# tail

![](https://3804023877-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LR7OsqPORtP86IQxs6E%2F-LWyDIvT7Z7M06Hdqznf%2F-LWNPOu-hh314qQErrMR%2Ftail.png?generation=1548305927027164\&alt=media)

The `in_tail` Input plugin allows Fluentd to read events from the tail of text files. Its behavior is similar to the `tail -F` command.

## Example Configuration

`in_tail` is included in Fluentd's core. No additional installation process is required.

```
<source>
  @type tail
  path /var/log/httpd-access.log
  pos_file /var/log/td-agent/httpd-access.log.pos
  tag apache.access
  format apache2
</source>
```

Please see the [Config File](https://docs.fluentd.org/0.12/configuration/config-file) article for the basic structure and syntax of the configuration file.

### How it Works

* When Fluentd is first configured with `in_tail`, it will start

  reading from the **tail** of that log, not the beginning.
* Once the log is rotated, Fluentd starts reading the new file from

  the beginning. It keeps track of the current inode number.
* If `td-agent` restarts, it starts reading from the last position

  td-agent read before the restart. This position is recorded in the

  position file specified by the pos\_file parameter.

## Parameters

### type (required)

The value must be `tail`.

### tag (required)

The tag of the event.

`*` can be used as a placeholder that expands to the actual file path, replacing '/' with '.'. For example, if you have the following configuration

```
path /path/to/file
tag foo.*
```

in\_tail emits the parsed events with the 'foo.path.to.file' tag.

### path (required)

The paths to read. Multiple paths can be specified, separated by ','.

`*` and strftime format can be included to add/remove watch file dynamically. At interval of `refresh_interval`, Fluentd refreshes the list of watch file.

```
path /path/to/%Y/%m/%d/*
```

If the date is 20140401, Fluentd starts to watch the files in /path/to/2014/04/01 directory. See also `read_from_head` parameter.

You should not use \\'\*\\' with log rotation because it may cause the log duplication. In such case, you should separate in\_tail plugin configuration.

### exclude\_path

The paths to exclude the files from watcher list. For example, if you want to remove compressed files, you can use following pattern.

```
path /path/to/*
exclude_path ["/path/to/*.gz", "/path/to/*.zip"]
```

### refresh\_interval

The interval of refreshing the list of watch file. Default is 60 seconds.

### limit\_recently\_modified

This parameter is available since v0.12.33.

Limit the watching files that the modification time is within the specified time range when use `*` in `path` parameter.

### skip\_refresh\_on\_startup

This parameter is available since v0.12.33.

Skip the refresh of watching list on startup. This reduces the start up time when use `*` in `path`.

### read\_from\_head

Start to read the logs from the head of file, not bottom. The default is `false`.

If you want to tail all contents with `*` or strftime dynamic path, set this parameter to `true`. Instead, you should guarantee that log rotation will not occur in `*` directory.

When this is true, in\_tail tries to read a file during start up phase. If target file is large, it takes long time and starting other plugins isn't executed until reading file is finished.

### encoding, from\_encoding

Specify the encoding of reading lines. The default is ASCII-8BIT.

By default, in\_tail emits string value as ASCII-8BIT encoding. These options change it.

* If specify only `encoding`, in\_tail changes string to `encoding`.

  This use ruby's

  [String#force\_encoding](https://docs.ruby-lang.org/en/trunk/String.html#method-i-force_encoding)
* If specify `encoding` and `from_encoding`, in\_tail tries to encode

  string from `from_encoding` to `encoding`. This uses ruby's

  [String#encode](https://docs.ruby-lang.org/en/trunk/String.html#method-i-encode)

You can get supported encoding list by typing following command:

```
$ ruby -e 'p Encoding.name_list.sort'
```

### read\_lines\_limit

The number of reading lines at each IO. Default is 1000 lines.

If you see "Size of the emitted data exceeds buffer\_chunk\_limit." log with in\_tail, set smaller value.

