## Contents

- [Start minimally: Initialize, and write logs to stderr](#start-minimally-initialize-and-write-logs-to-stderr)
- [Choose the log output channel](#choose-the-log-output-channel)
- [Choose the write mode](#choose-the-write-mode)
- [Influence the location and name of the log file](#influence-the-location-and-name-of-the-log-file)
- [Specify the format for the log lines explicitly](#specify-the-format-for-the-log-lines-explicitly)
- [Use a fixed log file, and truncate or append the file on each program start](#use-a-fixed-log-file-and-truncate-or-append-the-file-on-each-program-start)
- [Rotate the log file](#rotate-the-log-file)
- [Reconfigure the log specification programmatically](#reconfigure-the-log-specification-programmatically)
- [Reconfigure the log specification dynamically by editing a spec-file](#reconfigure-the-log-specification-dynamically-by-editing-a-spec-file)
- [Reconfigure the file log writer](#reconfigure-the-file-log-writer)
- [External file rotators](#external-file-rotators)
- [Miscellaneous](#miscellaneous)

## Start minimally: Initialize, and write logs to stderr

Initialize by choosing one of three options to specify which log output you want to see,
and call `start()` immediately:

- Provide the log specification in the environment variable `RUST_LOG`:

  ```rust
  # use flexi_logger::{Logger,FlexiLoggerError};
  # fn main() -> Result<(), FlexiLoggerError> {
  Logger::try_with_env()?.start()?;
  # Ok(())}
  ```

- Provide the log specification programmatically:

  ```rust
  # use flexi_logger::{Logger,FlexiLoggerError};
  # fn main() -> Result<(), FlexiLoggerError> {
  Logger::try_with_str("info")?.start()?;
  # Ok(())}
  ```

- Combine both options, with env having precendence over the given parameter value:

  ```rust
  # use flexi_logger::{Logger,FlexiLoggerError};
  # fn main() -> Result<(), FlexiLoggerError> {
  Logger::try_with_env_or_str("info")?.start()?;
  # Ok(())}
  ```

  or, even shorter, use:

  ```rust
  flexi_logger::init();
  ```

After that, you just use the log-macros from the log crate. Those log lines that match the
log specification are then written to the default output channel (stderr).

## Choose the log output channel

By default, logs are written to `stderr`.
With one of

- [`Logger::log_to_stdout`](crate::Logger::log_to_stdout),
- [`Logger::log_to_file`](crate::Logger::log_to_file),
- [`Logger::log_to_writer`](crate::Logger::log_to_writer),
- [`Logger::log_to_file_and_writer`](crate::Logger::log_to_file_and_writer),
- or [`Logger::do_not_log`](crate::Logger::do_not_log),

you can send the logs to other destinations, or write them not at all.

When writing to files or to a writer,
you sometimes want to see some parts of the log additionally on the terminal;
this can be achieved with
[`Logger::duplicate_to_stderr`](crate::Logger::duplicate_to_stderr) or
[`Logger::duplicate_to_stdout`](crate::Logger::duplicate_to_stdout),
which duplicate log messages to the terminal.

```rust
# use flexi_logger::{Logger,Duplicate, FileSpec};
# fn main() -> Result<(), Box<dyn std::error::Error>> {
Logger::try_with_str("info")?
    .log_to_file(FileSpec::default())         // write logs to file
    .duplicate_to_stderr(Duplicate::Warn)     // print warnings and errors also to the console
    .start()?;
# Ok(())
# }
```

## Choose the write mode

By default, every log line is directly written to the output, without buffering.
This allows seeing new log lines in real time.

With [`Logger::write_mode`](crate::Logger::write_mode)
you have some options to change this behavior, e.g.

- with [`WriteMode::BufferAndFlush`](crate::WriteMode::BufferAndFlush),
  or [`WriteMode::BufferAndFlushWith`](crate::WriteMode::BufferAndFlushWith),
  you can reduce the program's I/O overhead and thus increase overall performance,
  which can be relevant if logging is used heavily.
  In addition, to keep a short maximum wait time
  until a log line is visible in the output channel,
  an extra thread is created that flushes the buffers regularly.

  ```rust
  # use flexi_logger::{WriteMode,FileSpec,Logger,Duplicate};
  fn main() -> Result<(), Box<dyn std::error::Error>> {
      let _logger = Logger::try_with_str("info")?
         .log_to_file(FileSpec::default())
         .write_mode(WriteMode::BufferAndFlush)
         .start()?;
      // ... do all your work ...
      Ok(())
  }
  ```


