log-android - React Native

Learn how to view logs from Android devices and emulators for debugging React Native applications.

Overview

Android logging is handled through adb logcat, which displays system and application logs from connected Android devices and emulators.

Usage

Basic Logcat

adb logcat

Displays all system logs (very verbose).

Filter React Native Logs

adb logcat *:S ReactNative:V ReactNativeJS:V

Shows only React Native-related logs.

Your App’s Logs

adb logcat | grep "$(adb shell ps | grep com.yourapp | awk '{print $2}')"

Filter logs from your specific app.

Common Filters

By Tag

# React Native JavaScript logs
adb logcat -s ReactNativeJS

# Native module logs
adb logcat -s ReactNative

# Custom tag
adb logcat -s YourCustomTag

By Priority

# Error and above
adb logcat *:E

# Warning and above
adb logcat *:W

# Info and above
adb logcat *:I

# Debug and above
adb logcat *:D

# Verbose (all)
adb logcat *:V

Priority levels (lowest to highest):

V - Verbose
D - Debug
I - Info
W - Warning
E - Error
F - Fatal
S - Silent (suppress all)

Multiple Filters

adb logcat ReactNative:V ReactNativeJS:V *:S

Show ReactNative and ReactNativeJS at Verbose level, suppress everything else.

Output Formats

Brief (Default)

adb logcat -v brief

Output:

I/ReactNativeJS(12345): 'Hello from JavaScript!'

Time

adb logcat -v time

Output:

01-23 12:34:56.789 I/ReactNativeJS(12345): 'Hello from JavaScript!'

Threadtime

adb logcat -v threadtime

Output:

01-23 12:34:56.789 12345 12678 I ReactNativeJS: 'Hello from JavaScript!'

Long

adb logcat -v long

Most detailed format with full metadata.

Useful Commands

Clear Logs

adb logcat -c

Clears the log buffer.

Save to File

adb logcat -d > android-logs.txt

Follow Logs in Real-Time

adb logcat -v time | grep ReactNative

Last 100 Lines

adb logcat -t 100

Colored Output

Use pidcat for colored, filtered logs:

# Install pidcat
brew install pidcat  # macOS
# Or
pip install pidcat

# Use pidcat
pidcat com.yourapp

JavaScript Console Logs

console.log()

console.log('Hello from JS!');

Appears as:

I/ReactNativeJS: 'Hello from JS!'

console.warn()

console.warn('This is a warning');

Appears in logs and shows yellow box in app.

console.error()

console.error('This is an error');

Appears in logs and shows red screen in app.

console.debug()

console.debug('Debug information');

Native Module Logs

Android Log API

In native Android code:

import android.util.Log;

public class MyModule {
    private static final String TAG = "MyModule";
    
    public void myMethod() {
        Log.d(TAG, "Debug message");
        Log.i(TAG, "Info message");
        Log.w(TAG, "Warning message");
        Log.e(TAG, "Error message");
    }
}

View these logs:

adb logcat -s MyModule

React Native Bridge

import com.facebook.react.bridge.ReactContext;
import com.facebook.react.bridge.ReactApplicationContext;

ReactContext reactContext = getReactApplicationContext();
reactContext.getJSModule(DeviceEventManagerModule.RCTDeviceEventEmitter.class)
    .emit("eventName", data);

Filtering Techniques

Grep Patterns

# Case insensitive
adb logcat | grep -i "error"

# Multiple patterns
adb logcat | grep -E "error|warning"

# Exclude patterns
adb logcat | grep -v "noise"

By Process ID

# Get PID
adb shell ps | grep com.yourapp

# Filter by PID
adb logcat --pid=12345

By Package

adb logcat --pid=$(adb shell pidof -s com.yourapp)

Multiple Devices

List Devices

adb devices

Output:

List of devices attached
emulator-5554   device
3FEDA8BC0      device

Target Specific Device

# By serial
adb -s emulator-5554 logcat

# By transport
adb -e logcat  # Emulator
adb -d logcat  # USB device

Advanced Filtering

Custom Filter Script

Create filter-logs.sh:

filter-logs.sh

#!/bin/bash
adb logcat -v time | grep --line-buffered -E "ReactNative|ReactNativeJS|$1" | \
  grep --line-buffered -v "noise"

Use:

chmod +x filter-logs.sh
./filter-logs.sh MyTag

Color-coded Grep

adb logcat | grep --color=always -E "E/|W/|I/"

Performance Monitoring

Memory Logs

adb logcat -s dalvikvm

CPU and Threading

adb logcat -s ActivityManager

Crashes

adb logcat -s AndroidRuntime

Network

adb logcat -s NetworkScheduler:V

Crash Debugging

View Stack Traces

adb logcat -s AndroidRuntime:E

After Crash

adb logcat -d > crash-log.txt

Saves entire log buffer including crash information.

Native Crashes

adb logcat -s DEBUG

Shows native crash information and stack traces.

Integration with Development

React Native CLI

When running:

npx react-native run-android

Logs automatically appear in the terminal.

