Android Fastboot

Android Fastboot Operations

⚠️ DANGER ZONE - Fastboot operations write directly to device partitions. Wrong images or interrupted operations can brick devices. Always verify:

1. Correct device (check fastboot getvar product) 2. Correct images for exact device model 3. Bootloader is unlocked (for flashing) 4. Battery > 50%

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Entering Fastboot Mode

# From ADB (device running)
adb reboot bootloader

# Hardware buttons (most devices)
# Power + Volume Down (hold until fastboot screen)

Basic Commands

fastboot devices                    # List connected devices
fastboot getvar all                 # All device variables
fastboot reboot                     # Reboot to system
fastboot reboot bootloader          # Stay in bootloader
fastboot reboot recovery            # Boot to recovery
fastboot reboot fastboot            # Boot to fastbootd (Android 10+)

Check Before Flashing

# ALWAYS run these first
fastboot devices                    # Verify connection
fastboot getvar product             # Verify device model
fastboot getvar unlocked            # Must be "yes" for flashing
fastboot getvar current-slot        # Know current A/B slot
fastboot getvar battery-level       # Should be > 50

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Bootloader Unlocking

Google Pixel

# 1. Enable OEM unlocking in Developer Options
# 2. Boot to fastboot
adb reboot bootloader

# 3. Unlock
fastboot flashing unlock

# 4. Confirm on device with volume keys + power
# WARNING: This wipes all data!

Other Manufacturers

Check Unlock Status

fastboot getvar unlocked            # yes/no
fastboot getvar device-state        # locked/unlocked
fastboot getvar unlock_ability      # Can be unlocked?

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Flashing Partitions

Individual Partitions

fastboot flash boot boot.img
fastboot flash recovery recovery.img      # Non-A/B only
fastboot flash dtbo dtbo.img
fastboot flash vbmeta vbmeta.img

# Disable verification (for custom ROMs)
fastboot flash --disable-verity --disable-verification vbmeta vbmeta.img

Bootloader & Radio (Extra Careful!)

# These can hard-brick if wrong!
fastboot flash bootloader bootloader.img
fastboot reboot bootloader    # REQUIRED after bootloader flash

fastboot flash radio radio.img
fastboot reboot bootloader    # Recommended after radio flash

Temporary Boot (No Flash)

# Test before committing - reboots to normal after restart
fastboot boot recovery.img
fastboot boot patched_boot.img

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A/B Slot Devices

Modern devices have two copies of each partition (A and B).

# Check current slot
fastboot getvar current-slot        # Returns: a or b

# Set active slot
fastboot set_active a
fastboot set_active b

# Flash to specific slot
fastboot flash boot_a boot.img
fastboot flash boot_b boot.img

# Flash to both slots
fastboot --slot=all flash boot boot.img

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Dynamic Partitions (Android 10+)

Large partitions (system, vendor, product) live inside a "super" partition.

Fastbootd Mode

# Enter userspace fastboot
fastboot reboot fastboot

# Check if in fastbootd
fastboot getvar is-userspace        # Should return: yes

Flash Dynamic Partitions

# Must be in fastbootd mode!
fastboot reboot fastboot

fastboot flash system system.img
fastboot flash vendor vendor.img
fastboot flash product product.img

Virtual A/B Snapshots

# Check snapshot status
fastboot getvar snapshot-update-status

# Cancel stuck update (CAREFUL!)
fastboot snapshot-update cancel

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Recovery Operations

Enter Recovery

adb reboot recovery
# Or: Power + Volume Up (varies by device)

ADB Sideload

# 1. In recovery, select "Apply update from ADB"
# 2. Run:
adb sideload update.zip

Flash Custom Recovery (TWRP)

# Test first (temporary boot)
fastboot boot twrp.img

# If it works, flash permanently
# Non-A/B devices:
fastboot flash recovery twrp.img

# A/B devices (recovery in boot):
fastboot flash boot twrp.img

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Magisk (Root)

Installation

# 1. Extract boot.img from your firmware
# 2. Transfer to device, patch with Magisk app
# 3. Pull patched image
adb pull /sdcard/Download/magisk_patched_*.img

# 4. Flash patched boot
fastboot flash boot magisk_patched_*.img

# 5. Reboot
fastboot reboot

Useful Magisk Commands (after rooted)

adb shell su -c "magisk --version"
adb shell su -c "resetprop ro.debuggable 1"

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Factory Reset / Wipe

fastboot erase userdata             # Wipe user data
fastboot erase cache                # Wipe cache (if exists)
fastboot -w                         # Format data + cache

# Full flash with wipe
fastboot -w flashall
fastboot -w update image.zip

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Danger Levels

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Brick Prevention

Never:

• Flash images from different device models
• Interrupt a flash operation
• Lock bootloader with custom ROM installed
• Flash bootloader/radio without matching versions

Always:

• Verify device with fastboot getvar product
• Keep stock images available for recovery
• Use fastboot boot to test before fastboot flash
• Maintain battery > 50%

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Recovery from Soft Brick

# 1. Enter fastboot (Power + Vol Down)
fastboot devices

# 2. Flash stock boot
fastboot flash boot boot.img

# 3. For dynamic partitions
fastboot reboot fastboot
fastboot flash system system.img
fastboot flash vendor vendor.img

# 4. Wipe if needed
fastboot -w

# 5. Reboot
fastboot reboot

For hard bricks (no fastboot access), you need EDL mode (Qualcomm) or manufacturer service tools.