- with [`WriteMode::Async`](crate::WriteMode::Async)
  or [`WriteMode::AsyncWith`](crate::WriteMode::AsyncWith),
  logs are sent from your application threads through an unbounded channel
  to an output thread, which does the output (and the rotation and the cleanup, if applicable).
  Additionally, the output is buffered, and a bounded message pool is used
  to reduce allocations, and flushing is used to avoid long delays.
  If duplication is used, the messages to `stdout` or `stderr` are written synchronously.

  ```rust
  # use flexi_logger::{WriteMode, Duplicate, FileSpec, Logger};
  fn main() -> Result<(), Box<dyn std::error::Error>> {
  # #[cfg(feature="async")]
      let _logger = Logger::try_with_str("info")?
         .log_to_file(FileSpec::default())
         .write_mode(WriteMode::Async)
         .start()?;
      // ... do all your work ...
      Ok(())
  }
  ```


- with [`WriteMode::SupportCapture`](crate::WriteMode::SupportCapture) you allow
  `cargo test` to capture log output and print it only for failing tests.

Note that, with all write modes
except [`WriteMode::Direct`](crate::WriteMode::Direct) (which is the default) and
[`WriteMode::SupportCapture`](crate::WriteMode::SupportCapture),
you should **keep the [`LoggerHandle`](crate::LoggerHandle) alive**
up to the very end of your program, because, when its last instance is dropped
(in case you use `LoggerHandle::clone()` you can have multiple instances),
it will flush all writers to ensure that all buffered log lines are written
before the program terminates, and then it calls their shutdown method.

## Influence the location and name of the log file

By default, the log files are created in the current directory (where the program was started).
With [`FileSpec:directory`](crate::FileSpec::directory)
you can specify a concrete folder in which the files should be created.

Using [`FileSpec::discriminant`](crate::FileSpec::discriminant)
you can add a discriminating infix to the log file name.

With [`FileSpec::suffix`](crate::FileSpec::suffix)
you can change the suffix that is used for the log files.

When writing to files, especially when they are in a distant folder, you may want to let the
user know where the log file is.

[`Logger::print_message`](crate::Logger::print_message)
prints an info to `stdout` to which file the log is written.

[`Logger::create_symlink`](crate::Logger::create_symlink)
creates (on unix-systems only) a symbolic link at the specified path that points to the log file.

```rust
# use flexi_logger::{FileSpec,Logger};
# fn main() -> Result<(), Box<dyn std::error::Error>> {
Logger::try_with_str("info")?
    .log_to_file(
        FileSpec::default()
            .directory("log_files")          // create files in folder ./log_files
            .basename("foo")
            .discriminant("Sample4711A")     // use infix in log file name
            .suffix("trc")                   // use suffix .trc instead of .log
    )
    .print_message()                         //
    .create_symlink("current_run")           // create a symbolic link to the current log file
    .start()?;
# Ok(())
# }
```

This example will print a message like
"Log is written to `./log_files/foo_Sample4711A_2020-11-17_19-24-35.trc`"
and, on unix, create a symbolic link called `current_run`.

## Specify the format for the log lines explicitly

With [`Logger::format`](crate::Logger::format)
you set the format for all used output channels of `flexi_logger`.

`flexi_logger` provides a couple of format functions, and you can also create and use your own,
e.g. by copying and modifying one of the provided format functions (see [formats.rs](https://github.com/emabee/flexi_logger/blob/main/src/formats.rs)).

Here's an example that you could create somewhere in your code.
It also illustrates the signature the format function must have.

```rust,ignore
pub fn my_own_format(
    w: &mut dyn std::io::Write,
    now: &mut DeferredNow,
    record: &Record,
) -> Result<(), std::io::Error> {
    let level = record.level();
    write!(
        w,
        "{} [Thread {}] Severity {}, Message: {}",
        now.format(TS_DASHES_BLANK_COLONS_DOT_BLANK),
        thread::current().name().unwrap_or("<unnamed>"),
        record.level(),
        &record.args()
    )
}
```

Depending on the configuration, `flexi_logger` can write logs to multiple channels
(stdout, stderr, files, or additional writers)
at the same time. You can control the format for each output channel individually, using
[`Logger::format_for_files`](crate::Logger::format_for_files),
[`Logger::format_for_stderr`](crate::Logger::format_for_stderr),
[`Logger::format_for_stdout`](crate::Logger::format_for_stdout), or
[`Logger::format_for_writer`](crate::Logger::format_for_writer).

 As argument for these functions you can use one of the provided non-coloring format functions

- [`default_format`](crate::default_format)
- [`detailed_format`](crate::detailed_format)
- [`opt_format`](crate::opt_format)
- [`with_thread`](crate::with_thread),

or one of their coloring pendants

- [`colored_default_format`](crate::colored_default_format)
- [`colored_detailed_format`](crate::colored_detailed_format)
- [`colored_opt_format`](crate::colored_opt_format)
- [`colored_with_thread`](crate::colored_with_thread),

or your own method.

### Adaptive Coloring

You can use coloring for `stdout` and/or `stderr`
_conditionally_, such that colors

- are used when the output goes to a tty,
- are suppressed when you e.g. pipe the output to some other program.

