env_logger 0.2.0

A logger configured via an environment variable. ## Example ``` #[macro_use] extern crate log; extern crate env_logger; use log::LogLevel; fn main() { env_logger::init().unwrap(); debug!("this is a debug {}", "message"); error!("this is printed by default"); if log_enabled!(LogLevel::Info) { let x = 3i * 4i; // expensive computation info!("the answer was: {}", x); } } ``` Assumes the binary is `main`: ```{.bash} $ RUST_LOG=error ./main ERROR:main: this is printed by default ``` ```{.bash} $ RUST_LOG=info ./main ERROR:main: this is printed by default INFO:main: the answer was: 12 ``` ```{.bash} $ RUST_LOG=debug ./main DEBUG:main: this is a debug message ERROR:main: this is printed by default INFO:main: the answer was: 12 ``` You can also set the log level on a per module basis: ```{.bash} $ RUST_LOG=main=info ./main ERROR:main: this is printed by default INFO:main: the answer was: 12 ``` And enable all logging: ```{.bash} $ RUST_LOG=main ./main DEBUG:main: this is a debug message ERROR:main: this is printed by default INFO:main: the answer was: 12 ``` See the documentation for the log crate for more information about its API. ## Enabling logging Log levels are controlled on a per-module basis, and by default all logging is disabled except for `error!`. Logging is controlled via the `RUST_LOG` environment variable. The value of this environment variable is a comma-separated list of logging directives. A logging directive is of the form: ```text path::to::module=log_level ``` The path to the module is rooted in the name of the crate it was compiled for, so if your program is contained in a file `hello.rs`, for example, to turn on logging for this file you would use a value of `RUST_LOG=hello`. Furthermore, this path is a prefix-search, so all modules nested in the specified module will also have logging enabled. The actual `log_level` is optional to specify. If omitted, all logging will be enabled. If specified, it must be one of the strings `debug`, `error`, `info`, `warn`, or `trace`. As the log level for a module is optional, the module to enable logging for is also optional. If only a `log_level` is provided, then the global log level for all modules is set to this value. Some examples of valid values of `RUST_LOG` are: * `hello` turns on all logging for the 'hello' module * `info` turns on all info logging * `hello=debug` turns on debug logging for 'hello' * `hello,std::option` turns on hello, and std's option logging * `error,hello=warn` turn on global error logging and also warn for hello ## Filtering results A RUST_LOG directive may include a regex filter. The syntax is to append `/` followed by a regex. Each message is checked against the regex, and is only logged if it matches. Note that the matching is done after formatting the log string but before adding any logging meta-data. There is a single filter for all modules. Some examples: * `hello/foo` turns on all logging for the 'hello' module where the log message includes 'foo'. * `info/f.o` turns on all info logging where the log message includes 'foo', 'f1o', 'fao', etc. * `hello=debug/foo*foo` turns on debug logging for 'hello' where the log message includes 'foofoo' or 'fofoo' or 'fooooooofoo', etc. * `error,hello=warn/[0-9] scopes` turn on global error logging and also warn for hello. In both cases the log message must include a single digit number followed by 'scopes'.