TL;DR: Skip to the full Fastfile example below

I have an app written in react native that I’m using expo for. I recently switched to prebuild, and and wanted to preserve my fastlane release workflow. Here’s how I did it.

Prerequisites

Before you start, ensure you install fastlane into your project folder, and your app store connect account is already configured for your app’s bundle ID and signing certificates are installed. Running expo run:ios should configure some of this.

• If you have not, setup fastlane, there are instructions here.
• Also ensure you have read through Fastlane’s guide to beta deployment
• You should already have set up an app within app store connect, and have run expo prebuild to generate an initial version of the iOS Project.
• You should also initialize the bundle identifier field in your app.json, under the key expo.ios.buildNumber. You can set this to "1" or any number you’d like if migrating from another build system, if you wish to use the auto incrementing build number step below.

app.json Auto-incrementing build number

We can use jq to increment the build number within our app.json If you don’t yet have jq, it is available under that name in most package managers.

Fastlane’s sh module allows us to run a raw shell command. In the final script, this must be wrapped in an Dir.chdir("..") do … end block since shell scripts within fastlane run inside the fastlane/ folder as opposed to the project root.

This first edits it using jq, and then replaces the existing app.json with our modified version.

sh("cat app.json | jq -e '.expo.ios.buildNumber |= (tonumber +1 | tostring)' | tee app.json.tmp > /dev/null && mv app.json.tmp app.json")

Running Prebuild in Fastlane

This is fairly straightforward, next we can just tell prebuild to run to ensure everything is up to date:

sh("npx expo prebuild")

Putting it all together

fastlane/Fastfile:

# Uncomment the line if you want fastlane to automatically update itself
# update_fastlane

default_platform(:ios)

platform :ios do
  desc "Push a new beta build to TestFlight"
  lane :beta do
    begin

      # inside the app directory
      Dir.chdir("..") do
        # TASK: make sure `expo.ios.buildNumber` is set in your app.json first!
        # increment app.json build number
        sh("cat app.json | jq -e '.expo.ios.buildNumber |= (tonumber +1 | tostring)' | tee app.json.tmp > /dev/null && mv app.json.tmp app.json")

        # run expo prebuild to regenerate the native app
        sh("npx expo prebuild")
      end

      # build the scheme
      # TODO: edit the scheme / xcworkspace name with your own
      build_app(workspace: "./ios/MyAppName.xcworkspace", scheme: "MyAppName")

      # upload to testflight
      upload_to_testflight
    end
  end
end

fastlane/AppFile:

# TODO: customize these to match your team / app settings
app_identifier("com.example.appname") # The bundle identifier of your app
apple_id("you@example.com") # Your Apple Developer Portal username

itc_team_id("12345678") # App Store Connect Team ID
team_id("9H123S4TFE") # Developer Portal Team ID

# For more information about the Appfile, see:
#     https://docs.fastlane.tools/advanced/#appfile