Serilog.Sinks.File

Writes Serilog events to one or more text files.

Getting started

Install the Serilog.Sinks.File package from NuGet:

Install-Package Serilog.Sinks.File

To configure the sink in C# code, call WriteTo.File() during logger configuration:

var log = new LoggerConfiguration()
    .WriteTo.File("log.txt", rollingInterval: RollingInterval.Day)
    .CreateLogger();

This will append the time period to the filename, creating a file set like:

log20180631.txt
log20180701.txt
log20180702.txt

Important: By default, only one process may write to a log file at a given time. See Shared log files below for information on multi-process sharing.

Limits

To avoid bringing down apps with runaway disk usage the file sink limits file size to 1GB by default. Once the limit is reached, no further events will be written until the next roll point (see also: Rolling policies below).

The limit can be changed or removed using the fileSizeLimitBytes parameter.

    .WriteTo.File("log.txt", fileSizeLimitBytes: null)

For the same reason, only the most recent 31 files are retained by default (i.e. one long month). To change or remove this limit, pass the retainedFileCountLimit parameter.

    .WriteTo.File("log.txt", rollingInterval: RollingInterval.Day, retainedFileCountLimit: null)

Rolling policies

To create a log file per day or other time period, specify a rollingInterval as shown in the examples above.

To roll when the file reaches fileSizeLimitBytes, specify rollOnFileSizeLimit:

    .WriteTo.File("log.txt", rollOnFileSizeLimit: true)

This will create a file set like:

log.txt
log_001.txt
log_002.txt

Specifying both rollingInterval and rollOnFileSizeLimit will cause both policies to be applied, while specifying neither will result in all events being written to a single file.

Old files will be cleaned up as per retainedFileCountLimit - the default is 31.

XML `<appSettings>` configuration

To use the file sink with the Serilog.Settings.AppSettings package, first install that package if you haven't already done so:

Install-Package Serilog.Settings.AppSettings

Instead of configuring the logger in code, call ReadFrom.AppSettings():

var log = new LoggerConfiguration()
    .ReadFrom.AppSettings()
    .CreateLogger();

In your application's App.config or Web.config file, specify the file sink assembly and required path format under the <appSettings> node:

<configuration>
  <appSettings>
    <add key="serilog:using:File" value="Serilog.Sinks.File" />
    <add key="serilog:write-to:File.path" value="log.txt" />

The parameters that can be set through the serilog:write-to:File keys are the method parameters accepted by the WriteTo.File() configuration method. This means, for example, that the fileSizeLimitBytes parameter can be set with:

    <add key="serilog:write-to:File.fileSizeLimitBytes" value="1234567" />

Omitting the value will set the parameter to null:

    <add key="serilog:write-to:File.fileSizeLimitBytes" />

In XML and JSON configuration formats, environment variables can be used in setting values. This means, for instance, that the log file path can be based on TMP or APPDATA:

    <add key="serilog:write-to:File.path" value="%APPDATA%\MyApp\log.txt" />

JSON `appsettings.json` configuration

To use the file sink with Microsoft.Extensions.Configuration, for example with ASP.NET Core or .NET Core, use the Serilog.Settings.Configuration package. First install that package if you have not already done so:

Install-Package Serilog.Settings.Configuration

Instead of configuring the file directly in code, call ReadFrom.Configuration():

var configuration = new ConfigurationBuilder()
    .AddJsonFile("appsettings.json")
    .Build();

var logger = new LoggerConfiguration()
    .ReadFrom.Configuration(configuration)
    .CreateLogger();

In your appsettings.json file, under the Serilog node, :

{
  "Serilog": {
    "WriteTo": [
      { "Name": "File", "Args": { "path": "log.txt", "rollingInterval": "Day" } }
    ]
  }
}

See the XML <appSettings> example above for a discussion of available Args options.

Controlling event formatting

The file sink creates events in a fixed text format by default:

2018-07-06 09:02:17.148 +10:00 [INF] HTTP GET / responded 200 in 1994 ms

The format is controlled using an output template, which the file configuration method accepts as an outputTemplate parameter.

The default format above corresponds to an output template like:

  .WriteTo.File("log.txt",
    outputTemplate: "{Timestamp:yyyy-MM-dd HH:mm:ss.fff zzz} [{Level:u3}] {Message:lj}{NewLine}{Exception}")

JSON event formatting

To write events to the file in an alternative format such as JSON, pass an ITextFormatter as the first argument:

    // Install-Package Serilog.Formatting.Compact
    .WriteTo.File(new CompactJsonFormatter(), "log.txt")

Shared log files

To enable multi-process shared log files, set shared to true:

    .WriteTo.File("log.txt", shared: true)

Auditing

The file sink can operate as an audit file through AuditTo:

    .AuditTo.File("audit.txt")

Only a limited subset of configuration options are currently available in this mode.

Performance

By default, the file sink will flush each event written through it to disk. To improve write performance, specifying buffered: true will permit the underlying stream to buffer writes.

The Serilog.Sinks.Async package can be used to wrap the file sink and perform all disk access on a background worker thread.

Extensibility

FileLifecycleHooks provide an extensibility point that allows hooking into different parts of the life cycle of a log file.

You can create a hook by extending from FileLifecycleHooks and overriding the OnFileOpened and/or OnFileDeleting methods.

OnFileOpened provides access to the underlying stream that log events are written to, before Serilog begins writing events. You can use this to write your own data to the stream (for example, to write a header row), or to wrap the stream in another stream (for example, to add buffering, compression or encryption)
OnFileDeleting provides a means to work with obsolete rolling log files, before they are deleted by Serilog's retention mechanism - for example, to archive log files to another location

Available hooks:

serilog-sinks-file-header: writes a header to the start of each log file
serilog-sinks-file-gzip: compresses logs as they are written, using streaming GZIP compression
serilog-sinks-file-archive: archives obsolete rolling log files before they are deleted by Serilog's retention mechanism