Environment Variables - The Cargo Book (2024)

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

Environment variables Cargo reads

You can override these environment variables to change Cargo’s behavior on yoursystem:

• CARGO_LOG — Cargo uses the tracing crate to display debug log messages.The CARGO_LOG environment variable can be set to enable debug logging,with a value such as trace, debug, or warn.Usually it is only used during debugging. For more details refer to theDebug logging.
• CARGO_HOME — Cargo maintains a local cache of the registry index and ofgit checkouts of crates. By default these are stored under $HOME/.cargo(%USERPROFILE%\.cargo on Windows), but this variable overrides thelocation of this directory. Once a crate is cached it is not removed by theclean command.For more details refer to the guide.
• CARGO_TARGET_DIR — Location of where to place all generated artifacts,relative to the current working directory. See build.target-dir to setvia config.
• CARGO — If set, Cargo will forward this value instead of setting itto its own auto-detected path when it builds crates and when itexecutes build scripts and external subcommands. This value is notdirectly executed by Cargo, and should always point at a command thatbehaves exactly like cargo, as that’s what users of the variablewill be expecting.
• RUSTC — Instead of running rustc, Cargo will execute this specifiedcompiler instead. See build.rustc to set via config.
• RUSTC_WRAPPER — Instead of simply running rustc, Cargo will execute thisspecified wrapper, passing as its command-line arguments the rustcinvocation, with the first argument being the path to the actual rustc.Useful to set up a build cache tool such as sccache. Seebuild.rustc-wrapper to set via config. Setting this to the empty stringoverwrites the config and resets cargo to not use a wrapper.
• RUSTC_WORKSPACE_WRAPPER — Instead of simply running rustc, for workspace members Cargo willexecute this specified wrapper, passing as its command-line arguments the rustc invocation, withthe first argument being the path to the actual rustc. When building a single-package projectwithout workspaces, that package is considered to be the workspace. It affects the filename hashso that artifacts produced by the wrapper are cached separately. Seebuild.rustc-workspace-wrapper to set via config. Setting this to the empty string overwritesthe config and resets cargo to not use a wrapper for workspace members. If both RUSTC_WRAPPERand RUSTC_WORKSPACE_WRAPPER are set, then they will be nested: the final invocation is$RUSTC_WRAPPER $RUSTC_WORKSPACE_WRAPPER $RUSTC.
• RUSTDOC — Instead of running rustdoc, Cargo will execute this specifiedrustdoc instance instead. See build.rustdoc to set via config.
• RUSTDOCFLAGS — A space-separated list of custom flags to pass to all rustdocinvocations that Cargo performs. In contrast with cargo rustdoc, this isuseful for passing a flag to all rustdoc instances. Seebuild.rustdocflags for some more ways to set flags. This string issplit by whitespace; for a more robust encoding of multiple arguments,see CARGO_ENCODED_RUSTDOCFLAGS.
• CARGO_ENCODED_RUSTDOCFLAGS — A list of custom flags separated by 0x1f(ASCII Unit Separator) to pass to all rustdoc invocations that Cargo performs.
• RUSTFLAGS — A space-separated list of custom flags to pass to all compilerinvocations that Cargo performs. In contrast with cargo rustc, this isuseful for passing a flag to all compiler instances. Seebuild.rustflags for some more ways to set flags. This string issplit by whitespace; for a more robust encoding of multiple arguments,see CARGO_ENCODED_RUSTFLAGS.
• CARGO_ENCODED_RUSTFLAGS — A list of custom flags separated by 0x1f(ASCII Unit Separator) to pass to all compiler invocations that Cargo performs.
• CARGO_INCREMENTAL — If this is set to 1 then Cargo will force incrementalcompilation to be enabled for the current compilation, and when set to 0 itwill force disabling it. If this env var isn’t present then cargo’s defaultswill otherwise be used. See also build.incremental config value.
• CARGO_CACHE_RUSTC_INFO — If this is set to 0 then Cargo will not try to cachecompiler version information.
• HTTPS_PROXY or https_proxy or http_proxy — The HTTP proxy to use, seehttp.proxy for more detail.
• HTTP_TIMEOUT — The HTTP timeout in seconds, see http.timeout for moredetail.
• TERM — If this is set to dumb, it disables the progress bar.
• BROWSER — The web browser to execute to open documentation with cargo doc’s’ --open flag, see doc.browser for more details.
• RUSTFMT — Instead of running rustfmt,cargo fmt will execute this specifiedrustfmt instance instead.

