Download and build Mac OS 10.15 Catalina Installer Image

• Download installinstallmacos.py script:

  curl -O https://raw.githubusercontent.com/munki/macadmin-scripts/master/installinstallmacos.py
  

• Execute the script to download MacOS:

  sudo ./installinstallmacos.py
  

• You need to select Catalina from the list, for example:

   #    ProductID    Version    Build   Post Date  Title
   1    041-47723    10.14.4  18E2034  2019-03-25  macOS Mojave
   2    091-95155    10.13.6    17G66  2019-01-08  macOS High Sierra
   3    061-21551    10.14.6   18G103  2019-10-07  macOS Mojave
   4    061-18881      10.15   19A583  2019-10-07  macOS Catalina
   5    041-73257    10.14.5  18F2059  2019-07-31  macOS Mojave
  

  Select 4 from the list.

see also

• Using installinstallmacos.py to download macOS High Sierra installers