Command log

ffx log is a log-viewing utility built into ffx. This guide describes how to configure and use ffx log to view logs on your Fuchsia device.

To start the log viewer, run the following command:

ffx log

This command prints the current log contents and leaves the connection open to stream new log entries. To print the current contents of the log and exit, use dump:

ffx log dump

Proactive logging

The ffx daemon persists in the background after an ffx command is run. The daemon proactively discovers Fuchsia devices and connects to them as they become reachable.

With proactive logging, the ffx daemon begins reading logs from a target device in the background as soon as it's connected. The logs are cached on the host machine, up to a configured space limit. When the space limit is reached, logs are “rotated”: the oldest logs are deleted to make room for the newest ones.

This means that when you view logs using ffx log, logs are actually read from the cache on the host machine, not directly from the target device. In general, this should not add any noticeable delay to the log viewer, except in rare cases where the device is producing extremely large log volumes.

Note: Proactive logging is currently deprecated and is being removed.


Since logs are cached on the host machine, you can view logs that have been cached from a target device that are from previous boots of the device. For example, if a device crashes, you might be able to view the logs from the time just before the crash if they were cached in time.

You can use ffx log dump to view logs from a previous session. For example, to view logs from your device's previous boot:

ffx --target <NODENAME> log dump ~1

~1 identifies the session relative to the latest one you want to view, where 0 is reserved for the currently active session for that target device (whether or not a currently active session exists). You can view earlier boots by using ~2, ~3, and so on.


There are 3 configuration settings relevant to the proactive log cache:

  • proactive_log.max_sessions_per_target: The maximum number of boot sessions to keep cached on the host. Default is 5 (that is, after 6 reboots, the logs from the oldest boot session are deleted).
  • proactive_log.max_session_size_bytes: The maximum number of bytes to be cached for each session. Default is 100MB (that is, after 100MB of logs are on-disk, the oldest chunk of logs for that session are deleted)
  • proactive_log.max_log_size_bytes: The maximum number of bytes to be used in a single log chunk. You should not generally need to change this setting. Default is 1MB.


Logs are symbolized in the background as they are read from the device (before they are written to the host log cache). However, this background processing means that misconfigurations in the symbolizer host tool or with the symbol index can cause logs to be not symbolized without any visible warning. Errors encountered when setting up the symbolizer tool are logged to the ffx daemon log.

Users working with the Fuchsia source checkout setup do not need to perform any extra configuration; symbolization takes place automatically as in fx log. Users working without the Fuchsia source checkout setup need to configure the symbol index appropriate to their development environment.

The ffx log command tries to detect common misconfigurations in the symbolizer tool, but cannot detect all of them. If your logs are not being symbolized, please file a bug.

Configuring symbolizer

There are two configuration parameters relevant to symbolization:

  • proactive_log.symbolize.enabled: Toggles whether symbolization is attempted. Default is true.
  • proactive_log.symbolize.extra_args: A raw string of additional parameters passed directly to the symbolizer host tool. This can be used to, for example, configure remote symbol servers. Default is "".

Filtering logs

The ffx log command provides additional options to filter the logs captured from the target device. You can apply filters to the log based on timestamps, component, tags, or log level.

ffx log --filter hello-world --severity error

For a complete list of filtering options, see the reference documentation.

Log spam filter

You can filter spam entries from the output of the ffx log command. To filter the logs, you can define the path to a spam definition file in either the ffx configuration under the log_cmd.spam_filepath key or as a flag of the ffx log command --spam-list-path /path/to/spam/file.json. For more details, see the reference documentation.

This gives you the ability to define your own spam definitions to filter out logs that you may not find useful as you are developing.

JSON schema

The log spam definition file is stored in JSON format. This section describes the JSON schema used to serialized the data.

KeyValue TypeDescription
logSpamList of [LogSpamEntry] (#log-spam-entry)List of log spam entries

The following example represents an empty Log Spam Definition:

  "logSpam": []
KeyValue TypeDescription
file (optional)StringSource file path of log spam
line (optional)Number (64-bit)Source line number of log spam
payloadRegex (optional)StringRust Regex string that matches the log
: : : spam's content :

The following example represents a sample LogSpamEntry:

  "file": "path/to/",
  "line": 95,
  "payloadRegex": "Disconnected\\.*",

A LogSpamEntry is considered valid in any of the following cases (other combinations are ignored)

Identifies a log as spam if its source matches “file” and
: : : : “line” and its content matches “payloadRegex” :
Identifies a log as spam if its source matches “file” and
: : : : “line” :
Identifies a log as spam if it has no source information
: : : : and its content matches “payloadRegex” :

Log settings

Log filters modify how the captured logs are displayed by ffx log, but they do not affect the log entries emitted by components on the target device. Use the --select option to send a request to configure the log settings of specific components during the logging session. This adjusts the log level applied to any component matching the provided component selector for recording logs.

ffx log --select {{ '<var>' }}component-selector{{ '</var>' }}#{{ '<var>' }}log-level{{ '</var>' }}

You can use this to temporarily enable logs that are below the minimum severity configure by your component, such as DEBUG or TRACE logs, or to suppress noisy logs from a component to improve performance.

The following example enables debug logs for the core/audio component, and suppresses all log messages except errors from networking components:

$ ffx log --select core/audio#DEBUG --select core/network/**#ERROR

Note: Unlike the --severity option, which filters the view after logs are captured from the target device, this configures whether the target components emit logs of the given severity.