How to use the command logcat (with examples)

The logcat command is a useful tool for analyzing and troubleshooting issues in Android applications. By displaying system logs, it allows developers to identify errors, view stack traces, and monitor the behavior of their applications. In this article, we will explore several use cases of the logcat command, along with their respective examples and explanations.

Use case 1: Display system logs

Code:

logcat

Motivation: Displaying system logs can provide valuable insights into the behavior of an Android device and the applications running on it. By using the logcat command without any arguments, all system messages, including stack traces and logged messages, will be displayed.

Explanation: The logcat command without any arguments dumps a log of system messages and application-specific logs. It provides a real-time stream of logs, allowing developers to monitor the behavior of the system and the applications running on it.

Example output:

08-01 10:17:42.980 I/ActivityManager: Start proc com.example.app for activity com.example.app/.MainActivity: pid=12345 uid=67890 gids={1234, 5678}
08-01 10:17:43.120 D/PhoneWindow: onPanelClosed(0, com.android.internal.view.menu..)
08-01 10:17:43.123 I/ViewRootImpl: ViewRoot's Touch Event : Touch Down
08-01 10:17:43.456 D/ViewRootImpl: ViewPostImeInputStage processPointer 0

Use case 2: Write system logs to a file

Code:

logcat -f /path/to/file

Motivation: Writing system logs to a file is helpful when developers want to analyze the logs later or share them with team members. By using the -f option followed by the path to a file, the system logs will be written to the specified file instead of being displayed in the console.

Explanation: The -f option is used to specify the file path to which the system logs will be written. By providing a valid file path, developers can save the logs for further analysis or debugging purposes.

Example output: The logs will be written to the specified file (/path/to/file) without being displayed in the console.

Use case 3: Display lines that match a regular expression

Code:

logcat --regex regular_expression

Motivation: When analyzing large log files, it can be challenging to find specific entries. By using regular expressions, developers can filter the log output and display only the lines that match a specific pattern, making it easier to find relevant information.

Explanation: The --regex option is used to specify a regular expression. Logcat will then display only the log lines that match the provided regular expression. This is useful for focusing on specific log entries that are of interest.

Example output: If the regular expression is error|exception, logcat will display only the lines that contain either the word “error” or “exception”.

08-01 10:17:42.980 E/MyApp: Error occurred in method foo(): java.lang.NullPointerException
08-01 10:17:43.220 D/MyApp: Exception caught in method bar(): java.lang.IndexOutOfBoundsException

Use case 4: Display logs for a specific PID

Code:

logcat --pid=<pid>

Motivation: When multiple processes are running on an Android device, it can be challenging to track the logs of a specific process. By using the --pid option followed by the process ID (PID) of the desired process, developers can filter the log output and display only the logs related to that process.

Explanation: The --pid option allows developers to specify the process ID (PID) of the desired process. Logcat will then display only the logs associated with that process, making it easier to analyze the behavior and troubleshoot issues specific to that process.

Example output: Assuming the desired PID is 12345, logcat will display all the logs related to the process with that PID.

08-01 10:17:42.980 I/MyApp: Service started with PID 12345
08-01 10:17:43.220 D/MyApp: Processing request in process 12345
08-01 10:17:43.456 D/MyApp: Request completed successfully in process 12345

Use case 5: Display logs for the process of a specific package

Code:

logcat --pid=$(pidof -s <package>)

Motivation: In some cases, developers might need to monitor the logs of a specific application package running on an Android device. By using the pidof command and the --pid option, developers can easily retrieve the process ID (PID) of a package and display its logs.

Explanation: The pidof command retrieves the process ID (PID) of a running process for a given package name. By using command substitution ($()), the retrieved PID is then passed to the logcat command using the --pid option. This allows developers to display the logs of a specific package.

Example output: Assuming the desired package name is com.example.app, the pidof -s com.example.app command will retrieve the PID of the process associated with this package. The retrieved PID is then used with the logcat command to display the logs of that process.

08-01 10:17:42.980 I/com.example.app: Application started with PID 12345
08-01 10:17:43.220 D/com.example.app: Processing request in process 12345
08-01 10:17:43.456 D/com.example.app: Request completed successfully in process 12345

Conclusion:

The logcat command is a powerful tool for analyzing and troubleshooting Android applications. By using the various options provided by the command, developers can display and filter system logs, analyze stack traces, and monitor the behavior of their applications. These use cases demonstrate different scenarios in which the logcat command can be utilized, enabling developers to efficiently debug and resolve issues.