You achieve that
by providing one of the variants of [`AdaptiveFormat`](crate::AdaptiveFormat) to the respective
format method, e.g.

```rust
# use flexi_logger::AdaptiveFormat;
# fn main() -> Result<(), Box<dyn std::error::Error>> {
# {
      flexi_logger::Logger::try_with_str("info")?
          .adaptive_format_for_stderr(AdaptiveFormat::Detailed);
# }
#     Ok(())
# }
```

### Defaults

`flexi_logger` initializes by default equivalently to this:

```rust
# mod example {
# use flexi_logger::{Logger,AdaptiveFormat,default_format, FileSpec};
# use log::{debug, error, info, trace, warn};
# fn main() -> Result<(), Box<dyn std::error::Error>> {
# Logger::try_with_str("info")?      // Write all error, warn, and info messages
#     .log_to_file(FileSpec::default().directory(std::env::temp_dir()))
      // ...
      .adaptive_format_for_stderr(AdaptiveFormat::Default)
      .adaptive_format_for_stdout(AdaptiveFormat::Default)
      .format_for_files(default_format)
      .format_for_writer(default_format)
#     .start()?;
# error!("This is an error message");
# warn!("This is a warning");
# info!("This is an info message");
# debug!("This is a debug message - you must not see it!");
# trace!("This is a trace message - you must not see it!");
#  run()
# }
# fn run() -> Result<(), Box<dyn std::error::Error>> {Ok(())}
# }
```

## Use a fixed log file, and truncate or append the file on each program start

With [`Logger::log_to_file`](crate::Logger::log_to_file) and without rotation,
`flexi_logger` uses by default files with a timestamp in the name, like
`foo_2020-11-16_08-37-44.log` (for a program called `foo`), which are quite unique for each
program start.

With [`FileSpec::suppress_timestamp`](crate::FileSpec::suppress_timestamp)
you get a simple fixed filename, like `foo.log`.

In that case, a restart of the program will truncate an existing log file.

Use additionally [`Logger::append`](crate::Logger::append)
to append the logs of each new run to the existing file.

```rust
# use flexi_logger::{FileSpec, Logger};
# use log::{debug, error, info, trace, warn};
# fn main() -> Result<(), Box<dyn std::error::Error>> {
Logger::try_with_str("info")? // Write all error, warn, and info messages
    // use a simple filename without a timestamp
    .log_to_file(
        FileSpec::default().suppress_timestamp()
#           .directory(std::env::temp_dir())
    )
    // do not truncate the log file when the program is restarted
    .append()
    .start()?;

# error!("This is an error message");
# warn!("This is a warning");
# info!("This is an info message");
# debug!("This is a debug message - you must not see it!");
# trace!("This is a trace message - you must not see it!");
#  run()
# }
# fn run() -> Result<(), Box<dyn std::error::Error>> {Ok(())}
```

## Rotate the log file

With rotation, the logs are always written to a file
with the infix `rCURRENT`, like e.g. `foo_rCURRENT.log`.

[`Logger::rotate`](crate::Logger::rotate)
takes three enum arguments that define its behavior:

- [`Criterion`](crate::Criterion)
  - with [`Criterion::Age`](crate::Criterion::Age) the rotation happens
     when the clock switches to a new day, hour, minute, or second
  - with [`Criterion::Size`](crate::Criterion::Size) the rotation happens
     when the current log file exceeds the specified limit
  - with [`Criterion::AgeOrSize`](crate::Criterion::AgeOrSize) the rotation happens
     when either of the two limits is reached

- [`Naming`](crate::Naming)
The current file is then renamed
  - with [`Naming::Timestamps`](crate::Naming::Timestamps) to something
    like `foo_r2020-11-16_08-56-52.log`
  - with [`Naming::Numbers`](crate::Naming::Numbers) to something like `foo_r00000.log`

  and a fresh `rCURRENT` file is created.

