Environment Variables - The Cargo Book

## Environment Variables

Cargo sets and reads a number of environment variables which your code can detect or override. Here is a list of the variables Cargo sets, organized by when it interacts with them:

### Environment variables Cargo reads

You can override these environment variables to change Cargo's behavior on your system:

- `CARGO_HOME` — Cargo maintains a local cache of the registry index and of git checkouts of crates. By default these are stored under `$HOME/.cargo`, but this variable overrides the location of this directory. Once a crate is cached it is not removed by the clean command.
- `CARGO_TARGET_DIR` — Location of where to place all generated artifacts, relative to the current working directory.
- `RUSTC` — Instead of running `rustc`, Cargo will execute this specified compiler instead.
- `RUSTC_WRAPPER` — Instead of simply running `rustc`, Cargo will execute this specified wrapper instead, passing as its commandline arguments the rustc invocation, with the first argument being rustc.
- `RUSTDOC` — Instead of running `rustdoc`, Cargo will execute this specified `rustdoc` instance instead.
- `RUSTDOCFLAGS` — A space-separated list of custom flags to pass to all `rustdoc` invocations that Cargo performs. In contrast with `cargo rustdoc`, this is useful for passing a flag to all `rustdoc` instances.
- `RUSTFLAGS` — A space-separated list of custom flags to pass to all compiler invocations that Cargo performs. In contrast with `cargo rustc`, this is useful for passing a flag to all compiler instances.
- `CARGO_INCREMENTAL` — If this is set to 1 then Cargo will force incremental compilation to be enabled for the current compilation, and when set to 0 it will force disabling it. If this env var isn't present then cargo's defaults will otherwise be used.
- `CARGO_CACHE_RUSTC_INFO` — If this is set to 0 then Cargo will not try to cache compiler version information.

Note that Cargo will also read environment variables for `.cargo/config` configuration values, as described in that documentation

### Environment variables Cargo sets for crates

Cargo exposes these environment variables to your crate when it is compiled. Note that this applies for test binaries as well. To get the value of any of these variables in a Rust program, do this:

    # #![allow(unused_variables)]
    #fn main() {
    let version = env!("CARGO_PKG_VERSION");
    #}

`version` will now contain the value of `CARGO_PKG_VERSION`.

- `CARGO` - Path to the `cargo` binary performing the build.
- `CARGO_MANIFEST_DIR` - The directory containing the manifest of your package.
- `CARGO_PKG_VERSION` - The full version of your package.
- `CARGO_PKG_VERSION_MAJOR` - The major version of your package.
- `CARGO_PKG_VERSION_MINOR` - The minor version of your package.
- `CARGO_PKG_VERSION_PATCH` - The patch version of your package.
- `CARGO_PKG_VERSION_PRE` - The pre-release version of your package.
- `CARGO_PKG_AUTHORS` - Colon separated list of authors from the manifest of your package.
- `CARGO_PKG_NAME` - The name of your package.
- `CARGO_PKG_DESCRIPTION` - The description of your package.
- `CARGO_PKG_HOMEPAGE` - The home page of your package.
- `OUT_DIR` - If the package has a build script, this is set to the folder where the build script should place its output. See below for more information.

### Environment variables Cargo sets for build scripts

Cargo sets several environment variables when build scripts are run. Because these variables are not yet set when the build script is compiled, the above example using `env!` won't work and instead you'll need to retrieve the values when the build script is run:

    # #![allow(unused_variables)]
    #fn main() {
    use std::env;
    let out_dir = env::var("OUT_DIR").unwrap();
    #}

`out_dir` will now contain the value of `OUT_DIR`.

- `CARGO` - Path to the `cargo` binary performing the build.
- `CARGO_MANIFEST_DIR` - The directory containing the manifest for the package being built (the package containing the build script). Also note that this is the value of the current working directory of the build script when it starts.
- `CARGO_MANIFEST_LINKS` - the manifest `links` value.
- `CARGO_FEATURE_<name>` - For each activated feature of the package being built, this environment variable will be present where `<name>` is the name of the feature uppercased and having `-` translated to `_`.
- `CARGO_CFG_<cfg>` - For each configuration option of the package being built, this environment variable will contain the value of the configuration, where `<cfg>` is the name of the configuration uppercased and having `-` translated to `_`. Boolean configurations are present if they are set, and not present otherwise. Configurations with multiple values are joined to a single variable with the values delimited by `,`.
- `OUT_DIR` - the folder in which all output should be placed. This folder is inside the build directory for the package being built, and it is unique for the package in question.
- `TARGET` - the target triple that is being compiled for. Native code should be compiled for this triple. Some more information about target triples can be found in clang’s own documentation.
- `HOST` - the host triple of the rust compiler.
- `NUM_JOBS` - the parallelism specified as the top-level parallelism. This can be useful to pass a `-j` parameter to a system like `make`. Note that care should be taken when interpreting this environment variable. For historical purposes this is still provided but recent versions of Cargo, for example, do not need to run `make -j` as it'll automatically happen. Cargo implements its own jobserver and will allow build scripts to inherit this information, so programs compatible with GNU make jobservers will already have appropriately configured parallelism.
- `OPT_LEVEL`, `DEBUG` - values of the corresponding variables for the profile currently being built.
- `PROFILE` - `release` for release builds, `debug` for other builds.
- `DEP_<name>_<key>` - For more information about this set of environment variables, see build script documentation about `links`.
- `RUSTC`, `RUSTDOC` - the compiler and documentation generator that Cargo has resolved to use, passed to the build script so it might use it as well.
- `RUSTC_LINKER` - The path to the linker binary that Cargo has resolved to use for the current target, if specified. The linker can be changed by editing `.cargo/config`; see the documentation about cargo configuration for more information.

### Environment variables Cargo sets for 3rd party subcommands

Cargo exposes this environment variable to 3rd party subcommands (ie. programs named `cargo-foobar` placed in `$PATH`):

- `CARGO` - Path to the `cargo` binary performing the build.