Metro Bundler

When running:

npx react-native start

JavaScript logs appear in Metro terminal window.

Separate Log Window

# Terminal 1: Metro
npx react-native start

# Terminal 2: App
npx react-native run-android

# Terminal 3: Filtered logs
adb logcat -s ReactNativeJS

Debugging Tools

React Native Debugger

Standalone app with integrated Chrome DevTools:

brew install --cask react-native-debugger

Provides:

Console logs
Network inspector
Redux DevTools
React DevTools

Flipper

Desktop debugging platform:

brew install --cask flipper

Features:

Logs viewer
Network inspector
Layout inspector
Database viewer

Chrome DevTools

Open developer menu (Cmd+M or Ctrl+M)
Select “Debug”
Open Chrome DevTools
View Console tab

Log Management

Rotate Logs

# Set buffer size (in KB)
adb logcat -G 16M

# View buffer size
adb logcat -g

Multiple Buffers

# Main buffer (default)
adb logcat -b main

# System buffer
adb logcat -b system

# Events buffer
adb logcat -b events

# Crash buffer
adb logcat -b crash

# All buffers
adb logcat -b all

Troubleshooting

No Logs Appearing

Solution:

Check device connection:

adb devices

Restart ADB:

adb kill-server
adb start-server

Check log buffers:

adb logcat -g

Too Much Noise

Solution: Use more specific filters:

adb logcat ReactNativeJS:V *:S

Logs Cut Off

Solution: Increase buffer size:

adb logcat -G 16M

Can’t Find App Logs

Solution: Verify app package name:

adb shell pm list packages | grep yourapp

Then filter:

adb logcat | grep com.yourapp

Best Practices

Use Appropriate Log Levels

Use console.log() for general information
Use console.warn() for warnings
Use console.error() for errors
Use console.debug() for verbose debugging

Tag Your Native Logs

Always use descriptive tags in native code:

private static final String TAG = "MyFeature";
Log.d(TAG, "Clear description");

Clean Up Debug Logs

Remove or disable verbose logging before production:

if (__DEV__) {
  console.log('Debug info');
}

Use Proper Tools

Development: Metro bundler console
Debugging: React Native Debugger or Flipper
Native issues: adb logcat with filters
Production: Error tracking services (Sentry, Bugsnag)

Next Steps

Run Android

Build and run your Android app

iOS Logs

View iOS device logs

Debugging

Complete debugging guide

Troubleshooting

Common issues and solutions

​Overview

​Usage

​Basic Logcat

​Filter React Native Logs

​Your App’s Logs

​Common Filters

​By Tag

​By Priority

​Multiple Filters

​Output Formats

​Brief (Default)

​Time

​Threadtime

​Long

​Useful Commands

​Clear Logs

​Save to File

​Follow Logs in Real-Time

​Last 100 Lines

​Colored Output

​JavaScript Console Logs

​console.log()

​console.warn()

​console.error()

​console.debug()

​Native Module Logs

​Android Log API

​React Native Bridge

​Filtering Techniques

​Grep Patterns

​By Process ID

​By Package

​Multiple Devices

​List Devices

​Target Specific Device

​Advanced Filtering

​Custom Filter Script

​Color-coded Grep

​Performance Monitoring

​Memory Logs

​CPU and Threading

​Crashes

​Network

​Crash Debugging

​View Stack Traces

​After Crash

​Native Crashes

​Integration with Development

​React Native CLI

​Metro Bundler

​Separate Log Window

​Debugging Tools

​React Native Debugger

​Flipper

​Chrome DevTools

​Log Management

​Rotate Logs

​Multiple Buffers

​Troubleshooting

​No Logs Appearing

​Too Much Noise

​Logs Cut Off

​Can’t Find App Logs

​Best Practices

​Next Steps

Run Android

iOS Logs

Debugging

Troubleshooting

Overview

Usage

Basic Logcat

Filter React Native Logs

Your App’s Logs

Common Filters

By Tag

By Priority

Multiple Filters

Output Formats

Brief (Default)

Time

Threadtime

Long

Useful Commands

Clear Logs

Save to File

Follow Logs in Real-Time

Last 100 Lines

Colored Output

JavaScript Console Logs

console.log()

console.warn()

console.error()

console.debug()

Native Module Logs

Android Log API

React Native Bridge

Filtering Techniques

Grep Patterns

By Process ID

By Package

Multiple Devices

List Devices

Target Specific Device

Advanced Filtering

Custom Filter Script

Color-coded Grep

Performance Monitoring

Memory Logs

CPU and Threading

Crashes

Network

Crash Debugging

View Stack Traces

After Crash

Native Crashes

Integration with Development

React Native CLI

Metro Bundler

Separate Log Window

Debugging Tools

React Native Debugger

Flipper

Chrome DevTools

Log Management

Rotate Logs

Multiple Buffers

Troubleshooting

No Logs Appearing

Too Much Noise

Logs Cut Off

Can’t Find App Logs

Best Practices

Next Steps