Most of the time, you can re-download the current version of macOS via the Mac App Store, and older ones via these links:
However, I’ve run into a situation several times where the Software Update mechanism simply refuses to initiate a download:
Thankfully, macOS installers can be downloaded via Terminal in macOS Catalina. This command will download the most recent version of macOS, depositing it in your Applications folder:
softwareupdate command has some neat tricks up its sleeve, as pointed out by Armin Briegel:
--fetch-full-installerflag has a sub-flag:
--full-installer-versionwhich allows you to download a specific version.
During my testing in the Catalina beta version I was able to download 10.15, 10.14.6, 10.14.5, and 10.13.6. I was not able to test if 10.13.6 would download the hardware specific build of 10.13.6 for the 2018 MacBook Pro, since I do not have that hardware.
So, to pull 10.13.6 down, you’d use:
softwareupdate --fetch-full-installer --full-installer-version 10.13.6
I wish Apple would just have a support document up with direct downloads for all of this stuff, but this tool is not a bad alternative.
Update: Don’t miss this documentation from JAMF for more on the subject.