Logcat command-line tool

Logcat is a command-line tool that dumps a log of system messages, including stack traces when the device throws an error and messages that you have written from your app with the Log class.

This page is about the command-line logcat tool, but you can also view log messages from the Logcat window in Android Studio. For information about viewing and filtering logs from Android Studio, see Write and View Logs with Logcat.

Logging system overview

The Android logging system is a set of structured circular buffers maintained by the system process logd. The set of available buffers is fixed and defined by the system. The most relevant ones are: main, which stores most application logs, system, which stores messages originating from the Android OS, and crash, which stores crash logs. Each log entry has a priority (one of VERBOSE, DEBUG, INFO, WARNING, ERROR or FATAL), a tag that identifies the origin of the log, and the actual log message.

The primary interface to the logging system is the shared library liblog and its header <android/log.h>. All language-specific logging facilities eventually call the function __android_log_write. By default, it calls the function __android_log_logd_logger, which sends the log entry to logd using a socket. Starting with API level 30, the logging function can be changed by calling __android_set_log_writer. More information is available in the NDK documentation.

Logs displayed by adb logcat undergo four levels of filtering:

1. Compile-time filtering: depending on compilation settings, some logs may be completely removed from the binary. For example, Proguard can be configured to remove calls to Log.d from Java code.
2. System property filtering: liblog queries a set of system properties to determine the minimum severity level to be sent to logd. If your logs have the tag MyApp, the following properties are checked, and are expected to contain the first letter of the minimum severity (V, D, I, W, E, or S to disable all logs):
• log.tag.MyApp
• persist.log.tag.MyApp
• log.tag
• persist.log.tag
3. Application filtering: If none of the properties are set, liblog uses the minimum priority set by __android_log_set_minimum_priority. The default setting is INFO.
4. Display filtering: adb logcat supports additional filters that can reduce the amount of logs shown from logd. See below for details.

Command-line syntax

To run logcat through the adb shell, the general usage is:

[adb] logcat [<option>] ... [<filter-spec>] ...

You can run logcat as an adb command or directly in a shell prompt of your emulator or connected device. To view log output using adb, navigate to your SDK platform-tools/ directory and execute:

adb logcat

For logcat online help, start a device and then execute:

adb logcat --help

You can create a shell connection to a device and execute:

$ adb shell
# logcat

Options

The following table describes the command line options of logcat.

adb logcat -t '01-26 20:52:41.820'

adb logcat -t '01-26 20:52:41.820'

adb logcat -p

$ adb logcat -P '"<white_and_blacklist>"'

or

adb shell
$ logcat -P '<white_and_blacklist>'

adb logcat -P '"/32676 675 ~/32677 897"'

~! worst uid blacklist
~1000/! worst pid in system (1000)

Filtering log output

• The tag of a log message is a short string indicating the system component from which the message originates (for example, "View" for the view system).
• The priority is one of the following character values, ordered from lowest to highest priority:
• • V: Verbose (lowest priority)
  • D: Debug
  • I: Info
  • W: Warning
  • E: Error
  • F: Fatal
  • S: Silent (highest priority, on which nothing is ever printed)

You can obtain a list of tags used in the system, with priorities, by running logcat and observing the first two columns of each message, given as <priority>/<tag>.

The following is an example of brief logcat output obtained with the logcat -v brief output command. It shows that the message relates to priority level "I" and tag "ActivityManager":

I/ActivityManager(  585): Starting activity: Intent { action=android.intent.action...}

To reduce the log output to a manageable level, you can restrict log output using filter expressions. Filter expressions let you indicate to the system the tags-priority combinations that you are interested in — the system suppresses other messages for the specified tags.

A filter expression follows this format tag:priority ..., where tag indicates the tag of interest and priority indicates the minimum level of priority to report for that tag. Messages for that tag at or above the specified priority are written to the log. You can supply any number of tag:priority specifications in a single filter expression. The series of specifications is whitespace-delimited.

Here's an example of a filter expression that suppresses all log messages except those with the tag "ActivityManager", at priority "Info" or above, and all log messages with tag "MyApp", with priority "Debug" or above:

adb logcat ActivityManager:I MyApp:D *:S

The final element in the above expression, *:S, sets the priority level for all tags to "silent", thus ensuring only log messages with "ActivityManager" and "MyApp" are displayed. Using *:S is an excellent way to ensure that log output is restricted to the filters that you have explicitly specified — it lets your filters serve as an allowlist for log output.

