make them different.

i.e.

DotEnvBuilder::new().with_variable_substution(false).load()?

or

DotEnvBuilder::new().from_filename("my_filename").load()?

The existing methods would still work:
dotenvy::from_filename -> would just be DotEnvBuilder::new().from_filename("my_filename").load()? under the hood.

[sonro]

We are currently redesigning the API to allow for more extensible configuration. A builder pattern is likely to be part of it.

In the meantime, variable substitution is the same as in the vast majority of other "dotenv" implementations. You can escape it with a backslash or strong single quotes—the same as most shells' handling of environment variables.

KEY_ONE='$VAL'
ANOTHER_KEY=\$VAL

Both KEY_ONE and ANOTHER_KEY would be set to '$VAL'.

[allan2]

@phillipleblanc good idea. This will be supported in the next release.

[aidenfarley]

@allan2 Is there a PR in progress yet for the new API change, or just the discussion in #39. How can I contribute?

[allan2]

Hi @aidenfarley, I am working on a design locally. I haven't opened a PR yet, but it will come before the Rust 2024 edition. Stay tuned.

[allan2]

Hello, there is a new API on the v0.16 branch. PRs are welcome to that branch. I would suggest EnvLoader::substitution(true).