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Quick Reference

Android Partition Reference

Partition Types

Boot Partitions

System Partitions (Dynamic on Android 10+)

Data Partitions

Critical Partitions (⚠️ Careful!)

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A/B (Seamless) Updates

Modern devices have two copies of key partitions for seamless updates.

How It Works

1. System runs from slot A 2. Update installs to slot B (in background) 3. Reboot switches to slot B 4. If B fails, automatically falls back to A

Commands

fastboot getvar current-slot           # a or b
fastboot getvar slot-count             # Usually 2
fastboot getvar slot-successful:a      # yes/no
fastboot getvar slot-retry-count:a     # Remaining attempts

fastboot set_active a                  # Switch to A
fastboot set_active b                  # Switch to B

fastboot --slot=all flash boot boot.img   # Flash both slots
fastboot --slot=other flash boot boot.img # Flash inactive slot

Recovery on A/B

On A/B devices, recovery is part of boot.img, not a separate partition. To install custom recovery:

fastboot flash boot twrp-boot.img
# Or boot temporarily:
fastboot boot twrp.img

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Dynamic Partitions

Android 10+ uses dynamic partitions with a "super" partition.

Architecture

super (physical partition)
├── system_a (logical)
├── system_b (logical)
├── vendor_a (logical)
├── vendor_b (logical)
├── product_a (logical)
├── product_b (logical)
└── (free space for resizing)

Fastbootd Requirement

Regular fastboot (bootloader) cannot flash dynamic partitions. You must use fastbootd (userspace fastboot).

# Enter fastbootd
fastboot reboot fastboot

# Verify
fastboot getvar is-userspace           # Should be: yes

# Now flash
fastboot flash system system.img

Operations

# Create partition
fastboot create-logical-partition mypart 1073741824  # 1GB

# Delete partition
fastboot delete-logical-partition mypart

# Resize partition
fastboot resize-logical-partition system 3221225472  # 3GB

# Reset super to empty
fastboot wipe-super super_empty.img

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Virtual A/B (Snapshots)

Android 11+ adds Virtual A/B with copy-on-write snapshots.

How It Works

1. OTA writes changes as snapshots (not full partition copies) 2. After reboot, snapshots are "merged" into new slot 3. Merge happens in background after boot

Snapshot States

fastboot getvar snapshot-update-status
# none - No pending update
# snapshotted - Snapshots created, waiting for merge
# merging - Merge in progress

Dealing with Stuck Snapshots

# Cancel snapshot (may cause boot issues!)
fastboot snapshot-update cancel

# Then flash normally
fastboot reboot fastboot
fastboot flash system system.img

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Partition Variables

# Get partition info
fastboot getvar partition-type:boot     # raw, ext4, f2fs
fastboot getvar partition-size:boot     # Size in bytes
fastboot getvar is-logical:system       # yes/no
fastboot getvar has-slot:boot           # yes/no

# All variables
fastboot getvar all

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Partition Layout by Device Type

Non-A/B (Legacy)

bootloader, radio, boot, recovery, system, vendor, cache, userdata

A/B (Android 7+)

bootloader, radio, boot_a/b, system_a/b, vendor_a/b, userdata

A/B + Dynamic (Android 10+)

bootloader, radio, boot_a/b, vbmeta_a/b, dtbo_a/b,
super (contains system_a/b, vendor_a/b, product_a/b),
userdata, metadata

A/B + Virtual A/B + GKI (Android 13+)

bootloader, radio,
boot_a/b (kernel only),
init_boot_a/b (init ramdisk),
vendor_boot_a/b (vendor ramdisk),
vbmeta_a/b, dtbo_a/b,
super (system, vendor, product, system_ext, odm),
userdata, metadata

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Flashing Order

When flashing multiple partitions, order matters:

1. bootloader (reboot to bootloader after) 2. radio/modem (reboot to bootloader after) 3. vbmeta (disable verification if needed) 4. boot, dtbo, other boot partitions 5. super or individual dynamic partitions (in fastbootd) 6. userdata (wipe if needed)

# Example full flash
fastboot flash bootloader bootloader.img
fastboot reboot bootloader
fastboot flash radio radio.img
fastboot reboot bootloader
fastboot flash --disable-verity --disable-verification vbmeta vbmeta.img
fastboot flash boot boot.img
fastboot flash dtbo dtbo.img
fastboot reboot fastboot
fastboot flash system system.img
fastboot flash vendor vendor.img
fastboot -w
fastboot reboot

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GSI (Generic System Image)

GSI lets you run a generic Android build on any Treble-compatible device.

# 1. Unlock bootloader
# 2. Boot to fastbootd
adb reboot fastboot

# 3. Disable verification
fastboot flash vbmeta --disable-verity --disable-verification vbmeta.img

# 4. Optional: Delete product to make room
fastboot delete-logical-partition product_a
fastboot delete-logical-partition product_b

# 5. Flash GSI
fastboot flash system gsi-system.img

# 6. Wipe data (required!)
fastboot -w

# 7. Reboot
fastboot reboot