Android shell get foreground app package name

This worked for me:

adb shell dumpsys window windows | grep -E 'mCurrentFocus' | cut -d '/' -f1 | sed 's/.* //g'

com.facebook.katana

Updated answer for Android Q as mCurrentFocus was no longer working for me:

adb shell dumpsys activity recents | grep 'Recent #0' | cut -d= -f2 | sed 's| .*||' | cut -d '/' -f1

The accepted answer might give unexpected results in many cases.

Some UI elements (e.g., dialogs) will not show the package name on the mCurrentFocus (neither mFocusedApp) field. For example, when an app throws a dialog, the mCurrentFocus is often the dialog's title. Some apps show these on app start, making this approach unusable to detect if an app was successfully brought on foreground.

For example, the app com.imo.android.imoimbeta asks for the user country at start, and its current focus is:

$ adb shell dumpsys window windows | grep mCurrentFocus
  mCurrentFocus=Window{21e4cca8 u0 Choose a country}

The mFocusedApp is null in this case, so the only way to know which app package name originated this dialog is by checking its mOwnerUID:

Window #3 Window{21d12418 u0 Choose a country}:
    mDisplayId=0 mSession=Session{21cb88b8 5876:u0a10071} mClient=android.os.BinderProxy@21c32160
    mOwnerUid=10071 mShowToOwnerOnly=true package=com.imo.android.imoimbeta appop=NONE 

Depending on the use case the accepted solution might suffice but its worth mentioning its limitations.

A solution that i found to work so far:

window_output = %x(adb shell dumpsys window windows)
windows = Hash.new
app_window = nil

window_output.each_line do |line| 

    case line

      #matches the mCurrentFocus, so we can check the actual owner
      when /Window #\d+[^{]+({[^}]+})/ #New window
        app_window=$1 

      #owner of the current window
      when /mOwnerUid=[\d]+\s[^\s]+\spackage=([^\s]+)/ 
        app_package=$1

        #Lets store the respective app_package
        windows[app_window] = app_package

      when /mCurrentFocus=[^{]+({[^}]+})/
        app_focus=$1

        puts "Current Focus package name: #{windows[app_focus]}"

        break
    end
end