Configuration environment variables

Cargo reads environment variables for some configuration values.See the configuration chapter for more details.In summary, the supported environment variables are:

• CARGO_ALIAS_<name> — Command aliases, see alias.
• CARGO_BUILD_JOBS — Number of parallel jobs, see build.jobs.
• CARGO_BUILD_RUSTC — The rustc executable, see build.rustc.
• CARGO_BUILD_RUSTC_WRAPPER — The rustc wrapper, see build.rustc-wrapper.
• CARGO_BUILD_RUSTC_WORKSPACE_WRAPPER — The rustc wrapper for workspace members only, see build.rustc-workspace-wrapper.
• CARGO_BUILD_RUSTDOC — The rustdoc executable, see build.rustdoc.
• CARGO_BUILD_TARGET — The default target platform, see build.target.
• CARGO_BUILD_TARGET_DIR — The default output directory, see build.target-dir.
• CARGO_BUILD_RUSTFLAGS — Extra rustc flags, see build.rustflags.
• CARGO_BUILD_RUSTDOCFLAGS — Extra rustdoc flags, see build.rustdocflags.
• CARGO_BUILD_INCREMENTAL — Incremental compilation, see build.incremental.
• CARGO_BUILD_DEP_INFO_BASEDIR — Dep-info relative directory, see build.dep-info-basedir.
• CARGO_CARGO_NEW_VCS — The default source control system with cargo new, see cargo-new.vcs.
• CARGO_FUTURE_INCOMPAT_REPORT_FREQUENCY — How often we should generate a future incompat report notification, see future-incompat-report.frequency.
• CARGO_HTTP_DEBUG — Enables HTTP debugging, see http.debug.
• CARGO_HTTP_PROXY — Enables HTTP proxy, see http.proxy.
• CARGO_HTTP_TIMEOUT — The HTTP timeout, see http.timeout.
• CARGO_HTTP_CAINFO — The TLS certificate Certificate Authority file, see http.cainfo.
• CARGO_HTTP_CHECK_REVOKE — Disables TLS certificate revocation checks, see http.check-revoke.
• CARGO_HTTP_SSL_VERSION — The TLS version to use, see http.ssl-version.
• CARGO_HTTP_LOW_SPEED_LIMIT — The HTTP low-speed limit, see http.low-speed-limit.
• CARGO_HTTP_MULTIPLEXING — Whether HTTP/2 multiplexing is used, see http.multiplexing.
• CARGO_HTTP_USER_AGENT — The HTTP user-agent header, see http.user-agent.
• CARGO_INSTALL_ROOT — The default directory for cargo install, see install.root.
• CARGO_NET_RETRY — Number of times to retry network errors, see net.retry.
• CARGO_NET_GIT_FETCH_WITH_CLI — Enables the use of the git executable to fetch, see net.git-fetch-with-cli.
• CARGO_NET_OFFLINE — Offline mode, see net.offline.
• CARGO_PROFILE_<name>_BUILD_OVERRIDE_<key> — Override build script profile, see profile.<name>.build-override.
• CARGO_PROFILE_<name>_CODEGEN_UNITS — Set code generation units, see profile.<name>.codegen-units.
• CARGO_PROFILE_<name>_DEBUG — What kind of debug info to include, see profile.<name>.debug.
• CARGO_PROFILE_<name>_DEBUG_ASSERTIONS — Enable/disable debug assertions, see profile.<name>.debug-assertions.
• CARGO_PROFILE_<name>_INCREMENTAL — Enable/disable incremental compilation, see profile.<name>.incremental.
• CARGO_PROFILE_<name>_LTO — Link-time optimization, see profile.<name>.lto.
• CARGO_PROFILE_<name>_OVERFLOW_CHECKS — Enable/disable overflow checks, see profile.<name>.overflow-checks.
• CARGO_PROFILE_<name>_OPT_LEVEL — Set the optimization level, see profile.<name>.opt-level.
• CARGO_PROFILE_<name>_PANIC — The panic strategy to use, see profile.<name>.panic.
• CARGO_PROFILE_<name>_RPATH — The rpath linking option, see profile.<name>.rpath.
• CARGO_PROFILE_<name>_SPLIT_DEBUGINFO — Controls debug file output behavior, see profile.<name>.split-debuginfo.
• CARGO_PROFILE_<name>_STRIP — Controls stripping of symbols and/or debuginfos, see profile.<name>.strip.
• CARGO_REGISTRIES_<name>_CREDENTIAL_PROVIDER — Credential provider for a registry, see registries.<name>.credential-provider.
• CARGO_REGISTRIES_<name>_INDEX — URL of a registry index, see registries.<name>.index.
• CARGO_REGISTRIES_<name>_TOKEN — Authentication token of a registry, see registries.<name>.token.
• CARGO_REGISTRY_CREDENTIAL_PROVIDER — Credential provider for crates.io, see registry.credential-provider.
• CARGO_REGISTRY_DEFAULT — Default registry for the --registry flag, see registry.default.
• CARGO_REGISTRY_GLOBAL_CREDENTIAL_PROVIDERS — Credential providers for registries that do not have a specific provider defined. See registry.global-credential-providers.
• CARGO_REGISTRY_TOKEN — Authentication token for crates.io, see registry.token.
• CARGO_TARGET_<triple>_LINKER — The linker to use, see target.<triple>.linker. The triple must be converted to uppercase and underscores.
• CARGO_TARGET_<triple>_RUNNER — The executable runner, see target.<triple>.runner.
• CARGO_TARGET_<triple>_RUSTFLAGS — Extra rustc flags for a target, see target.<triple>.rustflags.
• CARGO_TERM_QUIET — Quiet mode, see term.quiet.
• CARGO_TERM_VERBOSE — The default terminal verbosity, see term.verbose.
• CARGO_TERM_COLOR — The default color mode, see term.color.
• CARGO_TERM_PROGRESS_WHEN — The default progress bar showing mode, see term.progress.when.
• CARGO_TERM_PROGRESS_WIDTH — The default progress bar width, see term.progress.width.