- [`Cleanup`](crate::Cleanup) defines if and how you
  avoid accumulating log files indefinitely:
  - with [`Cleanup::KeepLogFiles`](crate::Cleanup::KeepLogFiles) you specify
    the number of log files that should be retained;
    if there are more, the older ones are getting deleted
  - with [`Cleanup::KeepCompressedFiles`](crate::Cleanup::KeepCompressedFiles) you specify
    the number of log files that should be
    retained, and these are being compressed additionally
  - with [`Cleanup::KeepLogAndCompressedFiles`](crate::Cleanup::KeepLogAndCompressedFiles)
    you specify the number of log files that should be
    retained as is, and an additional number that are being compressed
  - with [`Cleanup::Never`](crate::Cleanup::Never) no cleanup is done, all files are retained.

```rust
# use flexi_logger::{Age, Cleanup, Criterion, FileSpec, Logger, Naming};
# use log::{debug, error, info, trace, warn};
# fn main() -> Result<(), Box<dyn std::error::Error>> {
Logger::try_with_str("info")?      // Write all error, warn, and info messages
    .log_to_file(
        FileSpec::default()
#           .directory(std::env::temp_dir())
    )
    .rotate(                      // If the program runs long enough,
        Criterion::Age(Age::Day), // - create a new file every day
        Naming::Timestamps,       // - let the rotated files have a timestamp in their name
        Cleanup::KeepLogFiles(7), // - keep at most 7 log files
    )
    .start()?;

#   error!("This is an error message");
#   warn!("This is a warning");
#   info!("This is an info message");
#   debug!("This is a debug message - you must not see it!");
#   trace!("This is a trace message - you must not see it!");
#    run()
# }
# fn run() -> Result<(), Box<dyn std::error::Error>> {Ok(())}
```

## Reconfigure the log specification programmatically

This can be especially handy in debugging situations where you want to see
log output only for a short instant.

Obtain the [`LoggerHandle`](crate::LoggerHandle)

```rust
# use flexi_logger::Logger;
let mut logger = Logger::try_with_str("info").unwrap()
    // ... logger configuration ...
    .start()
    .unwrap();
```

and modify the effective log specification from within your code:

```rust, ignore
# use flexi_logger::Logger;
# let mut logger = Logger::try_with_str("info").unwrap().start().unwrap();
// ...
logger.parse_and_push_temp_spec("info, critical_mod = trace");
// ... critical calls ...
logger.pop_temp_spec();
// ... continue with the log spec you had before.
```

## Reconfigure the log specification dynamically by editing a spec-file

If you start `flexi_logger` with a specfile,

```rust
# use flexi_logger::Logger;
# let logger =
Logger::try_with_str("info").unwrap()
    // ... logger configuration ...
# ;
# #[cfg(feature = "specfile")]
# logger
   .start_with_specfile("./server/config/logspec.toml")
   .unwrap();
```

then you can change the log specification dynamically, _while your program is running_,
by editing the specfile. This can be a great help e.g. if you want to get detailed log output
for _some_ requests to a long running server.

See [`Logger::start_with_specfile`](crate::Logger::start_with_specfile)
for more information.

## [Reconfigure the file log writer](#reconfigure-the-file-log-writer)

When using `Logger::log_to_file()`, you can change most of the properties of the
embedded `FileLogWriter` while the program is running using
[`Logger::reset_flw`](crate::LoggerHandle::reset_flw).

Obtain the [`LoggerHandle`](crate::LoggerHandle) when the program is started

```rust
use flexi_logger::{writers::FileLogWriter, Cleanup, Criterion, FileSpec, Naming};

# use std::error::Error;
# fn main() -> Result<(), Box<dyn Error>> {
let logger = flexi_logger::Logger::try_with_str("info")?
    .log_to_file(
        FileSpec::default()
            .basename("phase1")
            .directory("./log_files")
    )
    .start()?;

log::info!("start of phase 1");
# Ok(())
# }
```

and modify the file log writer later:

```rust
# use std::error::Error;
# use flexi_logger::{writers::FileLogWriter, Cleanup, Criterion, FileSpec, Naming};
# fn main() -> Result<(), Box<dyn Error>> {
#    let logger = flexi_logger::Logger::try_with_str("info")?
#       .log_to_file(FileSpec::default().basename("phase1").directory("./log_files"))
#       .start()?;
logger.reset_flw(
    &FileLogWriter::builder(
        FileSpec::default()
            .basename("phase2")
            .directory("./log_files")
    )
    .append()
    .rotate(
        Criterion::Size(1024 * 1000 * 1),
        Naming::Numbers,
        Cleanup::KeepLogFiles(3),
    ),
)?;

log::info!("start of phase 2");
# Ok(())
# }
```

## External file rotators

If the log is written to files, `flexi_logger` decides, based on your configuration,
to which file(s) the log is written, and expects that nobody else modifies these files.
It offers quite some functionality to rotate, compress, and clean up log files.

Alternatively, tools like linux' `logrotate` can be used to rotate, compress or remove
log files. But renaming or deleting the current output file e.g. might not stop
`flexi_logger` from writing to the now renamed file!
See [`LoggerHandle::reopen_outputfile`](../struct.LoggerHandle.html#method.reopen_outputfile)
to understand how to cope with external rotators.

## Miscellaneous

For the sake of completeness, we refer here to some more configuration methods.
See their documentation for more details.

[`Logger::set_palette`](crate::Logger::set_palette)

[`Logger::cleanup_in_background_thread`](crate::Logger::cleanup_in_background_thread)

[`Logger::use_windows_line_ending`](crate::Logger::use_windows_line_ending)

[`Logger::add_writer`](crate::Logger::add_writer)
