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* Allow environment variables to be hidden This change allows environment variables in Nushell to have a value of `Nothing`, which can be set by the user by passing `$nothing` to `let-env` and friends. Environment variables with a value of Nothing behave as if they are not set at all. This allows a user to shadow the value of an environment variable in a parent scope, effectively removing it from their current scope. This was not possible before, because a scope can not affect its parent scopes. This is a workaround for issues like #3920. Additionally, this allows a user to simultaneously set, change and remove multiple environment variables via `load-env`. Any environment variables set to $nothing will be hidden and thus act as if they are removed. This simplifies working with virtual environments, which rely on setting multiple environment variables, including PATH, to specific values, and remove/change them on deactivation. One surprising behavior is that an environment variable set to $nothing will act as if it is not set when querying it (via $nu.env.X), but it is still possible to remove it entirely via `unlet-env`. If the same environment variable is present in the parent scope, the value in the parent scope will be visible to the user. This might be surprising behavior to users who are not familiar with the implementation details. An additional corner case is the the shorthand form of `with-env` does not work with this feature. Using `X=$nothing` will set $nu.env.X to the string "$nothing". The long-form works as expected: `with-env [X $nothing] {...}`. * Remove unused import * Allow all primitives to be convert to strings |
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Nu-Engine
Nu-engine handles most of the core logic of nushell. For example, engine handles: - Passing of data between commands - Evaluating a commands return values - Loading of user configurations
Top level introduction
The following topics shall give the reader a top level understanding how various topics are handled in nushell.
How are environment variables handled?
Environment variables (or short envs) are stored in the Scope of the EvaluationContext. That means that environment variables are scoped by default and we don't use std::env to store envs (but make exceptions where convenient).
Nushell handles environment variables and their lifetime the following:
- At startup all existing environment variables are read and put into
Scope. (Nushell reads existing environment variables platform independent by asking theHost. They will most likely come fromstd::env::*) - Envs can also be loaded from config files. Each loaded config produces a new
ScopeFramewith the envs of the loaded config. - Nu-Script files and internal commands read and write env variables from / to the
Scope. External scripts and binaries can't interact with theScope. Therefore all env variables are read from theScopeand put into the external binaries environment-variables-memory area.