Environment variables Cargo sets for crates

Cargo exposes these environment variables to your crate when it is compiled.Note that this applies for running binaries with cargo run and cargo testas well. To get the value of any of these variables in a Rust program, dothis:

let version = env!("CARGO_PKG_VERSION");

version will now contain the value of CARGO_PKG_VERSION.

Note that if one of these values is not provided in the manifest, thecorresponding environment variable is set to the empty string, "".

• 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 from the manifest of your package.
• CARGO_PKG_HOMEPAGE — The home page from the manifest of your package.
• CARGO_PKG_REPOSITORY — The repository from the manifest of your package.
• CARGO_PKG_LICENSE — The license from the manifest of your package.
• CARGO_PKG_LICENSE_FILE — The license file from the manifest of your package.
• CARGO_PKG_RUST_VERSION — The Rust version from the manifest of your package.Note that this is the minimum Rust version supported by the package, not thecurrent Rust version.
• CARGO_PKG_README — Path to the README file of your package.
• CARGO_CRATE_NAME — The name of the crate that is currently being compiled. It is the name of the Cargo target with - converted to _, such as the name of the library, binary, example, integration test, or benchmark.
• CARGO_BIN_NAME — The name of the binary that is currently being compiled.Only set for binaries or binary examples. This name does not include anyfile extension, such as .exe.
• OUT_DIR — If the package has a build script, this is set to the folderwhere the build script should place its output. See below for more information.(Only set during compilation.)
• CARGO_BIN_EXE_<name> — The absolute path to a binary target’s executable.This is only set when building an integration test or benchmark. This maybe used with the env macro to find the executable to run for testingpurposes. The <name> is the name of the binary target, exactly as-is. Forexample, CARGO_BIN_EXE_my-program for a binary named my-program.Binaries are automatically built when the test is built, unless the binaryhas required features that are not enabled.
• CARGO_PRIMARY_PACKAGE — This environment variable will be set if thepackage being built is primary. Primary packages are the ones the userselected on the command-line, either with -p flags or the defaults basedon the current directory and the default workspace members. This environmentvariable will not be set when building dependencies. This is only set whencompiling the package (not when running binaries or tests).
• CARGO_TARGET_TMPDIR — Only set when building integration test or benchmark code.This is a path to a directory inside the target directorywhere integration tests or benchmarks are free to put any data needed bythe tests/benches. Cargo initially creates this directory but doesn’tmanage its content in any way, this is the responsibility of the test code.
• CARGO_RUSTC_CURRENT_DIR — This is a path that rustc is invoked from (nightly only).

Dynamic library paths

Cargo also sets the dynamic library path when compiling and running binarieswith commands like cargo run and cargo test. This helps with locatingshared libraries that are part of the build process. The variable name dependson the platform:

• Windows: PATH
• macOS: DYLD_FALLBACK_LIBRARY_PATH
• Unix: LD_LIBRARY_PATH
• AIX: LIBPATH

The value is extended from the existing value when Cargo starts. macOS hasspecial consideration where if DYLD_FALLBACK_LIBRARY_PATH is not alreadyset, it will add the default $HOME/lib:/usr/local/lib:/usr/lib.

Cargo includes the following paths:

• Search paths included from any build script with the rustc-link-searchinstruction. Paths outside of thetarget directory are removed. It is the responsibility of the user runningCargo to properly set the environment if additional libraries on the systemare needed in the search path.
• The base output directory, such as target/debug, and the “deps” directory.This is mostly for support of proc-macros.
• The rustc sysroot library path. This generally is not important to mostusers.