The following filter expression displays all log messages with priority level "warning" and higher, on all tags:

adb logcat *:W

If you're running logcat from your development computer (versus running it on a remote adb shell), you can also set a default filter expression by exporting a value for the environment variable ANDROID_LOG_TAGS:

export ANDROID_LOG_TAGS="ActivityManager:I MyApp:D *:S"

Note that ANDROID_LOG_TAGS filter is not exported to the emulator/device instance, if you are running logcat from a remote shell or using adb shell logcat.

Control log output format

Log messages contain a number of metadata fields, in addition to the tag and priority. You can modify the output format for messages so that they display a specific metadata field. To do so, you use the -v option and specify one of the supported output formats listed below.

• brief: Display priority, tag, and PID of the process issuing the message.
• long: Display all metadata fields and separate messages with blank lines.
• process: Display PID only.
• raw: Display the raw log message with no other metadata fields.
• tag: Display the priority and tag only.
• thread: A legacy format that shows priority, PID, and TID of the thread issuing the message.
• threadtime (default): Display the date, invocation time, priority, tag, PID, and TID of the thread issuing the message.
• time: Display the date, invocation time, priority, tag, and PID of the process issuing the message.

When starting logcat, you can specify the output format you want by using the -v option:

[adb] logcat [-v <format>]

Here's an example that shows how to generate messages in thread output format:

adb logcat -v thread

Note that you can only specify one output format with the -v option, but you can specify as many modifiers that make sense. Logcat ignores modifiers that do not make sense.

Format modifiers

Format modifiers change the logcat output in terms of any combination of one or more of the following modifiers. To specify a format modifier, use the -v option, as follows:

adb logcat -b all -v color -d

Every Android log message has a tag and a priority associated with it. You can combine any format modifier with any one of the following format options: brief, long, process, raw, tag, thread, threadtime, and time.

You can get the format modifier details by typing logcat -v --help at the command line.

• color: Show each priority level with a different color.
• descriptive: Show log buffer event descriptions. This modifier affects event log buffer messages only, and has no effect on the other non-binary buffers. The event descriptions come from the event-log-tags database.
• epoch: Display time in seconds starting from Jan 1, 1970.
• monotonic: Display time in CPU seconds starting from the last boot.
• printable: Ensure that any binary logging content is escaped.
• uid: If permitted by access controls, display the UID or Android ID of the logged process.
• usec: Display the time with precision down to microseconds.
• UTC: Display time as UTC.
• year: Add the year to the displayed time.
• zone: Add the local time zone to the displayed time.

Viewing alternative log buffers

The Android logging system keeps multiple circular buffers for log messages, and not all of the log messages are sent to the default circular buffer. To see additional log messages, you can run the logcat command with the -b option, to request viewing of an alternate circular buffer. You can view any of these alternate buffers:

• radio: View the buffer that contains radio/telephony related messages.
• events: View the interpreted binary system event buffer messages.
• main: View the main log buffer (default) does not contain system and crash log messages.
• system: View the system log buffer (default).
• crash: View the crash log buffer (default).
• all: View all buffers.
• default: Reports main, system, and crash buffers.

The usage of the -b option is:

[adb] logcat [-b <buffer>]

Here is an example of how to view a log buffer containing radio and telephony messages:

adb logcat -b radio

You can also specify multiple -b flags for all of the buffers you want to print, as follows:

logcat -b main -b radio -b events

You can specify a single -b flag with a comma-separated list of buffers, for example:

logcat -b main,radio,events

Logging from code

The Log class allows you to create log entries in your code that display in the logcat tool. Common logging methods include:

• Log.v(String, String) (verbose)
• Log.d(String, String) (debug)
• Log.i(String, String) (information)
• Log.w(String, String) (warning)
• Log.e(String, String) (error)

For example, using the following call:

Log.i("MyActivity", "MyClass.getView() — get item number $position")

Log.i("MyActivity", "MyClass.getView() — get item number " + position);

The logcat outputs something like:

I/MyActivity( 1557): MyClass.getView() — get item number 1