### multiline\_flush\_interval

The interval of flushing the buffer for multiline format. The default is disabled.

If you set `multiline_flush_interval 5s`, in\_tail flushes buffered event after 5 seconds from last emit. This option is useful when you use `format_firstline` option. Since v0.12.20 or later.

### pos\_file (highly recommended)

This parameter is highly recommended. Fluentd will record the position it last read into this file.

```
pos_file /var/log/td-agent/tmp/access.log.pos
```

`pos_file` handles multiple positions in one file so no need multiple `pos_file` parameters per `source`.

Don't share pos\_file between in\_tail configurations. It causes unexpected behavior, e.g. corrupt pos\_file content.

in\_tail removes untracked file position during startup phase. It means the content of pos\_file is growing until restart when you tails lots of files with dynamic path setting. I will fix this problem in the future. Check [this issue](https://github.com/fluent/fluentd/issues/1126).

### format (required)

The format of the log. `in_tail` uses parser plugin to parse the log. See [parser article](https://docs.fluentd.org/0.12/parser) for more detail.

### path\_key

Add watching file path to `path_key` field.

```
path /path/to/access.log
path_key tailed_path
```

With this config, generated events are like `{"tailed_path":"/path/to/access.log","k1":"v1",...,"kN":"vN"}`.

### rotate\_wait

in\_tail actually does a bit more than `tail -F` itself. When rotating a file, some data may still need to be written to the old file as opposed to the new one.

in\_tail takes care of this by keeping a reference to the old file (even after it has been rotated) for some time before transitioning completely to the new file. This helps prevent data designated for the old file from getting lost. By default, this time interval is 5 seconds.

The rotate\_wait parameter accepts a single integer representing the number of seconds you want this time interval to be.

### enable\_watch\_timer

Enable the additional watch timer. Setting this parameter to `false` will significantly reduce CPU and I/O consumption when tailing a large number of files on systems with inotify support. The default is `true` which results in an additional 1 second timer being used.

`in_tail` (via Cool.io) uses inotify on systems which support it. Earlier versions of libev on some platforms (eg Mac OS X) did not work properly; therefore, an explicit 1 second timer was used. Even on systems with inotify support, this results in additional I/O each second, for every file being tailed.

Early testing demonstrates that modern Cool.io and `in_tail` work properly without the additional watch timer. At some point in the future, depending on feedback and testing, the additional watch timer may be disabled by default.

### ignore\_repeated\_permission\_error

If you hard to exclude non-permision files from watching list, set this parameter to `true`. It suppress repeated permission error logs.

#### log\_level option

The `log_level` option allows the user to set different levels of logging for each plugin. The supported log levels are: `fatal`, `error`, `warn`, `info`, `debug`, and `trace`.

Please see the [logging article](https://docs.fluentd.org/0.12/deployment/logging) for further details.

## FAQ

### in\_tail doesn't start to read log file, why?

`in_tail` follows `tail -F` command behaviour by default, so `in_tail` reads only newer logs. If you want to read existing lines for batch use case, set `read_from_head true`.

### logrotate setting

`logrotate` has `nocreate` parameter and it doesn't create new file after triggered log rotation. It means `in_tail` can't find new file to tail.

This parameter doesn't fit typical application log cases, so check your `logrotate` setting which doesn't include `nocreate` parameter.

### What happens when in\_tail receives BufferQueueLimitError?

in\_tail stops reading new lines and pos file update until BufferQueueLimitError is resolved. After resolved BufferQueueLimitError, restart emitting new lines and pos file update.

If this article is incorrect or outdated, or omits critical information, please [let us know](https://github.com/fluent/fluentd-docs-gitbook/issues?state=open). [Fluentd](http://www.fluentd.org/) is a open source project under [Cloud Native Computing Foundation (CNCF)](https://cncf.io/). All components are available under the Apache 2 License.