Environment variables Cargo sets for build scripts

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

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 packagebeing built (the package containing the build script). Also note that this isthe value of the current working directory of the build script when it starts.
• CARGO_MANIFEST_LINKS — the manifest links value.
• CARGO_MAKEFLAGS — Contains parameters needed for Cargo’s jobserverimplementation to parallelize subprocesses. Rustc or cargo invocations frombuild.rs can already read CARGO_MAKEFLAGS, but GNU Make requires the flagsto be specified either directly as arguments, or through the MAKEFLAGSenvironment variable. Currently Cargo doesn’t set the MAKEFLAGS variable,but it’s free for build scripts invoking GNU Make to set it to the contentsof CARGO_MAKEFLAGS.
• CARGO_FEATURE_<name> — For each activated feature of the package being built,this environment variable will be present where <name> is the name of thefeature uppercased and having - translated to _.
• CARGO_CFG_<cfg> — For each configuration option of thepackage being built, this environment variable will contain the value of theconfiguration, where <cfg> is the name of the configuration uppercased andhaving - translated to _. Boolean configurations are present if they areset, and not present otherwise. Configurations with multiple values arejoined to a single variable with the values delimited by ,. This includesvalues built-in to the compiler (which can be seen with rustc --print=cfg)and values set by build scripts and extra flags passed to rustc (such asthose defined in RUSTFLAGS). Some examples of what these variables are:
  • CARGO_CFG_UNIX — Set on unix-like platforms.
  • CARGO_CFG_WINDOWS — Set on windows-like platforms.
  • CARGO_CFG_TARGET_FAMILY=unix,wasm — The target family.
  • CARGO_CFG_TARGET_OS=macos — The target operating system.
  • CARGO_CFG_TARGET_ARCH=x86_64 — The CPU target architecture.
  • CARGO_CFG_TARGET_VENDOR=apple — The target vendor.
  • CARGO_CFG_TARGET_ENV=gnu — The target environment ABI.
  • CARGO_CFG_TARGET_ABI=sim — The target ABI.
  • CARGO_CFG_TARGET_POINTER_WIDTH=64 — The CPU pointer width.
  • CARGO_CFG_TARGET_ENDIAN=little — The CPU target endianness.
  • CARGO_CFG_TARGET_FEATURE=mmx,sse — List of CPU target features enabled.

  Note that different target triples have different sets of cfg values,hence variables present in one target triple might not be available in the other.

• OUT_DIR — the folder in which all output and intermediate artifacts shouldbe 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 becompiled for this triple. See the Target Triple description for more information.
• HOST — the host triple of the Rust compiler.
• NUM_JOBS — the parallelism specified as the top-level parallelism. This canbe useful to pass a -j parameter to a system like make. Note that careshould be taken when interpreting this environment variable. For historicalpurposes this is still provided but recent versions of Cargo, for example, donot need to run make -j, and instead can set the MAKEFLAGS env var to thecontent of CARGO_MAKEFLAGS to activate the use of Cargo’s GNU Make compatiblejobserver for sub-make invocations.
• OPT_LEVEL, DEBUG — values of the corresponding variables for the profile currently being built.
• PROFILE — release for release builds, debug for other builds. This isdetermined based on if the profile inherits from the dev orrelease profile. Using this environment variable is not recommended.Using other environment variables like OPT_LEVEL provide a more correctview of the actual settings being used.
• 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 hasresolved to use, passed to the build script so it might use it as well.
• RUSTC_WRAPPER — the rustc wrapper, if any, that Cargo is using. See build.rustc-wrapper.
• RUSTC_WORKSPACE_WRAPPER — the rustc wrapper, if any, that Cargo is usingfor workspace members. See build.rustc-workspace-wrapper.
• RUSTC_LINKER — The path to the linker binary that Cargo has resolved to usefor the current target, if specified. The linker can be changed by editing.cargo/config.toml; see the documentation about cargo configurationfor more information.
• CARGO_ENCODED_RUSTFLAGS — extra flags that Cargo invokes rustc with,separated by a 0x1f character (ASCII Unit Separator). Seebuild.rustflags. Note that since Rust 1.55, RUSTFLAGS is removed fromthe environment; scripts should use CARGO_ENCODED_RUSTFLAGS instead.
• CARGO_PKG_<var> — The package information variables, with the same names and values as are provided during crate building.

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.
• CARGO_MAKEFLAGS — Contains parameters needed for Cargo’s jobserverimplementation to parallelize subprocesses.This is set only when Cargo detects the existence of a jobserver.

For extended information about your environment you may run cargo metadata.